The Blanco Telescope is equipped with an active optics system in which variable pressure is applied to the rear surface of the primary mirror via 33 pads. This system can modify the figure of the primary to improve image quality. Normally, the the calibration of this system is verified and, if necessary, modified using ImAn every few months.
The primary mirror is supported radially by 24 lever arms articulated to push on the mirror from below and to pull on it from above. These are not normally adjustable.
J.Baldwin, 15 October 1998
1.0 PRIMARY MIRROR CORRECTION FROM LOOKUP TABLE.
Corrections should normally be ON for all foci (prime, f/8).
The present table causes the primary mirror to be bent to correct for a systematic astigmatism effect which is due to problems with the primary mirror support system.
2.0 F/14 COMA CORRECTION FROM LOOKUP TABLE.
Should normally be OFF whenever f/14 is in use.
3.0 IMAGE ANALYZER.
3.1 Before start of night,
3.2 Just before using IMAN
3.3 Measuring a star.
4.0 TWEAKING.
5.0 ERROR RECOVERY
J.Baldwin, 14 November 1995
Last revised: M. Boccas, 3 February 2001
Active Optics systems are now in use on a number of telescopes (ESO NTT, WIYN) and are planned for all future large telescopes. They correct the shape and alignment of the telescope optics on a slow time scale (once every few minutes). This greatly simplifies getting the telescope properly tuned up to start with, and then allows the optics to be continually adjusted in order to compensate for flexure, etc. as the telescope moves around the sky. There is generally a lookup table which automatically changes the Active Optics corrections as a function of telescope position, and often also an image analyzer which uses fairly bright stars to make measurements during the night for additional fine tuning. Gemini will use an image analyzer in this mode almost all of the time.
The system on the 4m Blanco Telescope operates primarily from lookup tables which contain pre-calibrated correction values which can be interpolated to the present telescope position. There also is a provision for occasionally using an image analyzer on a bright star to fine tune (or "tweak") the corrections before observations for which high angular resolution is of special importance, but this will take enough telescope time (10 min?) that it is not expected to be the normal mode of operation.
Active Optics is not Adaptive Optics. Adaptive Optics refers to high speed corrections for seeing effects in real time. Sorry, all we offer is the boring low-speed stuff. (The f/14 secondary, now under development, offers high speed tip-tilt corrections to the image position to compensate for image motion arising from dome and atmospheric seeing and wind-shake of the telescope. This will be our first implementation of ADAPTIVE optics.)
The telescope intercepts light waves coming from distant objects and brings them to a focus. A wavefront is a locus of adjacent points where the electromagnetic wave has the same phase. Except for seeing, the incoming wavefronts, before striking the primary mirror, would be perfectly flat planes perpendicular to the direction to the object being observed. After bouncing off the mirror(s), when approaching the foci, the perfect wavefronts would be spherical in order to arrive at focus in phase.
However, there is seeing, and the telescope is not perfect, so the actual wavefronts are distorted. The distortions can be described as the amplitude A of the displacement of the wavefront, along the direction of travel, from where it should be in the perfect case. It is convenient to use a circular coordinate system oriented perpendicular to the direction of travel. Any point can be specified by radial coordinate r and angular coordinate phi.
The amplitude of the wavefront displacements, A, at that arbitrary point can then be described as a superposition of a series of terms of different radial and angular shapes; this is analogous to describing a complex sound as a sum of simple musical tones, or frequency spectrum.
The typical way to describe the wavefront errors is to use Zernike Polynomials. These are rather complicated functions, usually depending on more than one power of r, which have the nice property (among others) of being mathematically independent of each other (orthogonal). We don't do that. Instead, we follow the example of the ESO NTT (that's where we stole our software from), and describe the wavefront as:
A = c(1,1) * r * cos(phi) + c(2,2) * r2 * cos(2 phi) + ...
... + c(n,m) * rn * cos (m * phi)
summed over all possible values of the integers n and m. c(n,m) is a coefficient giving the amplitude of each term.
The individual terms in this series are called the "Quasi-Zernike Polynomials". The terms are not precisely orthogonal to each other, but under the real conditions in the real telescope, they are close enough.
The Active Optics System includes an image analyzer (IMAN) which measures the shape of the wavefront and then calculates a set of a few low-order quasi-Zernike functions which accurately represent the shape of the wavefront.
There are only a limited number of alignment or bending adjustments which we can make to the telescope's mirrors. Conveniently, each of these potential errors can be related to a different Quasi-Zernike mode. These are all low-spatial-frequency modes, with small values of m and n. The higher frequency modes are caused mostly by seeing and by small-scale polishing errors on the mirror surfaces; the active optics system cannot correct these because the mirror is too stiff.
Table 1 shows the low-order errors that we can measure with the image analyzer and how they are removed (cured) using the Active Optics System.
Table 1: Wavefront errors
| Aberration | Quasi-Zernike | Cure | Comments |
| Defocus | r2cos(0*pi) | Refocus | Easily confused with spherical |
| Spherical | r4cos(0*pi) | Bend Primary | Or move focal plane, change primary-secondary spacing |
| Decenter | r1cos(1*pi) | Repoint | Easily confused with telescope. astigmatism |
| Coma | r3cos(1*pi) | Translate or tilt secondary | |
| Astigmatism | r2cos(2*pi) | Bend primary | Easiest way to bend mirror |
| Trefoil | r3cos(2*pi) | Bend Primary | Usually print-through from hard points |
| Quadrafoil | r4cos(4*pi) | Bend primary | Not expected to be significant |
The Active Optics System has three main components: the 4m Active Primary system (4MAP), the Secondary Mirror Alignment System, and the Image Analyzer (IMAN).
The 4M Active Primary System (4MAP) is able to bend the mirror in modes which will correct for spherical aberration, astigmatism, trefoil and quadrafoil. For each aberration (but spherical aberration) and each focus, there is a lookup table containing corrections as a function of telescope position.
These tables are text files which are stored in /ut02/4map/ and are called: 4mapXY.cof, where X is the aberration (2 is astigmatism, 3 is trefoil and 4 is quadrafoil) and Y is the focus (pf, f8 or f14). Thus there are 9 '.cof' files overall. In addition, there is a file called zero.cof that is a null table (ie. filled with 0) which can be used to replace whatever 4mapXY.cof to cancel/zero the corrections whenever one doesn't want to use the lookup table (note that the telescope operator is instructed to ALWAYS use the lookup tables by selecting 'Corr ON focusxx' in the TCS menu). At a specific focus, the fact that you activate the M1 corrections means that 3 lookup tables -one for astigmatism, one for trefoil and one for quadrafoil- are under use, their values being added vectorially. Usually, only the astigmatism table actually contains numbers, the trefoil and quadrafoil tables beeing filled with 0 (this is because the telescope doesn't suffer from significant trefoil or quadrafoil aberrations). When the 4MAP PC boots, it first reads these 9 files in /ut02/4map/ in order to update its default files to the latest versions. This modification (putting the 4MAP PC on the network) was made in order to allow updating remotely the lookup tables, instead of having to physically seat in front of the 4MAP PC on the mountain as in the old days. Therefore, in order to make effective a newly-entered lookup table, one has to bring the telescope to zenith, turn off the air to M1, exit the 4MAP program and start it again (the 4MAP booting message will tell actually that it updated its .cof file). Correction values are automatically interpolated from this table (which contains 49 standard positions in the sky) to whatever is the current telescope position.
Optionally, we can also apply an additional small constant correction for each aberration. We call it the "tweak" correction. The lookup-table and tweak values are added together vectorally. The contents of the lookup table are only rarely changed (as an engineering-time activity), while the tweak values can be remeasured (using IMAN) each time the telescope is moved to a new part of the sky, if the astronomer wants to take the time. If the astronomer prefers to take the default image quality, using only the lookup tables, the tweak correction can be disabled.
The f/8 and f/14 secondary mirrors each have their own computer-controlled collimation system which tilts the mirror around a point near its vertex. This system permits the removal of coma, and is part of the Active Optics package. In addition, there is a system for manually translating the mirror sideways, intended as a rare daytime adjustment, to handle cases when the tilt adjustment does not have enough range.
Tests show that the collimation does not change significantly as the telescope moves around the sky during the night, but that it does occasionally change (for unknown reasons) over a period of weeks or months. The standard operating procedure therefore is to use the image analyzer on a regular once-per-week basis to check the collimation (and adjust it if necessary), but otherwise to leave it unchanged during routine operation. Any time the collimation value is changed, the new value should be entered in the Active Optics logbook and also written on the white board.
A coma lookup table is now implemented to take into account loss of optimum collimation (due to flexures) when the telescopes moves around the sky. That table is called Xtbl.cof (where X is the focus, either f14 or f8) and is stored in /ut20/tcp4m/tcp/. The coma lookup table is similar to the 4MAP lookup tables of the primary mirror, except that it acts only by producing a tilt adjustment (a 'tweak') of the secondary mirror on top of the nominal tilt values determined by the collimation procedure using IMAN (stored in Last Log Entry). Once you select that option, an 'ON' label will show up next to the focus number in the central window of the TCP blue status window. The label will say 'OFF' if the coma lookup table is not active. For the time being, it should normally always be OFF.
In addition, observers have the option of using the image analyzer to measure the collimation error at any time during their run, and then tilting the secondary to remove that error. This is the equivalent of making a tweak correction to the primary mirror, except that the new correction should be valid all over the sky. If the telescope is recollimated in this way, the new collimation value should be entered in the Active Optics logbook and also written on the white board, and should become the new default value until the next routine check is made.
The Image Analyzer (IMAN) consists of four components:
IMAN is always available at f/8 and f/14. It can be used by the night assistant at any time. It writes its results into a log file. With easy-to-use TCS commands, the night assistant can take results from this log file and use them as input for changing the collimation or the tweak values. There are options to take either the results from the most recent IMAN measurement, or to search through the log file and select some earlier result, or to type in values at the terminal. The IMAN program also makes a recommendation about which tweak values need to be changed and which do not. When tweak values are taken from the log file, the TCS program allows you to either follow these recommendations (the default) or override any of them.
This will be found in /ut22/iman/iman.log The results from a typical measurement will look like:
***************************************************************************
| UT 00:44 08/27/95 HA -01:14; DEC -31:23 f/8 ROT 90.0 | ||||||||||
| SECONDARY | PRIMARY | |||||||||
| coma3 | spher | astig | triang | quad | d80 | |||||
| um | d | um | um | d | um | d | um | d | arcsec | |
| 1 | 0.22 | 80 | -1.57 | 0.64 | 440 | 0.02 | 367 | 0.17 | 12 | 0.47 |
| 1 | 0.28 | 73 | -160 | 0.64 | 452 | 0.03 | 273 | 0.20 | 6 | 0.49 |
| 1 | 0.33 | -71 | -193 | 0.64 | 471 | 0.08 | 292 | 0.18 | 9 | 0.50 |
| Average | 0.09 | 36 | -1.70 | 0.63 | 94 | 0.04 | -64 | 0.18 | 9 | |
| Sigma | 0.15 | 0.17 | 0.02 | 0.03 | 0.01 | |||||
| d80 | 0.01 | 0.19 | 0.21 | 0.01 | 0.08 | |||||
| Tweak? | N | N | Y | N | N | |||||
| d80 (arcsec) |
TEL.FOCUS=172301 GDR: x=0.045 y=-0.04 |
||||||||
| npts | defoc | decen | init | coma | full | ||||
| 1 | 1 | 218 | 1.34 | 21.17 | 219 | 0.57 | 0.56 | 0.47 | |
| 2 | 2 | 218 | 1.24 | 24.00 | 216 | 0.56 | 0.56 | 0.49 | |
| 3 | 3 | 217 | 1.71 | 24.57 | 214 | 0.59 | 0.59 | 0.50 | |
The output first shows results for the three independent 30 sec measurements. Magnitudes of the aberrations are given in microns (um), and the position angles in degrees (d). The rightmost column shows the residual 80% encircled-energy diameter that the image would have after correcting for all of the fitted aberrations (this residual includes the effects of slowly changing dome seeing components, but most of the effects of atmospheric seeing have been averaged out).
The next line gives the vector average for each aberration. After that is a line giving the standard deviation (1 sigma) of the magnitude of each aberration, and then a line giving the 80% encircled image diameter (in arcsec) which would be expected from each average value.
The line labelled "Tweak?" gives a recommendation about whether or not a correction should be made for each aberration: yes (Y) ==> make a correction; no (N) ==> do not change anything. A tweak adjustment is generally recommended for aberrations producing d80 values in excess of 0.1 arcsec, unless there is large scatter in the individual measurements. However, the spherical aberration measurements tend to show huge scatter, and we currently do not recommend making a tweak adjustment for that under any circumstances.
Finally, additional information about each measurement is grouped at the bottom left of the output. "npts" is the number of spots used in the fit; "defoc" is the fitted defocus term (in microns); "decen" gives the fitted decentering term (in microns and degrees). The entries under "d80" are 80% encircled energy diameters at three different levels of correction: "init" is for no corrections; "coma" is with coma removed; "full" is with all fitted aberrations removed.
Commands are invoked by typing the first letter of the command, except for the STAR SEQUENCE and CAL SEQUENCE commands which are invoked with * and /, respectively.
| IMAGE ANALYZER | |
| CALIBRATION POSITION | |
| LARGE APERTURE | |
| SMALL APERTURE | |
| OBSERVE POSITION | (to power off the camera) |
| POWER ON CAMERA | |
| *STAR SEQUENCE | |
| MORE STARS | |
| / CAL SEQUENCE | |
| ABORT STAR SEQUENCE | |
| FLAT MIRROR (IN or OUT) | (IN for GUIDER; OUT for IMAN) |
| !! PELLICLE (IN or OUT) | (IN for IMAN; OUT for GUIDER) |
| IMAN COMMAND TO PC | |
| TILT SECONDARY | |
| INIT TILT | |
| RELATIVE TILT | (tilt to new value) |
| ABSOLUTE TILT | (tilt to new value) |
| LAST LOG ENTRY | (tilt to last value in IMAN log file) |
| OLD LOG ENTRY | (select any value from IMAN log file) |
| DISPLAY TILT | |
| !! ON/OFF AUTO TILT | (activate or not the Coma lookup table) |
| !! PERFORM AUTO TILT | (adjust the tilt to the value of the Coma lookup table for the current position) |
| !! SET TO REFERENCE TILT | (adjust to tilt stored in Last Log Entry) |
| PRIMARY MIRROR CONTROL | |
| GO | |
| HALT | |
| RESET ERRORS | |
| CORRECTIONS ON/OFF | (whether or not to use the Lookup Tables for each focus) |
| TWEAK ADJUST ON/OFF | (options are ENABLE, DISABLE, RESET) |
| SHOW TWEAK | (display entries for lookup table & tweak) |
| MIRROR ADJUST | |
| LAST LOG ENTRY | (set tweak to last value in IMAN log file) |
| OLD LOG ENTRY | (set tweak to any value from IMAN log file) |
| KEYBOARD ENTRY | (set tweak to values entered from keyboard) |
!! shows the REVISED TEXT (3Feb01).
When the LAST LOG ENTRY or OLD LOG ENTRY commands are used from the PRIMARY MIRROR CONTROL menu, the user is asked:
USE DEFAULTS ?
If Y, the changes TO THE PRIMARY MIRROR FIGURE recommended by the IMAN program will be made. This command cannot change the Secondary Mirror's tilt.
If N, then the user is asked:
SPHER :
ASTIG :
TREFOIL :
QUAD :
Answer Y to cause the corrections to be applied.
This accompanying document gives current instructions for:
The idea of the tweak correction is that if the adjustment of the optics is not quite right, you should use IMAN to measure the error and then change the adjustment by the required amount. Therefore, you want to add that change to whatever was the previous setting.
For the secondary mirror tilt, tweaking consists of applying a RELATIVE TILT correction (see Section 6.3), which is always a differential tilt correction from the mirror's present position.
In the case of the primary mirror, if the previous tweak values are not reset to zero (see below) at the time a new tweak command is sent out, the new tweak values get added (vectorially) to the old tweak values. If the lookup table is "ON", the total tweak corrections get added to the lookup table corrections. Normal use is to leave the Lookup Table "ON" (if the f/8 focus is being used; otherwise leave it OFF), but to reset the tweak values to zero (Section 6.2) before making an IMAN measurement to determine the tweak values in a new part of the sky.
This command is used to enable/disable/reset the tweak corrections. When the corrections are "enabled", the TCS Status Screen shows a flashing "TWEAK ON" message and whatever values are in the tweak table are applied to the primary mirror. When the tweak is "disabled", the values in the tweak table are left unchanged, but no tweak correction is applied to the primary mirror shape and the TCS status screen says "TWEAK OFF". When "reset" is selected, the values in the tweak table are set to zero, the tweak correction is disabled, and the TCS status screen says "TWEAK OFF".
up arrow
down arrow
PgUp
PgDn
CTRL-Home
CTRL-End
The lookup-table and tweak corrections can be individually toggled ON and OFF using the LOOKUP TABLE and TWEAK commands in the PRIMARY MIRROR menu. After a tweak correction is enabled, it's up to the astronomer or night assistant to decide when to (and remember to) turn it off. The telescope status screen tells whether LOOKUP TABLE and TWEAK are ON or OFF. "OFF" can mean that the tweak has either been disabled or reset to zero; use the SHOW TWEAK command if you need to know which.
The CCD camera head incorporates a Peltier electrical cooler of the same type as are used with the CCDTV. This is located *inside* the offset guider module, and generates a considerable amount of heat which can escape up the telescope's chimney, directly in the light path. The cooler is not always enabled, but when it is, leaving the IMAN power on for a long time is likely to generate bad seeing. The power is therefore remotely controlled, and should only be turned on for brief bursts when IMAN is actually in use. Use the menu command POWER ON CAMERA to turn it on; use OBSERVE POSITION to turn it off.
back to top
Check the list of error messages provided on the IMAN page.
This message usually indicates a failure in the NFS link between IMANPC and IMANSUN.
See Section 3.4 of the Iman Image Analyzer WWW page or manual.
Appears in a separate small blue box if a star sequence is aborted using the ABORT STAR SEQUENCE command in the IMAN menu. Use CTRL-F2 to clear the blue box from the screen.
| 1. | Menu Commands | |
| 2. | Command Mode commands | |
| 3. | Units, etc |
J.Baldwin, G. Schumacher, 14 November 1995
The f/7.8 secondary mirror is controlled by a CTIO "Smart Motor Controller". The mirror can be both focused and tilted by the operation of three computer-controlled jack screws which are spaced 120 degrees apart on the back of the mirror cell. Each screw is driven by by its own servo motor, which includes an incremental encoder. In addition, a Futaba linear encoder is mounted next to each of the jack screws, and gives an independent reading of the position of the jack screw to an accuracy of nominally 1 micron.
Control commands are normally issued from the "Tilt Secondary" and "Focus Secondary" menus on the TCS screen. The commands are:
TILT SECONDARY
INIT TILT
RELATIVE TILT (tilt to new value)
ABSOLUTE TILT (tilt to new value)
LAST LOG ENTRY (tilt to last value in IMAN log file)
OLD LOG ENTRY (select any value from IMAN log file)
DISPLAY TILT
FOCUS SECONDARY
INIT FOCUS (reset zero points of encoders, then return to present focus position)
MOVE TO VALUE
STEP FOCUS
Commands for the mirror can also be typed into the TCS using the Command mode:
sec encoder
Return readings of Futaba encoders A, B1 and B2, and differences, in the order A B1 B2 (B1-A) (B2-A). Units are microns of motion at the secondary mirror. All other commands dealing with focus motions use units of microns of movement of the focal plane.
sec tilt [tilt amplitude] [tilt azimuth]
Tilt secondary to specified absolute position.
sec focus [value]
Change focus by specified DIFFERENCE from present focus.
sec afocus [value]
Change focus to specified absolute value.
sec display
Show present values of tilt and azimuth.
sec fast [value]
Set fast focus speed, in arbitrary units. Default = 120.
sec slow [value]
Set slow focus speed, in arbitrary units. Default = 50.
sec init
Moves mirror to fiducial position and rezeros encoders. Does NOT restore previous focus value (unlike the menu command).
sec reset
Zeros out all registers and hardware; leaves smart motor controller ready to receive commands.
The three jack screws and their accomapnying Futaba encoders are labelled A, B1 and B2. A is on the South side of the secondary mirror when it is in its observing position; B1 is on the NW side and B2 is on the NE side.
Focus motions are achieved by driving all three jack screws by the same amount. Focus units are microns of travel of the focal plane, = 9.56819 times the motion at the secondary mirror (but Beware!, the command "sec encoder" returns the values at the secondary mirror). Positive focus changes move the focal plane upwards.
Tilt is produced by driving the three jack screws by differing amounts, so as to tilt the mirror about a point located 4.68 inches behind its vertex. This center of tilt was chosen because it is in the plane defined by the three rollers which provide the lateral support beween the inner mirror cell (which moves) and the outer mirror cel (which doesn't move).
Tilt units are microns of wavefront error for coma3 at the edge of the pupil (the units returned by IMAN), and the azimuth of the error. The mirror will then tilt so as to remove that amount of coma. This is to maintain consistency between values pulled from the IMAN log and values entered manually. The conversion to the actual angular tilt of the mirror is:
.0133 degrees of tilt = 1 micron of coma3.
The tilt azimuth is defined as 0 deg azimuth to the west, and then increasing as you go around to the north. Including the fact that the mirror is moved so as to remove the entered coma value, a tilt request containing a positive coma amplitude will cause the side of the secondary mirror in the specified PA to move closer to the primary mirror while at the same time the opposite side moves away from the primary mirror. Therefore, relative to the tilt=0 fiducial position:
| sec tilt 1 0 |
Lowers W side, raises E side through a 0.0133 degree tilt. |
|
| sec tilt 1 90 | Lowers N side, raises S side. |
The above tilt changes cause the absolute tilt (read out using the DISPLAY TILT menu command) to change by the requested amplitude but with a position angle which is the requested PA - 180 degrees. This will be added vectorially to the previous absolute tilt. For example, a relative tilt of:
sec tilt 1 90
will produce an absolute tilt of
| 1.0 micron PA 270 | if the starting point was abs. tilt = 0 0. | |
| 1.4 micron PA 225 | if the starting point was abs. tilt = 1 180 |
| Contents | ||
| 1.0 | Primary Mirror control Program description | |
| 2.0 | Command description | |
| 3.0 | Program Databases | |
| 4.0 | Program Maintanance | |
| 5.0 | Position Angle conventions | |
| 6.0 | Amplitude conventions | |
4MAP
G.Schumacher, 14 December 1995
FOR MORE INFO: Hard-copy manual "4M Active Primary Mirror Support System Operating Manual", by G.Perez et al.
Also: Calibration Positions for 4MAP Lookup Tables [2].
Controlling the primary mirror consists of applying a calculated pressure to the support pads. There are 33 pads distributed uniformly in two concentric rings: one called the outer ring, having 21 pads and the other called the inner ring, having 12 pads. In the outer ring there are also three hard points, separated at 120 degrees each, where the mirror sits when there is no pressure applied.
Associated to each pad, there is a pressure controller named MAMAC CONTROLLER, that outputs a pressure proportional to a voltage applied to it. The output pressure in turn is sensed and converted back to a voltage that is read by an analog to digital converter. Therefore, in order to control each MAMAC there is a DAC and an ADC device connected to it.
A control cycle consists then on calculating the pressure to apply to each pad, convert that pressure to a voltage, instruct the DAC to generate that voltage, and read back the voltage proportional to the output pressure sensed by the ADC. In between control cycles, the program constantly monitors each device and takes several actions on error conditions.
The control program is designed to operate in one of two modes. The first mode is called EMULATION MODE, and consists of emulating the behaviour of the old "passive" mechanical controllers. That behaviour is based on applying an equal pressure to every pad in each ring, proportional to the cosine of the zenith distance of the telescope. This pressure is called the Nominal Pressure and can be defined independently for the outer and inner ring. The second mode is called ACTIVE MODE and consists of adding different pressures to the basic Nominal ones, calculated based on known aberrations, parameterized in terms of tables of coefficients for each term of the distortion model. Switching between modes is done with the ACTIVE coefficients command.
The control program runs in one of five states: The START state, the HALT state, the ERROR state, the ADJUST state and the CHECK state.
An error condition causes the pressure to be dropped abruptly by activating the safety valves. A zero voltage is also written to the DAC's. In order to activate the control again, it is necessary to change to the HALT state by issuing the RESET command, followed by the GO command.
The ADJUST state is entered from the START state or from the CHECK state by an ADJ command. In this state, the pressures are calculated and then converted to voltages that are applied to the MAMACS. Before applying the new voltages a check is made to lower first all pressures that will be lower than the present ones and then raising all pressures that will be higher than the present ones. This is to avoid an intermediate situation in that the mirror might be lifted due to the total sum of pressures might be larger than the mirror weight.
The CHECK state is entered after a successfull adjust process. In this state the program continuously monitors the condition of the DGH modules and the MAMAC modules. In particular, the pressure is read back and checked against the requested one. If a pressure changes, no attempt is made to correct it, but the ERROR state is entered if the change is greater than a certain limit (presently 2 psi). The TCS link is also monitored. If no TCS command is received after 1 second has elapsed from the last one, it is assumed that the RS485 link or the TCS is broken and the pressure gets dropped by openning the safety valves.
The user interacts with the program by giving commands in a Command Window at the PC terminal. Several of the commands could also be issued from the TCP user interface. In that case, all commands should be preceded by the "box id". The id for the 4M Active Primary control PC is "4map".
act
This command turns on or off the calculation of active corrections for a given mirror position. By default, the program starts in the off state. Format:
act [on / off]
adj
This command causes the program to calculate a new set of pressures and apply them to the MAMAC controllers. This command takes as its arguments the present telescope hour angle (hours) and declination (degrees).
Format:
adj hour_angle declination
adj -1.23 -47.35
IT IS ILLEGAL AND DANGEROUS TO GIVE ADJUST COMMANDS WITH ERRONEOUS POSITION INFORMATION SINCE THE CONTROLLER WILL APPLY THE WRONG PRESSURES TO THE MIRROR.
c0, c2, c3, c4
Specify new values for the individula active corrections. c0 specifies spherical, c2 astigmatism, c3 trefoil and c4 quadrafoil.
These commands set the total value of the corresponding correction.
Format:
c0 amplitude(nm)
c0 2000
c2 amplitude(nm) PA(deg)
c2 1000 45
c3, c4 have same format as c2.
c0twk,c2twk,c3twk,c4twk
Specify values which will be added vectorially to the existing amplitude and PA of the corresponding correction. "twk" refers to the "tweak" command in the TCP. Format same as c0,c2,c3,c4.
din
This command reads the digital input port of a DGH module. The result is returned as an hex number.
Format:
din module_address
din Q
dout
This command writes to the digital output port of a DGH module. The value should be given as an hex number.
Format:
dout module_address value
dout x 2
go
This command activates the control cycle. The telescope must be at zenith with air on and the program must be in the HALT state. A test is made of all modules, and if satisfactory, the mirror gets supported with the proper pressures.
halt
This command halts the control cycle by sending a zero voltage to all the MAMACS. In this state, all periodical TCS communications ceases.
help
Lists help info on screen.
i
This command defines the DGH addresses for the inner ring pads. See the Nomenclature Diagram for the numbering scheme.
Format:
i pad_number DAC_address ADC_address
i 8 2 Y
o
This command defines the DGH addresses for the outer ring pads. See the Nomenclature Diagram for the numbering scheme.
Format:
o pad_number DAC_address ADC_address
o 8 J q
pin
This command changes the default Nominal Pressure for the inner ring. The pressure is given in units of psi.
Format:
pin [pressure]
pin 9.0
pout
This command changes the default Nominal Pressure for the outer ring. The pressure is given in units of psi.
Format:
pout [pressure]
pout 8.5
pp
This command calculates and prints the pressures on the screen. The command arguments are an hour angle and a declination. This command doesn't interfere with the normal calculations done with the adj command, so it's useful for debugging the active corrections.
pp hour_angle declination
reset
This command resets an error condition and place the program in the HALT state. This command must be given prior to go, after an error condition.status This command returns a textual description of the program status. Possible responses include:
OK CORRECTIONS ON/OFF
This message indicates that the system is active and no errors are present.
ERROR 5: HALT
This message indicates that the system is in the HALT state. To activate it, a go command must be given.
ERROR 5: MAMAC 12 BAD 2.351 0.000
This message indicates that when checking the MAMAC voltage (number 12 in this case), a difference of more than 0.5 volts was detected. This might be due to a bad DAC, a bad ADC or a bad MAMAC. To determine the offender, a specific test should be run for each module associated with that MAMAC unit (see the Nomenclature Diagram).
ERROR 5: DGH 2 NO RESPONSE
This message indicates that the specific module (2 in this case) is not responding to commands issued to it.
test
This command orders the execution of test for the DGH modules or MAMAC units. The tests are run on all modules or units. If you want to test a specific module, use the vin or vout commands. The responses are similar to the ones described under the status command.
Format:
test dgh/mamac
vin
This command reads the voltage of one DGH module or all ADC modules. The argument is the module address. If the address is '*' then read all ADC modules.
Format:
vin module_address (or *)
vout
This command outputs a voltage to one or all DAC modules. Be aware that this is an active command so if air is on, a pressure will be applied to the pads. Use with care and only if you know what you are doing. As a rule of thumb, the relation of voltage to pressure is close to 1 to 4 (i.e. 1 volt 4 psi).
Format:
vout module_address (or *) voltage
vout * 0.0
x
This command defines the DGH addresses for modules not related with the pads. This are the ones that act on the solenoid valves or receive information on the various switches.
Format:
x module_number module_address
zero
This command sets all voltages to zero. This is equivalent to vout * 0.0.
?
Lists help info on screen.
The program uses two databases for its proper functioning: a Parameters Database, called "4map.par" and a Coefficients Database, called "4map.cof". These are currently located on ctiot0, on the /ut02 disk, in the 4map directory.(8Jul04).
The Parameters Database is a collection of commands that defines the starting values for the program. Any valid command could be placed in this file, that gets executed at startup. In particular, the DGH addresses are to be found here so, if a module is changed the new address should be modified accordingly. A '*' character at the beggining of the line indicates a comment; therefore, the file is self documented.
The Coefficients Database contains the parameter values for the different aberrations, mapped around the sky. The map is made in terms of zenith distance and azimuth positions. Each line contains the values for a certain azimuth. Normally, there are 10 values per line, corresponding to zenith distances of 0°, 15°, 30°, 45° and 60°. The first 5 values corresponds to the amplitude parameter and the next 5 values corresponds to the angle parameter. The azimuth values span the range of 0° to 360°, in steps of 30°. Again, an '*' character at the beginning of the line indicates a comment. The hour angles and declinations at which the IMAN measurements for this table should be made are listed in "Calibration Positions for 4MAP Lookup Tables" [2].
All the source code resides in directory \AP\SOURCE on drive C: of the control PC. The program is entirely written in C and the MICROSOFT C/C++ compiler rev 8.0 is used to produce the object modules.
The process of making a new executable is automated by using the NMAKE utility. There is a MAKEFILE that declares all the files and libraries needed. The procedure then consist of editting the necessary files and then typing the command 'NMAKE'.
The active force patterns are generated by using the Mamac controllers to increase or decrease the air pressure in specific air bags, as compared to the nominal air pressure required to support the mirror at a given telescope position. A positive correction to the air pressure moves the corresponding part of the mirror upwards, while a negative correction lowers the corresponding part of the mirror.
4MAP can correct abberations with the azimuthal position cos(m*phi - phi0), for the following values of m:
| m | abberation |
| 0 | spherical |
| 2 | astigmatism |
| 3 | triangular (trefoil) |
| 4 | quadrafoil |
The corresponding cos(m*phi-phi0) force patterns are then superimposed on the mirror. The sinusoidal force pattern is repeated m times going around the mirror. An amplitude and a position angle must be specified in order to generate this pattern.
The position angle convention for phi0, when a positive amplitude is requested, is:
When TWEAK commands are entered through the TCP menu system, the requested amplitudes are accepted in the units measured by IMAN, and are then scaled by calibration factors before being passed on to 4MAP. The current calibration factors are:
| Abberation | m | Calibration factor |
| Spherical | 0 | 0.00288 |
| Astigmatism | 2 | 0.00101 |
| Trefoil | 3 | 0.00117 |
| Qaudrafoil | 4 | 0.00123 |
where the values entered through the menu commands are DIVIDED by the calibration factor before being passed on to 4MAP.
However, when force patterns requests are entered directly into the 4MAP PC using the commands c0,c2,c3 or c4, the amplitudes must be specified without the scale factors. The units then correspond to the deflections predicted by a simplified analysis of the mirror performed by Lothar Noethe at ESO.
The calibration factors also convert the micron units used by IMAN into the nanometer units used by 4MAP, and would be 0.001 if the calculations by Noethe had been perfect. So Lothar's analysis came really close on every abberation except spherical.
| Contents | |||
| 1.0 | INTRODUCTION | ||
| 2.0 | OPTICS | ||
| 3.0 | CAMERA | ||
| 3.1 | General Description | ||
| 3.2 | Thermoelectric Cooler | ||
| 3.3 | Camera Commands | ||
| 3.4 | Restart Procedure | ||
| 3.5 |
Hardware/Software requirements for IMAN PC |
||
| 4.0 | REDUCTION SYSTEM | ||
| 4.1 | Major Programs | ||
| 4.2 | Basic Subroutines | ||
| 4.3 | Tweak Recommendation | ||
| 4.4 | Sample Output | ||
| 4.5 | Log Files | ||
| 4.6 | Auxiliary Programs | ||
| 4.7 | Testing Iman | ||
| 4.8 |
Error Messages (and what to do about them) |
||
| 5.0 | CONTROL SYSTEM | ||
| 5.1 | Menu Commands | ||
| 5.2 | Cal sequence command | ||
| 5.3 | Star sequence command | ||
| 5.4 | More star command | ||
| 5.5 | Observe position command | ||
Jack Baldwin
26 April 1999
with edits by
B. Gregory (29 Nov 1999)
M. Boccas (18 Aug 2000, 19 Dec 2000)
R.Cantarutti (30 Jan 2001)
The image analyzer IMAN is integrated into the offset guider at the cassegrain focus of the 4m Blanco telescope. IMAN consists of four components:
IMAN is always available at f/8 and f/14. It can be used by the night assistant at any time. It writes its results into the log file /ut22/iman/iman.log. With easy-to-use TCS commands, the night assistant can take results from this log file and use them as input for adjusting the telescope optics.
The actual image analyzer is a 80 mm diameter x 200 mm long tube with a small CCD head mounted on the back. The tube fits inside the offset guider, in a space that was originally used for an image-dissector. Inside the optics tube is a collimator lens which views the telescope's focal plane, followed by a Shack-Hartmann lenslet array which reimages onto the CCD. This whole unit moves around with the guide probe; the CCD head includes only the CCD and a Peltier cooler, and is connected to an electronics unit mounted on the outside of the guider.
Light is fed into the image analyzer using the optical train originally intended to feed the image-dissector back in the days when it was the detector for the guider. Nowdays the detector for the guider is a CCD-TV system which is mounted on the outside of the offset guider shell, at a port originally intended for an eyepiece. A remotely movable pickoff mirror (the "flat mirror") can either divert light to the guider TV, or let it pass through to the image analyzer. A pellicle beamsplitter parallel and next to the flat mirror was installed (1998) and can be used: it will direct 10% of the light to the guider and 90% of the light to the image analyzer, allowing simultaneous guiding while running IMAN.
Figure 1 [3] shows the light path. Light coming from the telescope's secondary mirror first strikes a 45-deg diagonal pickoff mirror, then arrives at the position of the movable flat mirror. When the flat mirror is moved out of the way, the light passes through a folding prism, then through an aperture which is in the focal plane of the telescope, and finally into the IMAN optics tube which contains the collimator, lenslet array and CCD. The aperture is on a 3-postion wheel. The normal observing position is a 2.0mm diameter (13 arcsec) hole. The other positions are a much larger (133 arcsec) hole, and a calibration position which consists of a pinhole with an LED behind it.
The calibration position feeds a perfectly spherical wavefront into the image analyzer. The collimator converts this into a plane wavefront which then strikes the lenslet array. Each lenslet converts the light incident on it into a point image on the CCD. Thus an array of spots is formed (Figure 2 [4]). An imperfect wavefront coming from a star follows the same path, but each spot is displaced from the calibration position by an amount proportional to the inclination of the wavefront at the lenslet (Figure 3 [5]).
Both the flat mirror and the aperture wheel are remotely controlled from the TCS. They can be operated from the IMAGE ANALYZER menu using the following commands:
CALIBRATION POSITION
LARGE APERTURE
SMALL APERTURE
OBSERVE POSITION (move flat mir to GDR, ap to SMALL, CAMERA OFF)
FLAT MIRROR (set to GDR or IMAN)
PELLICLE (set to IN or OUT)
or they can operated from the command window using the following commands:
| iman cal | select calibration aperture; set flat mirror to iman position. |
| iman large | select large aperture; set flat mirror to iman position. |
| iman stop |
set flat mirror to guider position; small aperture; camera to idle; turn OFF camera power. |
| iman camera on | turn on power to camera electronics box. |
| iman camera off | turn off power to camera electronics box. |
| iman flat in | set flat mirror to guider position. |
| iman flat out | set flat mirror to iman position |
The Image Analyzer (IMAN) uses an upgraded version of one of our CCD-TV camera systems. A larger format CCD, a 12 bit serial A/D converter, a new sequencer, and a Coreco video processor board running in a 486-66 computer, were adopted for the IMAN.
The CCD is a 770 x 1152 CCD used in frame transfer mode, effectively yielding a 770 (H) x 576 (V) format. Pixels are 22.5 x 22.5 microns. This is a thick, front illuminated device produced by EEV in England. The chip can be cooled down to about -35 C by way of a Peltier cooler. This CCD is used in multiple pinned phase (MPP) mode, and it can also be operated at room temperature. We have actually taken 30 second integrations and observed an increase in the background level of less than 5% of the full dynamic range (4096 counts) for the A/D converter, while running the CCD at dome temperature (8 C).
The data are digitized before leaving the electronics in the Cass Cage and are sent as a serial stream of bits -where each CCD pixel is represented by 12 bits- to the computer in the Console room. Software commands replace the former User-Interface (CEU) front panel switches. The new design of the sequencer is based on a Xilinx field programmable gate array (FPGA) and an extended set of commands is available to the software. The software also controls the Coreco board which gets used to both process and display data.
For the long integrations, the gain is set to about 50 e/ADU, so that the maximum count possible of 4095 (for the 12 bit A/D converter) represents somewhat less than the CCD full well condition.
The camera head is equipped with a Peltier cooler. The cooler generates about 16 watts while it is operating, which is an unfortunately large amount of heat to dump into the offset guider (it presumably leads to hot air bubbles going up the stovepipe baffle, directly through the telescope's lightpath). For this reason, the power to the camera and cooler assembly is remotely controlled from the console room, and is normally turned off except when an IMAN observation is being made. The CCD cools down almost instantly when the cooler power is turned on. This is an adequate level of heat control when IMAN is used only occasionally during the night.
However, IMAN is sometimes used all night long, such as when the sky is being mapped to prepare new lookup tables for the active optics. Under these conditions, it is more convenient to leave the camera power switched on all night. We have found that IMAN actually works fine when the CCD is used uncooled, at ambient nighttime temperature at least as high as 8°C. However, we don't know whether or not cooling will be needed on warm summer nights.
To give a choice about whether or not to use the cooler, the on/off switch on the camera electronics box (on the side of the instrument rotator) has three positions:
This switch should normally be in the "ON (NO TEC)" position (camera ON, cooler OFF). During the summer months the "ON (ALL)" position (camera ON, cooler ON) may be required to suppress hot pixels. The middle position (camera OFF) should never be used; the camera power is remotely controlled.
These can either be typed in directly at the IMAN PC, or entered from the TCS Image Analyzer menu using the command "IMAN COMMAND TO PC", or entered in the TCS command mode (preceeding each command by "iman pc"; cf. type "iman pc histo").
| abort |
Abort the current integration |
| cal [integ time] |
Take cal frame and send it to Sun Default [int time] is 3 seconds |
| cc |
Stop whatever is doing, take cal frame and send it to Sun. Default [int time ] is 3 seconds. |
| cur | Display cursor on image screen. Only works when no grab is in process. L button to display cursor position and 9 data values. R button to quit. |
| e [on/off] | Camera erase on/off. Default is "off" |
| es | Initiate a star sequence. Takes three exposures and sends them to Sun. Current integration time is used. |
| fill [pixel value] | Fill coreco frame buffer with pixel value. |
| g [gain value] |
Set gain parameter. Legal values are 2,4,6,8,10,20. Default is 2. |
| grab | Initiate continuous image grabbing. |
| gstatus | Returns the grab status ok/busy |
| help | List help info on screen. |
| histo | Returns image statistics: min, max, mean, std deviation. |
| i [integ time] | Change integration time. If [integ time] ends in "m", the units are milliseconds. Otherwise, units are seconds. The default is to take 100 msec integrations in the grab mode. |
| o [offset value] | Change the offset parameter. The default is 231. |
| oi | Gets the contents of a coreco register |
| of | Toggle the olut on/off |
| olut mean stdv | Defines a new olut for display |
| one | Acquire one image frame. |
| os | Sets the contents of coreco register value |
| quit | Quit the program. |
| s | Stop a grab operation. |
| star | Initiate a star sequence. Takes three 30 sec exposures and sends them to Sun. |
| status | Returns program status: IDLE, GRAB, CAL or STAR. |
| sstatus |
Returns number of last star image sent to Sun (0 = none sent, or 1,2,3). |
| ? | List help info on screen. |
The IMAN PC should be restarted at the beginning of any night that IMAN will be used. This is the best way to ensure that the NFS link between the PC and the Sun will be working. To restart:
BUT...check to see if either of the following messages are buried in the lines of output written by gonfs:
"NSF216F-CTIO4m is not a PC-NFS authentication server."
or
"NSF216F-CTIO1m is not a PC-NFS authentication server."
If one of these messages appears, try reboot and gonfs one more time. If the message still appears, call the data system specialist, then go ahead and try the next step anyway... maybe things will work for a while.
The following information was provided by Ricardo Schmidt on 30 Nov 1995:
IMAN PC:
| DOS version: | uses DOS 6.1, although it should be non-critical. |
| PC computer: | 486/66 with 8 MB of RAM, 250MB Hard disk drive, 5.25 floppy drive (bad news: the 3.5 floppy drive fell through the cracks ...) |
| type of bus: | ISA |
| boards involved: | |
| ethernet adaptor: | 3COM 3C503, used with PC-NFS software. |
| video board: | uses a Viper, (non critical). |
| HD interface: | IDE |
| 485 interface: | RS422I-P, by Industrial Computer Source. |
| Special Boards: | |
|
COMEX (command extender (CTIO made), documentation in ERF8886). Coreco video processor: model Occulus F/64 (serial port for mouse) |
|
| Minimum software: | |
|
PC NFS All that is in directory ODX (Coreco related). Includes files in IMAN subdirectory. All that is in directory ODF64 (Coreco related). All that is in directory ODCI (Coreco related). Mouse related software Viper related software Autoexec.bat (special) Config.sys (special) (PCTools) |
|
| Application software backup: |
It would be best to back up directly from the PC (copy to another hard disk via Lap Link software?). German has the originals. |
| Hardware documentation: |
Full set of schematics (on Tololo) Description of IMAN (on Tololo) Hardware manual which includes additional technical notes. In progress (bug Ricardo). |
| Commercial software backup: | It would be best to back up directly from the PC (copy to another hard disk via Lap Link software? ). German has the originals. |
The Coreco board in the IMAN PC has to be correctly formatted to work with the detector. The formatting information is contained in files with extension .vid. The iman program uses the file user.vid. That and other format files of historical or technical interest can be found in the directory: C:\odf64 on the IMAN PC.
The .vid file is created and modified by the program camera.exe. Execute it in the directory C:\ODX by typing simply "camera". This will bring up a semi-self evident control panel. It comes up displaying the parameters in the current user.vid file. At the bottom of the display it shows "fwin ncols nrows". The correct format currently is 688 cols and 570 rows. These are computed from the total numbers of rows and columns and the numbers of blanked rows and columns.
Specifically, the current values are obtained as follows:
number of rows: 574 total - 4 blanked = 570 rows
number of columns: 768 total - 5*16 blanked = 688
These values should be entered in the appropriate spaces in the control panel*.
The number of blanked columns is a multiple of 16.
The Open command (type Capital O) looks for .vid files and displays them in a rolling list from where they can be selected and examined.
The data are modified using the lower part of the control screen, the display updates to show the effect of the changes. The results may be Saved (type Capital S) and you are prompted to name the file to which the parameters will be saved. Normally you before running the camera program, you should copy user.vid to some backup file. Then the new file created can be called user.vid and will be ready for use by the iman program.
* Brooke hopes this is sort of semi-right --- he has never actually done this.
The IMAN reduction programs are adaptations of the programs used at the NTT. They run on the machine which we designate the IMAN SUN; currently ctiot2. There are 3 Fortran programs:
IMANCAL processes a calibration image.
/ut22/iman/imancal file focus ut-date ut-time
where:
| file | =name of data file |
| focus | = 'f/8' or 'f/15' or 'f/30' |
| ut-date | = universal time, date |
| ut-time | = universal time, time |
writes ascii output to:
iman.cal
* via German's routine spout
iman.log
iman.log.star
IMANSTAR processes a star image.
/ut22/iman/imanstar file focus ut-date ut-time gdr-x gdr-y gdr-rot
where:
| file | = name of the data file |
| mode | ='cal' or 'star' |
| focus | ='f/8' or 'f/15' or 'f/30' |
| ut-date | =universal time, date |
| ut-time | =universal time, time |
| foc | = focus value |
| gdr-x | = x coordinate of guide probe (mm) |
| gdr-y | = y coordinate of guide probe (mm) |
| gdr-rot | = instrument rotator angle (deg) |
writes ascii output to:
* via German's routine "spout"
iman.log
iman.log.star
IMANAV produces average results for the images previously processed with imanstar.
/ut22/iman/imanav
(no arguments)
writes ascii output to:
* via German's routine "spout"
iman.log
All arguments are strings. The file argument is the only one that is really needed. Output to * normally gets redirected to the Sun screen in front of the night assistant.
Imanav and imanstar communicate through the file /ut22/iman/iman.sums
Three cshell scripts are also used. The script rmi initializes the sums used in the averages, rmc erases old calibration data, and rmd deletes old star data images.
rmd
rm /ut22/iman/imans*.bin
rmi
rm /ut22/iman/iman.sums
Imanstar and imancal call four subroutines which started life at ESO as four independent programs. These are:
The program IMANAV generates a recommendation about which aberrations should be corrected ("tweaked") and which shouldn't. This recommendation is presented as a "Y" (yes) or "N" (no) decision. The algorithim used is that the measured aberration must be above some minimum threshold, plus the average of the three individual measurements must be above some preset number of standard deviations of the individual measurements. The minimum threshold incorporates our experience with the measurement errors along with the criterion that any correction should be predicted to have at least some minimum effect on the predicted 80% encircled energy diameter of the image.
The present (3 Nov '95) settings for the Y/N criteria are:
| coma3 | spher | astig | triang | quad | |
| minimum d80 (arcsec) | 0.1 | 1.0 | 0.1 | 0.1 | 0.1 |
| min. std. deviations (um) | 2.0 | 3.0 | 2.0 | 2.0 | 2.0 |
| Scale factor to convert from wavefront error to d80: | |||||
| scale factor (arcsec/um) | 0.14 | 0.11 | 0.33 | 0.39 | 0.424m |
For most aberrations the minumum d80 is set at 0.1 arcsec. This is on the argument that if all five correctable aberrations have errors this size, they will combine in quadrature with a 0.5 arcsec image to produce 10% degradation in the observed d80. However, the spherical aberration measurements show such a huge scatter that the d80 threshold is set to 1.0 arcsec, effectively turning off this correction.
The output is sent to the TCS screen and also to a log file on the IMAN SUN called /ut22/iman/iman.log
8/17/00 UPDATE: the coreco board has exhibited problems for more than a year in the sense than about 50% of the time it doesn't update its buffer and doesn't transfer the last acquired image to the pc. Instead it keeps the last image and repeats it. That causes some errors in the averaging of the images for aberration calculations and induce the M1 tweak or M2 tilt correction to be unaccurate. The imanstar and imanav fortran programs were modified to include a comparaison test that does recognize any consecutive repeated frames and diregard them in the average calculation.
The results from a typical measurement will look like:
***************************************************************************
| UT 00:44 08/27/95 HA -01:14; DEC -31:23 f/8 ROT 90.0 | |||||||||||
| SECONDARY | PRIMARY | ||||||||||
| coma3 | spher | astig | triang | quad | d80 | ||||||
| um | d | um | um | d | um | d | um | d | arcsec | ||
| 1 | 0.22 | 80 | -1.57 | 0.64 | 440 | 0.02 | 367 | 0.17 | 12 | 0.47 | |
| 1 | 0.28 | 73 | -160 | 0.64 | 452 | 0.03 | 273 | 0.20 | 6 | 0.49 | |
| 1 | 0.33 | -71 | -193 | 0.64 | 471 | 0.08 | 292 | 0.18 | 9 | 0.50 | |
| Average | 0.09 | 36 | -1.70 | 0.63 | 94 | 0.04 | -64 | 0.18 | 9 | ||
| Sigma | 0.15 | 0.17 | 0.02 | 0.03 | 0.01 | ||||||
| d80 | 0.01 | 0.19 | 0.21 | 0.01 | 0.08 | ||||||
| Tweak? | N | N | Y | N | N | ||||||
| d80 (arcsec) |
TEL.FOCUS=172301 GDR:x=0.045 y=-0.04 |
||||||||
| npts | defoc | decen | init | coma | full | ||||
| 1 | 1 | 218 | 1.34 | 21.17 | 219 | 0.57 | 0.56 | 0.47 | |
| 2 | 2 | 218 | 1.24 | 24.00 | 216 | 0.56 | 0.56 | 0.49 | |
| 3 | 3 | 217 | 1.71 | 24.57 | 214 | 0.59 | 0.59 | 0.50 | |
The output first shows results for the three independent 30 sec measurements. Magnitudes of the aberrations are given in microns (um), and the position angles in degrees (d). The rightmost column shows the residual 80% encircled-energy diameter that the image would have after correcting for all of the fitted aberrations (this residual includes the effects of slowly changing dome seeing components, but most of the effects of atmospheric seeing have been averaged out).
The next line gives the vector average for each aberration. After that is a line giving the standard deviation (1 sigma) of the magnitude of each aberration, and then a line giving the 80% encircled image diameter (in arcsec) which would be expected from each average value.
The line labelled "Tweak?" gives a recommendation about whether or not a correction should be made for each aberration: yes (Y) ==> make a correction; no (N) ==> do not change anything. A tweak adjustment is generally recommended for aberrations producing d80 values in excess of 0.1 arcsec, unless there is large scatter in the individual measurements. However, the spherical aberration measurements tend to show huge scatter, and we currently do not recommend making a tweak adjustment for that under any circumstances.
Finally, additional information about each measurement is grouped at the bottom left of the output. The first index increments with each frames analyzed, the second index (new at 8/17/00) shows the corresponding image number (i.e. 1,2 or 3) within a sequence allowing you to see which images were repeated/corrupted, "npts" is the number of spots used in the fit; "defoc" is the fitted defocus term (in microns); "decen" gives the fitted decentering term (in microns and degrees). The entries under "d80" are 80% encircled energy diameters at three different levels of correction: "init" is for no corrections; "coma" is with coma removed; "full" is with all fitted aberrations removed.
The iman reduction programs write output onto a number of different log files:
This is intended for creating the input for the auxilary programs "listav" and "listmap" (see Section 4.6, below), and would typically be deleted at the start of an engineering night when the sky is being mapped with IMAN, etc.
Example of iman.log.av: a 'star sequence' (3 images ok) followed by a 'more star' (2 images ok) averaging 5 different frames to calculate the aberrations
| UT | UD | HA | Dec | rot pa |
#red av |
#red used |
def | dec |
dec pa |
| 14:47 | 08/18/20 | 00:00 | -30:08 | 106.7 | 3 | 3 | 0.04 | 0.16 | -36 |
| 14:47 | 08/18/20 | 00:00 | -30:08 | 106.7 | 5 | 2 | 0.05 | 0.15 | -36 |
| UT | UD | HA | Dec | coma |
coma pa |
sph | astig |
astig pa |
tref | tref pa | quad | quad pa |
| 14:47 | 08/18/20 | 00:00 | -30:08 | 0.01 | -149 | 0.00 | 0.03 | -164 | 0.03 | -117 | 0.01 | -119 |
| 14:47 | 08/18/20 | 00:00 | -30:08 | 0.01 | -137 | 0.00 | 0.03 | -164 | 0.03 | -118 | 0.02 | -128 |
References to individual entries in iman.log and iman.log.av are by the time stamp; so be sure to record the UT time and date in any handwritten logs you may also be keeping.
When imancal, imanstar and imanav are run from the TCS, they write into the versions of these log files which are in the directory /ut22/iman. When the auxilary programs such as testseq are run, they write into versions of these log files which are in the current directory.
The statements for opening files which are normally used by imancal, imanstar and imanav have the full path name to /ut22/iman hardwired into them, and will crash unless run through German's calling procedure from the TCS. Each of these programs has a separate test mode which lets them access files in whatever directory they are run from. The test mode is activated by entering the word "test" as the third argument for imancal or imanstar, or as the first argument for imanav.
To make this mode easy to use, there are three c-shell scripts called testcal, teststar and testav, which directly call imancal, imanstar and imanav, respectively. They should be called as follows:
testcal [image] [f/ratio] [name]
teststar [image] [f/ratio] [name]
testav (no arguments)
The argument [image] is the name of the disk file containing the binary ccd image. [focus] should be either "f/8" or "f/14"; f/8 is assumed if no value is given. [name] can be any one-word name; it will be written into the header part of the output record.
To make it easier to save and re-analyse data, three additional c-shell scripts are provided:
| savecal [id] |
save the last calibration exposure into the current dis directory. [id] is an arbitrary number; the saved file will be called "cal[id].bin" (cf. cal3.bin). The file iman.log.cal will also be saved, with the name "iman.log.cal[id]". |
| saveseq [id] |
save the last sequence of three star exposures into the current disk directory. [id] is an arbitrary number; the saved files will be called "r[id]s1.bin", "r[id]s2.bin" and "r[id]s3.bin" (cf. r25s1.bin, etc.). The file iman.log.star will also be saved, with the name "r[id].log". |
| testseq [id] [f/ratio] |
process the saved star sequence [id]. f/ratio is assumed to be f/8 unless f/14 is entered. This script calls teststar and testav. |
There are also four auxilary programs which process the output contained in the file iman.log.av. That file contains one line of information for each of the three-exposure sequences, listing the time, telescope position and average values of the aberrations. The programs for further processing are:
| listav | calculates average aberrations for a list of iman.log.av entries. Input file is "list.in". Output is to screen unless redirected (eg. "listav > listav.out" or "listav | lpr"). |
| listmap |
plots aberration values as a function of telescope position. Input file is "list.in". Output is to an interactively-selected pgplot device (typically /te, /xwin or [file]/ps; [file] can then be printed out). Program will ask which aberration should be plotted. |
| listspher |
plots spherical aberration vs. defocus. Input file is "list.in". Output is to an interactiively-selected pgplot device (typically /te or [file]/ps; [file] can then be printed out). |
| listall |
performs listav, listmap for all aberrations, and listsphere. Output is sent to the default printer. Postscript files of the plots are left on disk, with names like astig.plt ... you can look at these on the CRT using the Page View tool before deleting them, if you wish. |
The input file "list.in" must be in the same format as the file "iman.log.av". The intention is for you to copy iman.log.av into "list.in", and then to edit out any parts that you do not want to include in a specific reduction run.
All of these auxiliary scripts and program executables are found in the directory /ut22/iman. Modified versions of 'listav' and 'listmap' that will work with the new -as of 8/17/00- format of iman.log.av are to be found at /ua76/boccas/4m/iman/. To set aliases for them in your current directory, type "source /www/4m/iman-alias".
The best first-order check of whether or not the full IMAN system is working is to take a CALIBRATION SEQUENCE, followed by a STAR SEQUENCE with the aperture wheel in the CALIBRATION POSITION. The calibration sequence should execute all of the way through and finish by telling you that a new calibration has been stored on disk. The star sequence should produce a saturated comparison spot pattern on the IMAN display, and should execute all the way through and return small aberrations as its result (~0.1 nm in magnitude).
Further subtle errors can occur which are most easily spotted by a closer examination of the IMAN images. Samples of good images can be found in /ut22/iman/samples. Some techniques for using IRAF to look at IMAN images in detail are described in /ut22/iman/samples/README.
ERROR -- STAR IS TOO BRIGHT. From CGRV. Too few spots have been found and more than 1000 pixels (average of about 5 per spot) have signal levels of 4095 (CCD saturation). Find a fainter star.
ERROR -- STAR TOO FAINT. From CGRV. Too few spots have been found and less than 500 pixels are more than 150 ADU above the background. Find a brighter star.
ERROR -- BACKGROUND TOO BRIGHT? From CGRV. Too few spots have been found and the average ADU/pixel is more than half the saturation value. Find a darker sky.
ERROR -- FOUND TOO FEW SPOTS. From CGRV. Too few spots have been found and none of the 3 previous errors have been detected. Take another star sequence and watch the IMAN image display monitor as the 30 second exposures are read out. Is the star way off center? Does the image turn to noise half-way through the picture?
ERROR -- CANNOT FIND SPOTS ABOVE THRESHOLD. From CGRV. Signal too weak or background is too bright. Find a brighter star.
ERROR--CANNOT REDUCE MORE THAN 99 IMAN IMAGES. The arrays in IMANSTAR are dimensioned to hold data for only 99 images when the MORE STARS command is used. Control yourself.
ERROR--COULD NOT ALIGN OBJECT AND CAL GRIDS. From COMB. Unable to identify a dark spot in the star pattern with one in the calibration pattern. Try moving the star in the aperture until the donut image includes a dark spot with a bright spot on each of it's four sides.
ERROR -- COULD NOT FIND ALL 3 DARK SPOTS IN CALIBRATION IMAGE. From COMB. When processing a calibration image (but not a star image) the system requires that all three dark spots be detected. (A detail... there are actually four Shack-Hartmann lenslets that are blacked out, but the program only knows about three of them). Recovery... try taking another calibration frame. If that fails, just use the old calibration, which should still be available for use.
ERROR -- COULD NOT OPEN CALIBRATION FILE. From IMANSTAR. The file /ut22/iman/iman.cal does not exist. Take a new calibration.
ERROR -- COULD NOT READ iman.sums FILE. From IMANSTAR. Error encountered while reading iman.sums. Start the star sequence again.
ERROR -- COULD NOT READ STAR DATA FILE. From IMANSTAR. The Sun did not receive the data image from the IMAN PC. Follow the IMAN PC restart procedure.
ERROR -- DATA FILE NOT FOUND ON SUN. From CGRV. The image file which was supposed to be sent from the IMAN PC could not be opened. Follow the IMAN PC restart procedure.
ERROR -- GRID COULD NOT BE IDENTIFIED. From NUMH. It was not possible to organize the spots into a square grid pattern. Sometimes caused by cosmic ray hits adding spurious spots. Try taking another star sequence.
ERROR. IMAN calibration not saved. From IMANCAL. General warning that a new calibration was not produced. The previous calibration should still be on disk ready to use.
ERROR -- NO CALIBRATION IMAGE ON SUN DISK. From IMANCAL. The Sun did not receive the calibration image from the IMAN PC. Follow the IMAN PC restart procedure.
ERROR -- NOT ENOUGH POINTS IN GRID. From NUMH. Grid was identified, but it contained fewer than 150 spots. Try recentering star in aperture.
ERROR -- SIGNAL TOO WEAK. From CGRV. Fewer than 500 pixels have signal level above 150 counts (as compared to typical background level of ~100 counts). Find a brighter star.
IMAGE ANALYZER
| CALIBRATION POSITION | aperture wheel to cal. source, LED on |
| LARGE APERTURE | move to large aperture. Cal LED off. |
| SMALL APERTURE | move to large aperture. Cal LED off. |
| OBSERVE POSITION | camera power off, flat to GDR, small ap. |
| POWER ON CAMERA | camera power on |
| * STAR SEQUENCE | take and analyze 3 star observations. |
| ABORT STAR SEQUENCE | abort STAR or MORE STARS sequence. |
| MORE STARS | take 3 more star frames, add into average. |
| / CAL SEQUENCE | take and analyze cal frame. |
| FLAT MIRROR | set to GDR (IN) or IMAN (OUT) |
| PELLICLE | set to IN (IMAN) or OUT (GUIDER) |
| IMAN COMMAND TO PC | send command described in Section 3.3 |
The IMAN control system is the section of code within the TCS software which accepts the above commands from the telescope operator and then translates them into other commands which are issued to the IMAN optics, camera and reduction systems in the correct sequence. The interactions between the different elements of the IMAN system are sketched in figure 4 [6].
Some of the above commands cause only one operation, but others are converted into long sequences of commands to different devices. Typical sequences are given below. Commands starting with "iman" are sent to the iman optics, those starting with "iman pc" are sent to the IMAN PC, and those starting with "tcp" are sent to the TCP program which then sends them on to the IMAN SUN.
The TCP commands are followed by an integer 1-5 which selects follow-on actions after completion of the SUN task which appears as their arguement. In particular "tcp 4" starts the reduction of image imans3.bin (as specified in the argument), waits for completion of the imanstar task, then initiates the imanav task on the SUN.
iman pc s
iman pc status (WAIT IN LOOP UNTIL "IDLE" IS RETURNED)
iman pc cal
iman pc status (WAIT IN LOOP UNTIL "GRAB" IS RETURNED)
tcp 5 /ut22/iman/imancal cal001.bin
| iman flat out | |
| iman pc s | |
| iman pc status | (WAIT IN LOOP UNTIL "IDLE" IS RETURNED) |
| iman pc star | |
| tcp 1 /ut22/iman/rmi | |
| tcp 1 /ut22/iman/rmd | |
| iman pc sstatus | (WAIT IN LOOP UNTIL "1" IS RETURNED) |
| tcp 2 /ut22/iman/imanstar imans1.bin f/8 01/03/1995 21:32:17 -02:20:10 -30:00:00 180000 1.200 1.320 90.0 | |
| iman pc sstatus | (WAIT IN LOOP UNTIL "2" IS RETURNED) |
| tcp 3 /ut22/iman/imanstar imans2.bin f/8 01/03/1995 21:32:17 -02:20:09 -30:00:00 180000 1.200 1.320 90.0 | |
| iman pc sstatus | (WAIT IN LOOP UNTIL "3" IS RETURNED) |
| tcp 4 /ut22/iman/imanstar imans3.bin f/8 01/03/1995 21:32:17 -02:20:08 -30:00:00 180000 1.200 1.320 90.0 | |
| (TCP 4 INITIATES /ut22/iman/imanav) | |
Same as STAR SEQUENCE command, except that the following command is not sent:
tcp 1 /ut22/iman/rmi
Not sending this command has the effect of not clearing the sums and counters used to compute the averages and standard deviations of the aberrations. Thus, additional sets of three stars can be incorporated into the running averages. A maximum of 33 sets of 3 star observations each (99 observations total) can be averaged together.
iman pc s
iman flat in
iman camera off
iman aperture small
German Schumacher
5 December 1995
The following table lists the hour angles and declinations at which IMAN measurements should be taken in order to calibrate the lookup tables for 4MAP (the 4M Active Primary mirror support system). The HA and Dec entries are chosen to give an equally spaced grid in azimuth and zenith distance; azimuth varies as you move vertically through the table and zenith distance as you move horizontally.
Start and end at zenith. In between, do the following:
|
|
AZIM/ZD | 15 | 30 | 45 | 60 | |
| HA (h m) | 0 | 0 00 | 0 00 | 0 00 | 0 00 | |
| DEC (d m) | -15 09 | -00 09 | 14 50 | 29 50 | ||
| WEST | 30 | 0 31 | 0 58 | 1 24 | 1 52 | |
| -16 57 | 03 29 | 10 01 | 23 24 | |||
| 60 | 0 55 | 1 45 | 2 31 | 3 16 | ||
| -21 55 | -12 39 | -02 50 | 07 04 | |||
| 90 | 1 08 | 2 14 | 3 16 | 4 13 | ||
| -29 01 | -25 47 | -20 48 | -14 33 | |||
| 120 | 1 04 | 2 19 | 3 38 | 4 56 | ||
| -36 40 | -40 38 | -41 22 | -38 43 | |||
| 150 | 0 40 | 1 40 | 3 17 | 5 30 | ||
| -42 46 | -54 03 | -62 13 | -64 06 | |||
| 180 | 0 00 | 0 00 | 0 00 | 0 00 | ||
| -45 09 | -60 09 | -75 09 | -89 50 | |||
| EAST | 210 | -0 40 | -1 40 | -3 17 | -5 30 | |
| -42 46 | -54 03 | -62 13 | -64 06 | |||
| 240 | -1 04 | -2 19 | -3 38 | -4 56 | ||
| -36 40 | -40 38 | -41 22 | -38 43 | |||
| 270 | -1 08 | -2 14 | -3 16 | -4 13 | ||
| -29 02 | -25 47 | -20 48 | -14 33 | |||
| 300 | -0 55 | -1 45 | -2 31 | -3 16 | ||
| -21 55 | -12 39 | -02 50 | 07 04 | |||
| 330 | -0 31 | -0 58 | -1 24 | -1 52 | ||
| -16 57 | -03 29 | 10 01 | 23 24 |
T. E. Ingerson
(1997)
Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories
Cerro Tololo Interamerican Observatory, Casilla 603, La Serena, Chile
The National Optical Astronomy Observatories are operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.
Abstract:
A new set of corrector optics incorporating atmospheric dispersion compensation is now in routine use at the prime focus of the 4m Victor M. Blanco telescope at CTIO. This corrector is described and direct photographic measurements of the optical field angle distortion (OFAD) coefficients are compared with the values predicted from the optical design. The results are used to quantify the baseline behavior of this corrector and then extended to provide predictions of the telescope's performance with the new optics under conditions which have not been directly measured.
When the 4m Blanco telescope at CTIO was designed in the late 1960's, it is doubtful that the designers anticipated that it would ever be used at prime focus with any detector other than photographic plates. Wide field imaging was to be done with a camera using a pair of non-achromatic triplet correctors, optimized for use in red and blue light.
The telescope has changed greatly since then, as have other large telescopes constructed during the same era. Imaging is now done almost exclusively with CCDs. Image quality has been significantly improved by careful control of environmental variables and upgrading the optics where feasible (Baldwin et al. (1996)).
A new corrector, the Prime Focus Atmospheric Dispersion Compensator (PFADC) has been installed to take advantage of the telescope's improved imaging capability. The PFADC provides high-quality, wide-field achromatic imaging at prime focus and incorporates atmospheric dispersion compensation (ADC). It is used mainly with a CCD imager and a fiber-fed, multi-object spectrograph known as Argus. Direct photography is still supported, though this option is now little used.
In principle, everything there is to know about a system like this can be computed directly from the optical design. However, there are at least 70 independent variables involved in the design and fabrication of this set of optics, such as spacing, radii, tilts, decenterings and refractive indices. Sufficient error in any one of these is capable of rendering the system's image quality unacceptable.
Each of the parameters can be measured, though always with some uncertainty. The corrector cannot be tested as a unit except on the telescope where the only variables which can be accurately measured are the image size and the optical field angle distortion (OFAD). Photographic plates are the classical and still the most appropriate method of directly measuring the OFAD. The large detector area, flatness, continuous nature of the detecting medium and high dimensional stability of plates makes them ideal for the job. Monolithic CCDs of the requisite size, flatness and number of pixels still lie in the future.
The OFAD coefficients for the old CTIO prime focus UBK-7 triplets were determined experimentally using plates by Cudworth & Rees (1991) and Guo et al. (1993). A similar photographic determination of the OFAD for the PFADC has been made recently by Guo et al. (1996).
Photographic measurements are not sufficient to fully characterize the optics. At CTIO, several instruments with differing optical configurations are used at prime focus and some of the elements of the PFADC are moveable. It is impractical to directly measure the OFAD under all possible permutations. What we have done here is to carefully compare the empirically determined OFAD under a single known set of conditions to the predicted performance under the same conditions and quantify a baseline behavior.
Monte Carlo simulations permit us to show that the observed performance of the optics is within the range which would be expected to occur as a result of normal manufacturing tolerances. This gives us confidence that we understand the corrector and allows us to make useful predictions as to how it can be expected to work in other configurations. The analysis represents a synthesis of theory and measurement and results in a better characterization of the corrector's behavior than would have been possible using the information provided by either computer modeling or direct measurement alone.
The results presented here are intrinsically interesting and not merely to potential users of this corrector. We certainly have benefitted from the exercise. Even the answer to such an apparently mundane question as "What happens when a filter is changed?" can be more interesting and significant than one might think. This kind of sub-arcsecond absolute astrometry will also be necessary for modeling a second ADC corrector now under construction. It will be used with "Hydra-CTIO" (Bardeen 1991), a new multiple object fiber-fed spectrograph now being constructed at NOAO-Tucson.
For the moment, our extrapolations of the PFADC's behavior only involve changes of the optical parameters which deal with the effect of changing filters and the corrector back focal distance. We do not yet have enough information to allow us to do comparisons of direct measurement with theoretical models of the ADC function. This is a different and more complex problem which we hope to study in the near future.
The PFADC was designed by Richard Bingham at University College London under contract to CTIO. It is important to emphasize that the values shown in Table 1 are those of the nominal design, but of the system as built and measured. The theoretical performance of the final configuration is essentially identical to that of the original design.
Table1: Optical Design of PFADC for PFCCD on 4M Blanco Telescope
| Surface | Radius of Curvature | Axial Separation | Material | Clear Dia. |
| 1 | 21369.00 | 9991.8 | Air | 4000 |
| 2 | 2920.60 | 25.0 | LLF1T | 400 |
| 3 | 1466.49 | 39.3 | PSK3T | 392 |
| 4 | -12682.9 | 2.9 | Air | 385 |
| 5 | 303.97 | 25.0 | PSK1T | 356 |
| 6 | 289.60 | 31.3 | LLF1T | 339 |
| 7 | 298.62 | 122.9 | Air | 321 |
| 8 | 748.58 | 15.0 | BK7T | 273 |
| 9 | 244.89 | 328.2 | Air | 255 |
| 10 | 294.17 | 27.8 | BK5T | 213 |
| 11 | 1715.95 | 91.9 | Air | 209 |
| 12 |
∞
|
|||
|
All parameters as measured. All dimensions in mm. Hyperbolic primary: conic constant=-1.09863 Surface 3 cemented with .1mm RTV: Inclined at 1.17 degrees Surface 6 cemented with .1mm RTV: Inclined at 1.37 degrees 91.9mm in space 11 includes 4mm filter and 6mm window "T" suffix glasses are adjusted fro melt variations Both doublets are free to rotate over 360° |
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This corrector is a descendant of the triplets originally provided with the Blanco telescope (Wynne, 1968). Addition of a fourth element provides broadband color correction and significantly improves image quality. The basic optical configuration is similar to a 4-element design first described by Wynne (1967) for the Hale 5m. Though the 1967 design is for a classical Cassegrain optical system, Wynne (1987) later showed that it could be adapted for use on a Ritchey-Chretièn telescope.
Wynne and Worswick (1988) then demonstrated that an ADC version could be built by putting a pair of rotating, curved, zero-deviation, Risley-like prisms with an oiled mating surface in front of the basic 4-element configuration. Bingham (1988) soon produced a simpler design in which a pair of rotating ADC prisms with an oiled, flat, rotating contact surface served as the first element of a 4-element corrector. This reduced the number of elements from 8 to 7. Glass-air interfaces were decreased from 10 to 8.
While designing the PFADC for CTIO and a similar corrector for the WHT, Bingham was able to further improve the design by replacing both of the front two elements of the 4-element configuration with doublets having shapes similar to those in the corresponding elements of the basic 4-element corrector. Each doublet is made of glasses (LLF1 and PSK3) which have almost the same indices of refraction but different dispersions. The cemented surfaces of the doublets are slightly inclined, so both act like zero-deviation prisms with a small dispersive power. When the axes of the prisms are 180° out of phase, their dispersions cancel and the system has essentially the same image quality as the basic 4-element design. The final optical system contains 6 pieces of glass and 8 glass-air interfaces. The rotating surfaces are not in contact.
Both doublets can rotate independently over 360°, allowing an artificial dispersion of variable magnitude to be added in any direction. This permits the corrector to compensate for atmospheric distortion with very little image degradation at any azimuth and at zenith angles to 70°. The optical design provides excellent unvignetted images at all wavelengths from 3400A to past 10000A over 48 arcmin field. There is little image shift with ADC. Chromatic effects are small. The quality of imaging at all air masses is primarily seeing-limited.
The four surfaces on the two singlets have been coated with broad-band anti-reflection coatings having high transmission from 3500- 10000A. The four surfaces of the doublets were coated with MgF2 instead of the broad-band coatings. Use of these new coatings was felt to involve too much risk because their long-term characteristics were not well known. So far they appear to be stable and robust.
Transmission of the corrector including coatings and glasses is 85% or higher at all wavelengths from 3700A to 8700A, falling to 75% at 3650A and 10000A and 54% at 3500A. Excellent BVRI photometry can be done using the PFADC. The short wavelength transmission limit makes photometric calibrations somewhat more difficult in U, though good results have been obtained in this band.
The original design specification called for image quality of .25'' full width half maximum (fwhm) in the center of the field and .5'' fwhm at the edge. The corrector meets this specification. However, the images produced by ADC correctors tend to have irregular profiles which often makes fwhm a misleading representation of image size. In the rest of this paper, we will refer to image size by specifying the diameter of a circle in which 70% of the incident energy is contained (D70). This is a somewhat more stringent specification for image quality than the original. For various reasons, we believe that D70 provides as accurate a quantification of the useful image quality of the instrument as can be provided by a single number. For the purpose of theoretical OFAD modeling, the images are considered to lie at the centroid of the spot diagram.
The PFADC was planned with a CCD imager as the default instrument. The design assumed that a 4mm BK7 filter would normally be placed within the back focal space (distance 11 in Table 1 [7]) in front of a 6mm window of fused silica. With the corrector as built, theory predicts the best imaging with the focal plane of the detector 91.9mm behind the rear face of the nominal corrector. Under these conditions, the focal plane is very nearly flat and the system is achromatic. So long as the surfaces of the window and filter are flat, their precise locations within the back focal distance have almost no effect on the optical behavior of the system.
Argus places its fiber tips directly in the image plane. As a result, there is 10mm less refractive material in the system than the design calls for, which causes an image shift and a small amount of chromatism. To maintain the same optical distance between corrector and detector, the Argus fibers must be placed closer (88.6mm at 4400A) to the rear surface of the nominal corrector. Over the entire field, more than 90% of the transmitted light at all wavelengths from 3500A to 10000A falls into a .7 arcsecond circle when the system is focussed through a B filter. This image quality is sufficient to do efficient broadband spectroscopy through Argus' 1.86'' diameter fibers.
The nominal thickness of the filters used in the Prime Focus Camera is 2mm. For best imaging with 2mm filters, the film surface should be 89.3mm from the nominal corrector. The filters normally used for photography have different thicknesses and compositions. The image quality is much better than was achievable with the old triplet correctors.
In all three cases, the instruments have been mounted with back focal distances which are nearly optimum for the respective configurations in the nominal design. The actual "as built'' and measured distances (+/-.1mm) between the rear of the corrector and detector plane are 91.6mm, 88.5mm and 89.2mm respectively for the PFCCD, Argus and the PF Camera.
In an astrograph, the spherical sky is presumed to be projected onto a flat plate perpendicular to the beam of the astrograph. In this ideal case, the distance r0 from the optical axis to a point on a plate is given by r0=f tan(A), where f is the focal length of the astrograph and A the angular distance from the optical axis to the same point.
Correctors such as the PFADC deviate from this model via radial pincushion distortion, called optical field angle distortion (OFAD), which varies as a function of the distance from the optical axis. We will represent distortion using the model in Chiu's 1976 paper
r = f tan(A)[1+d3 tan2(A) +d5 tan4(A)]
Here, r is the measured distance from the center of the field to the image in the detector plane. Distortion is modeled via the third and fifth order dimensionless distortion coefficients d3 and d5.
The inverted model
r0 = r b3 r3 +b5 r5
is usually preferred for analyzing plates, along with the image scale S, usually expressed in arcsec/mm. The two models are equivalent for our purpose and easily interchangeable via the simple relations
f = 206265/S, d3 = -b3f2 and d5 = (3b32 -b5) f4
In Table 2, results of theoretical analysis of the OFAD for the "as built'' PF camera using Zemax tm are presented along with the values empirically determined by Guo et al.(1996) from plates taken in 1995. In both cases, the two ADC doublets were set in a "neutral'' position with their dispersive axes opposed in the north-south direction. UBVRI bands in the observations are approximated respectively by the conventional wavelengths of 3600A, 4400A, 5500A, 7000A and 9000A.
Table 2: Theoretical and Experimental OFAD Coefficients for the PF Camera
| Source | Band | f.l. | d3 | d5 | Comments |
| Guo | U | 11465.4 | 360.0 | 775000 | Measured |
| Guo | B | 11467.3 | 360.2 | 695000 | Measured |
| Guo | V | 11467.8 | 357.6 | 687000 | Measured |
| Zemax | U | 11461.9 | 360.2 | 880000 | Predicted |
| Zemax | B | 11464.2 | 357.4 | 840000 | Predicted |
| Zemax | V | 11465.6 | 354.7 | 820000 | Predicted |
| Zemax | R | 11466.2 | 352.4 | 810000 | Predicted |
| Zemax | I | 11466.9 | 350.7 | 800000 | Predicted |
|
d3 and d5 are dimensionless Images have been refocussed for each passband All values are from 2mm BK7 filters |
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The photographic exposures were made through UG-5, GG385 and GG485 filters with thicknesses of 2.73mm, 1.96mm and 1.84mm respectively. The focal lengths in Table 2 have been corrected to the values they would have had if the filters had all been 2mm thick and made of BK7.
These two sets of models predict star positions with an rms difference between theoretical and empirical positions of less than 13µ over the field in all three colors. Nowhere does the predicted position of a star image differ from its measured location by more than 17µ (.31 arcsec). Still, the measured focal lengths of the system are slightly greater than the theoretical values. The rms difference between measured and predicted positions can be reduced to 4µ by adjusting the focal lengths of the system to be 1.8mm greater than predicted. As we will show, an adjustment of this order is what might be expected as a result of manufacturing tolerances.
Theory and experiment agree that there is a gradual increase in f with increasing wavelength while d3 and d5 decrease. Most of this variation of the OFAD is caused by secondary chromatism coming from color dependencies in the image distortion. Moving away from the optical axis, the blue images at first fall slightly closer to the center than those in the red so the focal lengths are lower in the blue. Farther out in the field, the shorter wavelength images begin to be displaced more because of larger values of d3 and d5, passing the longer wavelength images near the edge. The U images are as much as .36'' from the I images in parts of the field. This shift is the reason the broad band images often required by Argus are somewhat larger than the narrower band images generally used for CCD exposures. With Argus and the PF Camera, there is a small additional shift of image position with color resulting from primary chromatism caused by the incorrect filter thickness.
The photographic calibrations used here are based on eight plates of two fields, all taken on the night of February 23/24, 1995. These fields are called LP543 and M68 by Guo et al. The measured OFAD coefficients in Table 2 [8] are averages in which equal weight is given to all plates.
The focal lengths measured from the LP543 plates are approximately 3mm longer than those derived from the M68 series while the focal length determined from plates taken in the same color of the same field differed by less than a millimeter. This difference is difficult to explain since the images were taken in uninterrupted sequence during the same night. The air mass at which the LP543 plates were exposed is somewhat higher than for those of M68 so an error may have been made in correcting for atmospheric refraction.
The errors can be put into perspective by observing that the models determined for each plate predict measured image positions on that particular plate with an rms precision of 1.5-2micron. The measured values of the OFAD fluctuate between plates within the same series by approximately 3-4micron of position uncertainty, roughly the same as the difference between the theoretical and experimental models one a correction has been applied to the focal length. The rms position difference caused by the 3mm focal length difference between the LP543 and M68 plates is 14micron . This means that uncertainty in focal length is the principal source of error in our knowledge of the corrector's behavior. The experimental focal lengths in Table 2 [8] are an average of the two plate sets.
The values for the paraxial focal lengths predicted by Zemax at all wavelengths were taken to be the "f'' terms in the predicted OFAD. Values for d3 and d5 were then obtained by fitting the OFAD model to the positions given by Zemax for the centroids of a number of images distributed uniformly throughout the field. This procedure yields models which predict Zemax's theoretical image locations with an rms error of less than 4µ in U and 2µ in all other colors.
As previously mentioned, the parameters given in Table 2 [8]are not those of the corrector as it was designed, but come from measurements made after construction. To estimate the effect of any error in these measurements, Monte Carlo (MC) calculations were done during which the mechanical and optical parameters of the system were varied at random within the tolerances which were maintained during manufacture and final assembly. Every MC iteration creates a new, slightly different optical system which represents the way the corrector might have been put together. Each MC design was modeled in the same way as the nominal configuration, optimizing the image quality by refocussing after each iteration.
Image quality after a MC perturbation always remained within or very near to the design specifications, i.e. with the monochromatic D70 not exceeding .25'' in the center and .50'' at the edge. The image center moved by as much as several hundred µs as a result of tilts introduced by the Monte Carlo process, but after recentering, a new OFAD model could always be found which was able to predict the new theoretical positions to an rms precision of 6µ or less. The system focal length varied from the nominal value by an average of +/-5mm from after each Monte Carlo calculation. The MC perturbations changed d5 by an average of 30,000 units. Increases of d5 were seen more often than decreases. Changes in d5 were usually accompanied by changes of d3 in the opposite sense.
Theory should accurately predict the shape of the distortion curve, yet the measured values of d5 were consistently approximately 100,000 units smaller than expected. According to the Monte Carlo calculations, d5 is unlikely to have decreased as a result of manufacturing.
A difference of 100,000 in values of d5 causes a maximum difference in image positions of 17micron (.3 arcsec) at the edge of the field. When the theoretical and empirical models are adjusted to coincide as well as possible, larger values of theoretical d5 produce the best fit with slightly lower, compensating values of f and d3, reducing the residual errors to about rms 4micron, roughly the same as the intrinsic errors of the measuring process. Thus, the difference between the experimental and theoretical values of d5 is not significant here and can be safely ignored for the present, but it is unclear why this discrepancy exists. The most likely explanation is that it is some kind of systematic difference in how positions are predicted with a computer and measured photographically.
The MC calculations indicate that due to fabrication tolerances, the measured focal length might vary by be as much as 5mm from the predicted values. As previously mentioned, the best agreement occurs when the focal lengths are shortened by 1.8mm. The fact that a correction of this degree is sufficient to minimize the difference between experiment and theory strongly suggests that the corrector was assembled within specifications.
Adjusting the focal length by reduces the rms difference between the predicted and measured image positions to less than 4micron (.08 arcsec), which is comprable to the experimental error in the positions predicted by the measured OFAD. Once this adjustment has been made, the measured and predicted values tabulated for the OFAD of the PF camera in Table 2 [8] are essentially indistinguishable.
This focal length adjustment can be put into further perspective by noting that the theoretical focal length agrees almost perfectly with the plate scale derived from the M68 plates while it differs by 3mm from the scale on the LP543 plates. This provides some support to the supposition that the M68 scale is more likely to be correct and that an error may have been made correcting for refraction on the LP543 plates.
Summarizing, the theoretical OFAD coefficients are probably the more reliable, certainly for determining how d3 and d5 vary with wavelength. The photographic modeling gives us assurance that the true image scale is within the expected range. However the errors in fabrication appear to have been smaller than those made in the measurement of the OFAD. The theoretical values appear to be the best predictors we have of the corrector's behavior until we can obtain another, more accurate measurement of the paraxial focal length.
Table 3: OFAD Coefficients for Nominal PFCCD
| Band | f.l (mm) | d3 | d5 |
| U | 11466.5 | 359.5 | 900000 |
| B | 11468.3 | 355.9 | 875000 |
| V | 11469.3 | 353.8 | 840000 |
| R | 11469.8 | 351.3 | 830000 |
| I | 11470.3 | 350.2 | 810000 |
|
With 4mm filter and 6mm window d3 and d5 are dimensionless Errors are mainly in focal length: see text |
|||
Zemax can now be used to calculate the OFAD to use for the PFCCD and Argus. The OFAD for the nominal PFCCD with a 4mm BK7 filter and 6mm fused silica window are given in Table 3. Argus should be focused through a blue filter and the B band OFAD used, i.e. f=11466.5mm, d3 = 357.9 and d5 =835000.
The focal lengths for the nominal PFCCD are approximately 1.5mm longer than for the PF Camera while d3 and d5 are negligibly different. The focal length difference is because the PFCCD is .3mm from the nominal position while the PF Camera is within .1mm of the best location. It is also interesting to note that there is a slightly greater variation of focal length with color for the PF camera. As previously mentioned, this comes from a small amount of chromatic aberration in Argus and the photographic camera caused by the incorrect thickness of the filters.
Recent measurements indicate that .70'' fwhm images have been observed with the PFADC in the center of the field under very good conditions of seeing. These images are undersampled because the CCD currently used has a scale of .43"/mm. At present we are unable to determine if the actual image size is smaller than this reported value.
This observation implies that with perfect seeing the PFADC is probably capable of producing central images with D70 well under .5''. The images are clearly very good, though we are unable to measure how closely the central images approach the level of D70 < .25" that theory predicts they should.
Changing a filter is generally equivalent to moving the instrument with respect to the corrector because different filters will generally have different optical thicknesses. Obviously the system has to be adjusted to compensate. One naturally reaches for the "focus" control to perform this operation.
Focusing the Blanco telescope moves the prime focus pedestal up and down. The pedestal is a rigid assembly which moves the corrector and detector as a unit. This is an inappropriate way to adjust for an error in back focal distance.
Such a movement forces a change in the back focal distance by relocating the corrector assembly with respect to the primary mirror. This refocusses the images, but does so by moving the corrector away from the optimum location. This degrades image quality and makes a significant change in the telescope's effective focal length.
This is not a serious problem with Argus nor with the Prime Focus camera. Argus is permanently mounted with its fiber tips in the correct plane. Observations with the prime focus camera are made with a set of filters which are nominally 2mm thick. Even though these filters actually vary in thickness from 1.8 to 3mm, the variation is small enough so there is no significant image degradation, though there is a noticeable change in focal length with wavelength and filter thickness.
The Prime Focus CCD (PFCCD) is another matter. Currently, the detector is permanently mounted 91.6mm behind the corrector. This is close to the optimum distance (91.9mm) assuming it has the 4mm filter and 6mm window for which the corrector was designed. However, the system as built uses a fused silica window 8.85mm thick. The window is a meniscus lens which compensates for curvature of the CCD. This lens acts as a Barlow, significantly increasing the focal length. With the nominal dimensions, this predicts an increase of 66.5mm in the focal length to 11542.9mm in V with a 5.1mm filter. The measured value of the focal length is 11531.5mm, the difference in the offset coming from the fact that we currently lack precise knowledge of all the dimensions, including the exact CCD pixel size at the working temperature. For the rest of this paper, we will consider the dewar window to be a plane quartz window 8.85mm thick with a flat detector.
The PFCCD normally uses filters which are between 5mm and 10mm thick, meaning further excess material is placed in the back focal space. This extra material moves the detector optically closer to the corrector. Compensating for these back focus errors by moving the pedestal causes image degradation and a substantial change in the effective focal length of the telescope.
In principle, the proper way to compensate for the problems introduced by changing filters would be to have two focussing mechanisms; one like the present pedestal height control to focus the telescope and a second adjustment which moves the detector with respect to the corrector to maintain the back focal distance at the optimum value. There is no mechanism like this currently on the PFCCD, nor are there any plans to install one.
Table 4 shows what happens when back focal distance is wrong and has been corrected by moving the pedestal. Image size and focal length as a function of back focus error (BFE) are listed. The table begins by showing the behavior of the main instruments as they now exist. As can be seen, Argus and the photographic camera are mounted in very nearly the optimal locations, while the PFCCD has a rather substantial BFE.
| Instrument | BFE mm | f.l. (B) mm | D70(center) | D70 (32" dia) |
| Photographic Camera | 0.0 | 11467.0 | 0.20" | 0.25" |
| Argus (Broad Band) | 0.1 | 11466.5 | 0.50" | 0.60" |
| PFCCD (5mm filter) | -1.3 | 11476.4 | 0.30" | 0.35" |
| PFCCD (10mm filter) | -3.0 | 11489.4 | 0.50" | 0.50" |
| PFCCD (Nom. +3mm) | 3.0 | 11443.4 | 0.65" | 0.65" |
| PFCCD (Nom. +2mm) | 2.0 | 11451.1 | 0.50" | 0.50" |
| PFCCD (Nom. +1mm) | 1.0 | 11458.8 | 0.25" | 0.30" |
| PFCCD (Nominal) | 0.0 | 11466.4 | 0.20" | 0.25" |
| PFCCD (Nom. -1mm) | -1.0 | 11474.1 | 0.25" | 0.30" |
| PFCCD (Nom. -2mm) | -2.0 | 11481.7 | 0.35" | 0.40" |
| PFCCD (Nom. -3mm) | -3.0 | 11489.4 | 0.50" | 0.50" |
|
Focal lenghts are give fro B band. Increase focal lenghs by 54.3mm when using meniscus CCD. Images sizes are given to nearest .05". |
||||
In the second part of the table, the theoretical behavior of the PFCCD is shown with the detector at incremental locations one mm apart, beginning with the detector 3mm farther from the corrector than optimum and ending with it 3mm closer. This listing clearly shows that BFE greater than 1mm should be avoided and BFE of more than 3mm causes serious image degradation.
As can be seen in Table 4, theory predicts a linear change in effective focal length as a function of BFE at a rate of -7.67 mm of focal length change per mm change in BFE. This change could either be produced directly by physical change in BFE or by the insertion of an extra thickness of a refractive material within the back focal space. For a material of thickness T and refractive index n, this causes a back focal shift of T(1-1/n) and a change in focal length of 7.67 times this value. The variation in n with wavelength can cause significant chromatism. About .5mm of the 4.4mm focal shift with wavelength in Table 2 is caused by this effect.
This -7.67mm difference in focal length per mm of back focus change should not be confused with the classical shift in the "focus" of the telescope. The two are strictly proportional, but the pedestal only needs to move by -.86mm to cause one mm of back focus change, which in turn changes the focal length of the telescope by 7.67mm. This is probably the reason no one seems to have paid attention to this problem in the past. It is not intuitively obvious that refocussing by moving the pedestal by 1mm will cause the focal length of the telescope to change by almost 9mm. This relationship was used to calculate the effect on focal length caused by differences in the thicknesses of the filters used to measure the values in Table 2 [8].
Due to a fortuitous error, we are able to demonstrate that these predictions are accurate. Plates were first taken in 1993 to measure the OFAD. After our observations, we realized that the camera had been incorrectly mounted with a back focus of 91.5mm. The focal plane was lowered by 2.11mm before a second run in 1995. This increased the measured focal length by 15.9mm. This change is almost exactly the 16.2mm predicted for the increase by the BFE/focal length relationship, indicating that the focal length shift occurred as predicted. As expected, the change did not change the measured distortion coefficients.
The quality of the images also varies with wavelength. An estimate of the magnitude of this effect is also given in Table 4. These estimates are only approximate because image shape varies wildly, but they are nonetheless interesting. The values shown are based on theoretical analysis of the spot diagrams combined with some subjective weighting to attempt to make them reflect the real situation as well as is possible in a single number.
In summary, our results are consistent with the hypothesis that the corrector was fabricated and assembled quite well. The 2m difference between the measured and predicted focal lengths is a slight focal change caused by random manufacturing variations of the parameters of the optics within the specified tolerances. Image quality is excellent, as far as we are able to tell, with intrinsic image quality in perfect seeing of .5" or better in the center of the field.
Table 3 [9] gives the final OFAD to use for the PFCCD in the default location with the correct window and filter thicknesses. It is straightforward to extrapolate these results to determine the focal length and estimate image quality for other configurations by using the offsets from the B focal lengths in Table 4 [10] and the -7.67 mm/mm relationship between e.f.l. and BFE. These changes should not have a significant effect on d3 and d5.
The image plane of the PFCCD in its proper configuration is achromatic even though there is a 3.7mm shift in focal length from U to I. This is a manifestation of chromatic difference in distortion, not change in focal plane. No focus change with color is required if the filters are all the standard thickness and composition. In Table 2 [8] the PF Camera has a 4.4mm focal length shift over the same range, of which .5mm is a color shift in the focal plane caused by the filter and the window being thinner than the design calls for. With filters of differing thicknesses and composition the focal length shifts will be different from those shown here. The proper values are easy to compute if filter thicknesses and indices of refraction are known.
It is clear from Table 4 [10] that the image plane of the PFCCD should be moved to a better location. Installing a spacer 2.1mm thick will keep the BFE to under 1mm with any filter in current use and not cause significant image degradation, though the focal length will obviously change as filter thickness is varied. This solution to the back focus problem is reliable and easy to implement. It will significantly improve image quality and will provide a new fixed focal plane location which can be used to attempt a more precise determination of the system's focal length than we have been able to obtain so far.
Moving the detector farther from the corrector in this way will change the telescope's focal length to a value very near to the optimum for filters 7.5mm thick. There will be a very slight focal shift with color. This shift will have the opposite sign of the shift in the photographic camera because the PFCCD has too much refractive material in the beam rather than too little, as is the case from the PF camera.
The author would like to thank members of the CTIO staff, especially Daniel Maturana, John Filhaber, Gabriel Pérez, Nick Suntzeff and Alistair Walker for their invaluable help in accumulating and presenting the data given here.
1
Baldwin, J. et al. NOAO Newsletter 45, Mar. 1996
2
Bardeen, S. et al.
3
Bingham, R.G. 1988, Proceedings of the ESO Conference on Very Large Telescopes and Their Instrumentation, ed. L.B. Robinson (Springer-Verlag, New York), 1167
4
Cudworth, K. M., & Rees, R. F. 1991, PASP, 103, 470
5
Chiu, L.-T. G. 1976, PASP, 88, 803
6
Guo, X. 1995, PhD Thesis, Yale University
7
Guo, X., Girard, T. M., van Altena, W. F., & López, C. E. 1993, AJ, 105, 2182
8
Guo, X. et al. To be published in PASP, 1996 as a companion paper to this one.
9
Wynne, C.G. 1967, Ap. J., 152, 675
10
Wynne, C.G. 1968, Ap. J., 152, 675
11
Wynne, C.G. 1987, Observatory, 107, 31
12
Wynne, C.G. and Worswick, S.P. 1998, MNRAS, 230, 457
Empirical and Theoretical Modeling of the PFADC Corrector on the Blanco 4m Telescope
The original document was generated using the LaTeX2HTML translator Version 96.1 (Feb 5, 1996) Copyright © 1993, 1994, 1995, 1996, Nikos Drakos, Computer Based Learning Unit, University of Leeds.
The command line arguments were:
latex2html pfadc_tei.tex.
The translation was initiated by t.ingerson x292 on Thu Jan 2 12:28:34 CDT 1997
For wide-field use, especially with Hydra, a new corrector has been installed at the R/C focus of the Blanco Telescope. It is referred to as the "RCADC" (Ritchey-Chrètien Atmospheric Dispersion Compensator) corrector. It is located in the telescope chimney. Click here [11] to see just where it is.
Hydra MUST be used with this corrector. Any optical R/C instrument can use the RCADC if desired, but there is not much justification. The R/C and echelle spectrographs do not need its wide field though they sometimes might benefit from the ADC function. The RCADC can only be installed by Observer Support personnel via a motorized system operated from the Cass. cage.
The RCADC has six elements in four groups. This Optical Diagram [12] shows its configuration. It contains two meniscus "corrector" elements of fused silica at the front and back surfaces of the assembly. They provide images with D80 less than .3 arcsec over the entire 42 arcminute Hydra field. The corrector also makes the image "telecentric", which means that the pupil is located at the center of curvature of the field so that the optical axis of the images is perpendicular to the focal surface over the entire field. This minimizes light lost due to focal ratio degradation (FRD) in the fibers.
Between the two corrector elements, there are two cemented doublet prisms of silica and a light flint glass (LLF6). All surfaces of these prisms are plane, inclined appropriately so that the light passes through with zero deviation at an intermediate wavelength (4200A). Each prism provides a small amount of dispersion and rotates under control of the TCS though an angle of 360 degrees. When the two elements are oriented 180 degrees apart, their dispersions cancel so that the prisms have essentially no effect on the images. Orienting them at different angles can provide an artifical dispersion in any direction desired, which can compensate for atmospheric dispersion up to the limiting power of the prisms which in the case of the RCADC is at Air Mass 2.4 (65 degrees zenith angle).
The RCADC is coated with sol-gel over MgF2 on all eight surfaces. Sol-gel over MgF2 has very low reflectivity over a broad wavelength range. Although it has not been directly measured, the overall transmission of the corrector is believed to be above 95% at all wavelengths from 4000-10000A. Transmission falls in the UV due to the LLF6 elements in the ADC prism. Throughput of the corrector is roughly 85% at 3500A, 60% at 3340A and 20% at 3200A.
If dispersion correction is not desired or has been disabled for some reason, the ADC elements MUST be set in the neutral position. This is easily done via the TCS. Zero and 180 degrees is the standard setting but any orientation of the prisms 180o apart is equivalent. Normally, the ADC is left on and dispersion correction is automatic.
Observers sometimes ask when ADC should be used. The safest answer is "always". If there is significant dispersion in the field, correcting for it will improve the efficiency of the observation. It will never make it worse. The only reason not to use the ADC function is to avoid any possible effect on the pointing accuracy or if the control system is malfunctioning. Optical analysis indicates that rotating the elements does not significantly alter the field model, though for lack of time this has not been explicitly verified.
The expected effect of atmospheric refraction on the observing efficiency can be estimated from the following diagrams.
1. Differential refraction at 2km altitude [13]
2. Flux captured by Hydra fibers as a function of seeing and centering [14]
3. Image movement during exposure caused by refraction [15]
The first diagram quantifies the effect of refraction while the second lists fiber efficiency using the standard (Wolff) model of the profile of images degraded by seeing. Using the two diagrams it is relatively easy to estimate the effect of refraction on system efficiency.
For example, when the seeing is 1.0 arcsec, 85% of the incident light will enter a perfectly centered Hydra fiber. If the image is decentered by 0.5 arcsec, the efficiency falls to 73%. A decenter of 1.0 arcsec decreases the efficiency to 39%. Thus, if refraction decenters a star by 0.5 arcsec, in 1" seeing, overall system efficiency will decrease by approximately 14% (.85-.73/.85). Correspondingly at this seeing the efficiency will decrease by 54% if there is a 1 arcsec centering error.)
One can study the table as a function of seeing and estimate how much effect seeing might have on overall efficiency in a particular observing situation. If (say) the 10-15% efficiency degradation produced by an .5 arcsec offset is deemed acceptable, then an overall dispersion of 1 arcsec could be tolerated. Diagram 1 then tells us that someone observing from 3500-5000A could observe to an air mass of 1.3 without using the corrector. Observations from 4000-6000A could be done to an air mass of 1.45 while observations from 6000-9500A could be made at any air mass up to 2.40.
Important! Note that these tables can be used to determine the optimum central wavelength for positioning the fibers. If the corrector is not used, centering the guide star(s) on the wrong wavelength will offset the entire field. Either a filter must be used in the FOPS guide camera or FOPS stars of appropriate color must be used. Of course if the ADC function is enabled, no filter need be installed in the guide camera and the spectral type of the FOPS stars will have no significant effect on the positioning accuracy of the system.
Yet another consequence of refraction is to cause an apparent relative movement of points in the image as the field moves across the sky. This effect is quantified in the third diagram above.
Here, the Hydra field is shown with the locations of star images at 9 points in the field at -70 degrees during ten hours as the telescope tracks from 5 hours east to 5 hours west of the meridian. As can be seen, the image appears to rotate about a point approximately on the edge of the field with an amplitude of about .5 arcsec per hour of telescope motion at the other side of the field and the images drift with respect to the overall rotaion.
This effect is relatively small though it can be significant under some circumstances. Differential motion is the reason that Hydra asks for the approximate time of the middle of the exposure before positioning the fibers. In some situations it is desirable to reposition the Hydra fibers between exposures and to select the location of the guide star(s) in the field with care.
Deciding on what, if anything to do about this effect is up to the observer. One can use the information given in the first diagram to make an estimate as to the relative size of refraction effects at different zenith angles. The information in the third diagram can be used to make an educated guess as to how the images will drift during and between exposures. Since the effect is small, this is all that is ever necessary.
29 May 2000
by T. Ingerson
Differential Atmospheric Refraction at an Altitude of 2KM relative to a reference wavelength of 5000A
| Sec Z | 3000A | 3500A | 4000A | 4500A | 5000A | 5500A | 6000A | 6500A |
| 1.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
| 1.05 | 0.68 | 0.38 | 0.20 | 0.08 | 0.00 | -0.06 | -0.11 | -0.14 |
| 1.10 | 0.97 | 0.55 | 0.29 | 0.12 | 0.00 | -0.09 | -0.15 | -0.20 |
| 1.15 | 1.20 | 0.68 | 0.36 | 0.15 | 0.00 | -0.11 | -0.19 | -0.25 |
| 1.20 | 1.40 | 0.80 | 0.42 | 0.17 | 0.00 | -0.13 | -0.22 | -0.30 |
| 1.25 | 1.59 | 0.90 | 0.48 | 0.20 | 0.00 | -0.14 | -0.25 | -0.33 |
| 1.30 | 1.76 | 1.00 | 0.53 | 0.22 | 0.00 | -0.16 | -0.28 | -0.37 |
| 1.35 | 1.92 | 1.09 | 0.58 | 0.24 | 0.00 | -0.17 | -0.30 | -0.40 |
| 1.40 | 2.07 | 1.18 | 0.62 | 0.26 | 0.00 | -0.19 | -0.33 | -0.44 |
| 1.45 | 2.22 | 1.26 | 0.67 | 0.28 | 0.00 | -0.20 | -0.35 | -0.47 |
| 1.50 | 2.37 | 1.34 | 0.71 | 0.29 | 0.00 | -0.21 | -0.37 | -0.50 |
| 1.55 | 2.51 | 1.42 | 0.75 | 0.31 | 0.00 | -0.23 | -0.40 | -0.53 |
| 1.60 | 2.64 | 1.50 | 0.80 | 0.33 | 0.00 | -0.24 | -0.42 | -0.56 |
| 1.65 | 2.78 | 1.58 | 0.84 | 0.34 | 0.00 | -0.25 | -0.44 | -0.59 |
| 1.70 | 2.91 | 1.65 | 0.88 | 0.36 | 0.00 | -0.26 | -0.46 | -0.61 |
| 1.75 | 3.04 | 1.73 | 0.92 | 0.38 | 0.00 | -0.27 | -0.48 | -0.64 |
| 1.80 | 3.17 | 1.80 | 0.95 | 0.39 | 0.00 | -0.29 | -0.50 | -0.67 |
| 1.85 | 3.29 | 1.87 | 0.99 | 0.41 | 0.00 | -0.30 | -0.52 | -0.69 |
| 1.90 | 3.42 | 1.94 | 1.03 | 0.42 | 0.00 | -0.31 | -0.54 | -0.72 |
| 2.00 | 3.54 | 2.01 | 1.07 | 0.44 | 0.00 | -0.32 | -0.56 | -0.75 |
| 2.05 | 3.67 | 2.08 | 1.10 | 0.45 | 0.00 | -0.33 | -0.58 | -0.77 |
| 2.10 | 3.91 | 2.22 | 1.18 | 0.48 | 0.00 | -0.35 | -0.62 | -0.82 |
| 2.20 | 4.15 | 2.36 | 1.25 | 0.51 | 0.00 | -0.37 | -0.66 | -0.87 |
| 2.30 | 4.38 | 2.49 | 1.32 | 0.54 | 0.00 | -0.40 | -0.69 | -0.92 |
| 2.40 | 4.62 | 2.62 | 1.39 | 0.57 | 0.00 | -0.42 | -0.73 | -0.97 |
| Sec Z | 7000A | 7500A | 8000A | 8500A | 9000A | 9500A | 10000A |
| 1.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
| 1.05 | -0.17 | -0.19 | -0.21 | -0.23 | -0.24 | -0.25 | -0.26 |
| 1.10 | -0.24 | -0.28 | -0.30 | -0.32 | -0.34 | -0.36 | -0.37 |
| 1.15 | -0.30 | -0.34 | -0.38 | -0.40 | -0.42 | -0.44 | -0.46 |
| 1.20 | -0.35 | -0.40 | -0.44 | -0.47 | -0.50 | -0.52 | -0.54 |
| 1.25 | -0.40 | -0.45 | -0.50 | -0.53 | -0.56 | -0.59 | -0.61 |
| 1.30 | -0.44 | -0.50 | -0.66 | -0.59 | -0.62 | -0.65 | -0.67 |
| 1.35 | -0.48 | -0.55 | -0.60 | -0.64 | -0.68 | -0.71 | -0.73 |
| 1.40 | -0.52 | -0.59 | -0.65 | -0.69 | -0.73 | -0.77 | -0.79 |
| 1.45 | -0.56 | -0.63 | -0.69 | -0.74 | -0.79 | -0.82 | -0.85 |
| 1.50 | -0.60 | -0.68 | -0.74 | -0.79 | -0.84 | -0.87 | -0.91 |
| 1.55 | -0.63 | -0.72 | -0.78 | -0.84 | -0.89 | -0.93 | -0.96 |
| 1.60 | -0.67 | -0.75 | -0.83 | -0.88 | -0.93 | -0.98 | -1.01 |
| 1.65 | -0.70 | -0.79 | -0.87 | -0.93 | -0.98 | -1.03 | -1.06 |
| 1.70 | -0.73 | -0.83 | -0.91 | -0.97 | -1.03 | -1.07 | -1.11 |
| 1.75 | -0.77 | -0.87 | -0.95 | -1.02 | -1.07 | -1.12 | -1.16 |
| 1.80 | -0.80 | -0.90 | -0.99 | -1.06 | -1.12 | -1.17 | -1.21 |
| 1.85 | -0.83 | -0.94 | -1.03 | -1.10 | -1.16 | -1.22 | -1.26 |
| 1.90 | -0.86 | -0.98 | -1.07 | -1.14 | -1.21 | -1.26 | -1.31 |
| 2.00 | -0.89 | -1.01 | -1.11 | -1.19 | -1.25 | -1.31 | -1.36 |
| 2.05 | -0.92 | -1.05 | -1.15 | -1.23 | -1.30 | -1.35 | -1.40 |
| 2.10 | -0.99 | -1.12 | -1.22 | -1.31 | -1.38 | -1.44 | -1.50 |
| 2.20 | -1.05 | -1.18 | -1.30 | -1.39 | -1.47 | -1.53 | -1.59 |
| 2.30 | -1.11 | -1.25 | -1.37 | -1.47 | -1.55 | -1.62 | -1.68 |
| 2.40 | -1.16 | -1.32 | -1.44 | -1.55 | -1.63 | -1.70 | -1.77 |
Notes:
1. Sec Z is essentially equal to air mass over this range.
2. The CFADC can correct for atmospheric dispersion up to Sec Z=2.4 (65 degrees zenith angle)
3. The useful short wavelength limit of the Hydra corrector is 3350A
4. Source of this table: ESO
as a funtion if seeing and offset
|
SEEING (arcsec) |
OFFSET (arcsec) |
||||||||||||
| 0.00 | 0.10 | 0.20 | 0.30 | 0.40 | 0.50 | 0.60 | 0.70 | 0.80 | 0.90 | 1.00 | 1.10 | 1.20 | |
| 0.0: | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 0.50 | 0.00 | 0.00 |
| 0.1: | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 0.97 | 0.49 | 0.02 | 0.00 |
| 0.2: | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 0.97 | 0.83 | 0.48 | 0.14 | 0.02 |
| 0.3: | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 0.99 | 0.97 | 0.89 | 0.73 | 0.47 | 0.22 | 0.08 |
| 0.4: | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 0.99 | 0.96 | 0.91 | 0.81 | 0.66 | 0.46 | 0.27 | 0.14 |
| 0.5: | 1.00 | 1.00 | 1.00 | 0.99 | 0.98 | 0.96 | 0.92 | 0.85 | 0.75 | 0.61 | 0.45 | 0.30 | 0.18 |
| 0.6: | 0.99 | 0.99 | 0.98 | 0.97 | 0.95 | 0.92 | 0.87 | 0.79 | 0.69 | 0.57 | 0.44 | 0.31 | 0.21 |
| 0.7: | 0.97 | 0.97 | 0.96 | 0.94 | 0.91 | 0.87 | 0.82 | 0.74 | 0.65 | 0.54 | 0.43 | 0.32 | 0.23 |
| 0.8: | 0.94 | 0.94 | 0.92 | 0.90 | 0.87 | 0.83 | 0.77 | 0.69 | 0.61 | 0.52 | 0.42 | 0.32 | 0.24 |
| 0.9: | 0.90 | 0.89 | 0.88 | 0.86 | 0.82 | 0.78 | 0.72 | 0.65 | 0.58 | 0.49 | 0.41 | 0.32 | 0.25 |
| 1.0: | 0.85 | 0.84 | 0.83 | 0.81 | 0.77 | 0.73 | 0.67 | 0.61 | 0.54 | 0.47 | 0.39 | 0.32 | 0.26 |
| 1.1: | 0.80 | 0.79 | 0.78 | 0.75 | 0.72 | 0.68 | 0.63 | 0.58 | 0.51 | 0.45 | 0.38 | 0.32 | 0.26 |
| 1.2: | 0.74 | 0.74 | 0.72 | 0.70 | 0.67 | 0.64 | 0.59 | 0.54 | 0.49 | 0.43 | 0.37 | 0.31 | 0.26 |
| 1.3: | 0.69 | 0.69 | 0.67 | 0.65 | 0.63 | 0.59 | 0.55 | 0.51 | 0.46 | 0.41 | 0.36 | 0.31 | 0.26 |
| 1.4: | 0.64 | 0.64 | 0.63 | 0.61 | 0.58 | 0.55 | 0.52 | 0.48 | 0.44 | 0.39 | 0.34 | 0.30 | 0.26 |
| 1.5: | 0.59 | 0.59 | 0.58 | 0.56 | 0.54 | 0.52 | 0.49 | 0.45 | 0.41 | 0.37 | 0.33 | 0.29 | 0.25 |
| 1.6: | 0.55 | 0.55 | 0.54 | 0.52 | 0.51 | 0.48 | 0.45 | 0.42 | 0.39 | 0.35 | 0.32 | 0.28 | 0.25 |
| 1.7: | 0.51 | 0.51 | 0.50 | 0.49 | 0.47 | 0.45 | 0.43 | 0.40 | 0.37 | 0.34 | 0.31 | 0.27 | 0.24 |
| 1.8: | 0.47 | 0.47 | 0.46 | 0.45 | 0.44 | 0.42 | 0.40 | 0.37 | 0.35 | 0.32 | 0.29 | 0.26 | 0.23 |
| 1.9: | 0.44 | 0.44 | 0.43 | 0.42 | 0.41 | 0.39 | 0.37 | 0.35 | 0.33 | 0.30 | 0.28 | 0.25 | 0.23 |
| 2.0: | 0.41 | 0.41 | 0.40 | 0.39 | 0.38 | 0.37 | 0.35 | 0.33 | 0.31 | 0.29 | 0.27 | 0.24 | 0.22 |
| 2.1: | 0.38 | 0.38 | 0.37 | 0.37 | 0.36 | 0.34 | 0.33 | 0.31 | 0.29 | 0.28 | 0.25 | 0.23 | 0.21 |
| 2.2: | 0.35 | 0.35 | 0.35 | 0.34 | 0.33 | 0.32 | 0.31 | 0.30 | 0.28 | 0.26 | 0.24 | 0.22 | 0.21 |
| 2.3: | 0.33 | 0.33 | 0.33 | 0.32 | 0.31 | 0.30 | 0.29 | 0.28 | 0.26 | 0.25 | 0.23 | 0.22 | 0.20 |
| 2.4: | 0.31 | 0.31 | 0.31 | 0.30 | 0.29 | 0.29 | 0.27 | 0.26 | 0.25 | 0.24 | 0.22 | 0.21 | 0.19 |
Links:
[1] http://www.ctio.noao.edu/noao/es/content/short-instructions-normal-use
[2] http://www.ctio.noao.edu/noao/es/content/calibrations-positions-4map-lookup-tables
[3] http://www.ctio.noao.edu/noao/sites/default/files/telescopes/iman_fig1.gif
[4] http://www.ctio.noao.edu/noao/sites/default/files/telescopes/iman_fig2.gif
[5] http://www.ctio.noao.edu/noao/sites/default/files/telescopes/iman_fig3.gif
[6] http://www.ctio.noao.edu/noao/sites/default/files/telescopes/iman_fig4.gif
[7] http://www.ctio.noao.edu/noao/es/content/optical-design#table1
[8] http://www.ctio.noao.edu/noao/es/content/measured-and-predicted-ofad#table2
[9] http://www.ctio.noao.edu/noao/es/content/estimation-most-probable-values-ofad#table3
[10] http://www.ctio.noao.edu/noao/es/content/effect-changing-back-focal-distance#table4
[11] http://www.ctio.noao.edu/noao/sites/default/files/telescopes/chimney_layout.gif
[12] http://www.ctio.noao.edu/noao/sites/default/files/telescopes/rcadc3.gif
[13] http://www.ctio.noao.edu/noao/content/differential-atmospheric-refraction
[14] http://www.ctio.noao.edu/noao/content/flux-captured-hydra
[15] http://www.ctio.noao.edu/noao/sites/default/files/telescopes/lutz_s.gif
