INSTRUMENTS


Contents


A. SPECTROGRAPHS

In this section, the following spectrographs will be described:

Detectors which may be used with the above spectrographs are described in Section V.

1. THE 4.0-METER TELESCOPE RITCHEY-CHRETIEN SPECTROGRAPH

a) Introduction

The R-C grating spectrograph is used at the f/7.8 R-C focus of the 4.0-meter telescope where the scale is 6.56 arcsec/mm. It is identical to the one at Kitt Peak National Observatory, in construction and as far as possible, in operation. The distinguishing feature of these 4.0-meter spectrographs is their large beam size. As a result, higher dispersion than usual for Cassegrain spectrographs is available, along with excellent spatial resolution for observations of extended objects. All functions of the spectrograph can be controlled remotely by the data acquisition computer, permitting convenient and efficient operation.

b) Acquisition and guiding

In normal operation, slit viewing is accomplished by means of a TV mounted on the instrument rotator. Two positions of the instrument rotator mirror carriage provide for direct viewing of the field, for target acquisition and for viewing light reflected from the spectrograph slit. The field of view in slit viewing mode is somewhat restricted, so that only the central arc minute of the spectrograph slit is visible on the TV. Auto guiding is normally accomplished using a separate TV camera, optically coupled to the offset guide probe, mounted in the instrument rotator, in conjunction with a leaky guider module. It is also possible to connect the same leaky guider to the slit viewing TV in order to permit auto guiding on light reflected from the slit jaws. Full details of the acquisition TV system and auto-guiders can be found in Section III. The spectrograph itself has both front-slit and rear-slit viewing optics.

c) Spectrograph rotation

The spectrograph may be rotated by up to 180 under remote control from the console room, with the telescope in any position. Observer support personnel should be notified of your intention to do so at the start of your run so that they can ensure that all cables have been routed to permit this. Normally the spectrograph is mounted with the slit E-W (PA=90 ).

d) Optical layout

The optical diagram for the spectrograph is given in Figure IV-1. The following descriptive sections are listed in the order which light passes through the spectrograph.

e) Decker

The decker plate for defining the length of the slit has five stellar/comparison pairs, a pointer for marking the center of the slit, a position for holding a calibration wedge, and a fully open position. The relevant dimensions are listed in Table IV-1.

f) Slit

The entrance slit has a length of 50 mm. Its width has a range from closure to 50 mm. One second of arc corresponds to approximately 150 microns. The size of the slit projected on the detector can be calculated from the information given in Figure IV-2. The conventional slit can be replaced by an aperture plate, permitting the acquisition of spectra of several objects distributed over an approximately 5 arcmin diameter field. Aperture plates are currently prepared in advance at NOAO Tucson. A TV camera can be attached to the rear-slit viewing periscope to assist in registering the apertures with the program objects. Observers with a potential interest in this system should contact CTIO staff well in advance of their run.

g) Multiple aperture plate

The multiple aperture plate contains 55 holes each 150 wide (1 arcsec) at 1.0 mm (6.6 arcsec) intervals along the slit. Exposures of the quartz lamp or white spot taken through these multi-holes are used to calibrate the geometric distortions introduced by the detector/camera combination for long slit work.

h) Filters

Two filter bolts are available each containing four filters plus a clear position. The lower filter bolt always contains neutral density filters; the upper-filter bolt either contains further neutral density filters or order sorting filters. An additional holder to contain a single order sorting filter can be manually placed in the beam if required. Note that this can only be done with the image tubes turned off if the 2D-Frutti is in use. Since all these filters are below the slit, the appropriate filters should be in place when the spectro- graph is focussed. The available filters are listed in Table IV-2 .

i) Shutter-Newall mask

A four-position plate acts as a shutter and Newall mask. The positions are: open, closed, mask north half and mask south half. The latter two are used for quantitative focusing of the spectrograph. Successive exposures of a comparison source are taken through each mask. At perfect focus, the positions of the arc lines in these exposures coincide exactly. Increasing departures from focus lead to greater displacements between the images. An additional high speed computer controlled shutter is used to obtain precisely timed exposures.

j) Collimator

The collimator mirror is an off-axis paraboloid of 225 mm diameter and 1161 mm focal length. The point-source beam size is 152 mm. The collimator has a travel of 38.1 mm for focusing the image of the slit onto the detector.

k) Gratings

At present, there are thirteen 203 by 254 mm gratings available. Their nominal specifications are listed in the Table IV-3. Table IV-4 lists the resolution and coverage available with various detectors. Tables IV- 5, IV-6, and IV-7 list grating efficiencies.

l) Cameras

Two Air Schmidt cameras are available. The Air Schmidt camera is a field-flattened Schmidt camera of 229 mm focal length and 229 mm clear aperture. This camera is exclusively used with a CCD detector, which together with its special dewar, forms an integral part of the camera. All optical surfaces of the "red" Air Schmidt are coated to give optimal performance in the red and as a result, this camera cannot be used blueward of 4500 , the "blue" Air Schmidt can be used at all optical wavelengths (3000 - 10000 ) but is less efficient that the "red" Air Schmidt redward of 5000 .

The Folded Schmidt camera is also a field-flattened Schmidt camera with 229 mm focal length and optical characteristics almost identical to the Air Schmidt. The corrector is optimized for blue and near UV wavelengths. The incorporation of a folding flat in the optical train brings the focal plane outside the body of the camera, permitting its use with the 2D-Frutti photon counting system or the CCDs mounted in standard "direct" dewars. This causes the camera to have somewhat less clear aperture and consequently greater vignetting than the Air Schmidt. On-axis, the Folded Schmidt has nearly the same throughput as the Air Schmidt. For distances of greater than 10 mm off-axis, the through-put begins to fall off significantly. The uv transmitting optics provide for excellent throughput at wavelengths all the way down to the atmospheric cutoff.

m) Calibration sources

Two calibration sources, Th-Ar and He-Ne-Ar, are mounted in the instrument rotator. It is possible to turn on the He-Ar and the Ne lamps separately. A quartz continuum lamp for flat-field calibration is also available. With a manually operated filter bolt in the rotator, it is possible to attenuate the comparison source light by a factor of either three or ten. A lens is moved over the slit of the spectrograph to provide an f/7.6 beam for comparison exposures.

2. THE 4.0-METER ECHELLE SPECTROGRAPH

a) Introduction

The echelle spectrograph for the 4.0-meter telescope provides high spectral resolution at the f/7.8 Ritchey- -Chretien focus where the scale is 6.56 arcsec/mm. The instrument is essentially identical to its counterpart at KPNO. A resolving power R 20,000 can be achieved by using the GEC CCD on the 229 mm focal length Schmidt cameras. Higher resolution (R 50,000) is available by using a variety of CCDs with the Long focus cameras. Full details of these various detector systems can be found in Section V.

b) Acquisition and guiding

In normal operation, slit viewing is accomplished by means of a TV mounted on the instrument rotator. Two positions of the instrument rotator mirror carriage provide for direct viewing of

the field, for target acquisition and for viewing light reflected from the spectrograph slit. The spectrograph itself has both front-slit and rear-slit viewing optics; however, the front-slit viewer pick-off mirror is normally removed when the spectrograph is on the telescope because it would otherwise block the TV. The rear-slit viewer pick-off mirror is manually placed in the beam when desired, it blocks the beam and must be out to expose. A TV system is available which provides for both wide field viewing for target acquisition, and the viewing of light reflected from the slit jaws. Auto guiding is normally accomplished using a separate TV camera, optically coupled to the offset guide probe, mounted in the instrument rotator, in conjunction with a leaky guider module. It is also possible to connect the same leaky guider to the slit viewing TV in order to permit auto guiding on light reflected from the slit jaws. Full details of the acquisition TV system and auto- guiders can be found in Section III. The spectrograph itself has both front-slit and rear-slit viewing optics.

c) Spectrograph rotation

The spectrograph may be rotated by up to 180 under remote control from the console room, with the telescope in any position. Observer support personnel should be notified of your intention to

do so at the start of your run so that they can ensure that all cables have been routed to permit this. Normally the spectrograph is mounted with the slit E-W (PA=270 ).

d) Control of functions

It is possible to control the comparison lamp illumination system from the console room. All other components of the spectrograph have to be operated manually from the cassegrain cage.

e) Optical Layout

An optical diagram is shown in Figure IV-3. The Newtonian diagonal flat, collimator and cross-dispersion grating are duplicated for the blue or red spectral regions.

f) Decker

The dimensions of the decker for defining the length of the slit are given in Table IV-8.

g) Filters

A pre-slit wheel for 2 by 2 inch filters, and a post-slit wheel for 1 by 1 inch plus one larger filter are available. However, the pre-slit wheel is normally not installed since it partially obstructs the slit-viewing TV. In principle it can be installed through a special hatch when the spectrograph is on the telescope. The filters listed in Table IV-9 are normally mounted in the post-slit wheel. The post-slit filters affect the spectrograph focus and they must be in place when focusing.

h) Shutter-Newall mask

A four-position plate acts as the shutter and Newall mask. The positions are: open, closed, mask east half and mask west half. The latter two are used for focusing the collimator. Successive exposures of a comparison source are taken through each mask. At perfect focus, the positions of the arc line in these exposures coincide exactly. Increasing departures from focus lead to greater displacements between the images. An additional high speed computer controlled shutter is used to obtain precisely timed exposures.

i) Newtonian diagonal flats

The Newtonian diagonal flats redirect the beam so that it becomes perpendicular to the telescope optical axis. Following the echelle grating, the blue and red optimized flats are both in the spectrograph at all times, mounted on a slide which can be set to predetermined positions by means of stops.

j) Collimators

The f/8 collimators have focal lengths of 1017 mm and produce a 125 mm diameter beam. The mirrors are suspended on an axial rod and balanced by adjustable counterweights to compensate any systematic gravitational flexure effects. Hence the assemblies are very delicate and must not be jarred.

k) Echelle gratings

The echelle spectrograph can be used with any of the three echelle gratings (31.6 l/mm, 79.0 l/mm or 316 l/mm). Three cross dispersion gratings are also available. Two of these are 226 l/mm; one blazed at 6300 and the other at 8000 The other is 100 l/mm blazed at 6300 . All of the possible combinations of echelle and cross disperser give essentially the same spectral resolution. The choice between them hinges on the desired wavelength coverage and the desired efficiency at a given wavelength. The dispersion decreases linearly with increasing blaze wavelength (i.e, toward lower orders). The dispersions with the 31.6 l/mm and 79 l/mm echelles are similar, but the 79 l/mm operates in lower orders with correspondingly greater free spectral range (and order separation) at a given wavelength. On the other hand, the 31.6 l/mm will in general provide more complete spectral coverage in a single exposure. With 316 l/mm echelle, only a short section of a small number of orders fit on the detector; it is primarily of use for work on extended objects.

The echelle dispersions at 5000 are as follows:



                                 Air Schmidt or 
            Long Camera ( /mm)    Folded Schmidt
             (A/mm)                Cameras (A/mm)

31.6 l/mm      2.50                    6.44 
79.0 l/mm      2.47                    6.36
316 l/mm       2.50                    6.54 

The tilt is set by means of a mechanical counter; two slip-clutch clamps must be released before adjusting the tilt and engaged afterward. The counter readings to center on the echelle blaze are as follows:


               31.6 l/mm       79.0 l/mm
Left port        573             561
Right port       577             565

l) Cross-dispersion gratings

Two 226 l/mm cross-dispersers are available, No. 3 blazed for 6300 in first order and No. 2 for 8000 in first order. No. 3 is used in the first-order yellow-red region and second-order near UV and No. 2 in the first-order red or the second-order blue-violet in addition a 100 l/mm, 6300 blazed cross disperser is available. The cross-disperser tilt readings on the mechanical counter are given in Table IV-10.

There is a clamp accessed through a hatch on the bottom of the spectrograph which must be released before adjusting the cross-disperser grating tilt, and engaged before observing. In addition to the tilt, when a change is made between camera ports, the cross-disperser blaze direction must be changed by rotating it 180 about an axis perpendicular to the grating surface.

m) Light baffle

A movable baffle blocks the camera port not in use.

n) Cameras

The Air Schmidt and Folded Schmidt cameras are discussed in the description of the camera section of the 4.0-meter R-C Spectrograph.

The Long cameras are blue-visual and red-infrared folded Schmidts with focal lengths of 590 mm. The Red camera is significantly faster than the blue camera at wavelengths red-ward of 6000 while the blue camera is to be preferred at wavelengths shortward of 4000 . At intermediate wavelengths the two cameras offer similar throughput.

o) Comparison sources

A Thorium-Argon comparison source, located on the instrument rotator is generally used with the echelle spectrograph. The mirror carriage in the instrument rotator has two operating positions, one of which permits both TV field acquisition and comparison illumination, while the other provides TV viewing of the slit during exposure. In addition there is a comparison lens in the spectrograph which is driven over the slit head to simulate the telescope beam during comparison exposures. A quartz lamp source is also available on the instrument rotator to aid intensity calibration of the echelle orders.

3. THE 4.0-METER MULTIPLE OBJECT SPECTROGRAPH-ARGUS

a) Introduction

A fiber-fed, multiple object spectrograph, called "Argus", is available at the 4.0-meter telescope. Argus uses computer controlled positioners to carry light from fibers directed at targets and their adjacent sky in the prime focus field of the 4.0-meter telescope to a bench mounted spectrograph 34 meters away. For more current information, visitors should consult recent issues of the NOAO Newsletter and/or contact CTIO staff members.

b) Argus system overview

Argus is located at prime focus of the 4.0-meter telescope. It has 24 arms equally spaced around in the telescope's prime focus field. This field is flat, 50 arc minutes (160 mm) in diameter and is fed at f/2.76. Top and side views of Argus are shown in Figure IV-4. In the top view, positioners are shown at the center of their range at the edge of the field. The sector accessible to a single representative positioner is shown.

Fibers can approach each other to a center-to-center distance of 10 arcsec. Sky fibers are 30 arcsec from their respective object fibers. They are located radially inward toward the field edge from the probe tip. In the top view from Figure IV-4, a shaded area is shown which indicates the light directed via a pickoff mirror to a television camera mounted on the prime focus pedestal. This camera, mirror and lens assembly provide a view of a 3 x 4 arc minute region at the edge of the field for acquisition and guiding. With the present optics and television camera, objects as dim as 21st magnitude have been seen under optimum conditions.

The "periscope" is an optical system on a moveable arm extending into the field from the side opposite from the television camera. It can access any point within the field and provides the user with a view of any 1 x 1 arc minute subfield within the main field. Fibers can be illuminated from behind and, via a pellicle arrangement in the periscope, any fiber's position may be seen superimposed on the field by directing the periscope to a position below it.

The fibers are routed for 34 meters from the prime focus cage to a bench-mounted spectrograph located near the base of the 4.0-meter telescope. This room is thermally and mechanically isolated from the telescope. The bench-mounted spectrograph uses the same gratings as the existing 4.0-meter R/C spectrograph and can be used with either the red or blue Air Schmidt or the Folded Schmidt cameras (all 229 mm f.l.), which are also used in the 4.0-meter R/C spectrograph. The actual designs of the two spectrographs are quite different. The bench-mounted spectrograph is fed with a much faster beam than the R/C spectrograph (f/2.5 instead of f/7.8). It uses a 520 mm f.l. Schmidt optical system as a collimator. The bench spectrograph is 2-3 orders of magnitude more stable than the R/C spectrograph.

c) Fibers

The fibers used in Argus have 100 micron cores (1.8 arcsec at the 4.0-meter prime focus scale). They are "hydrogenated" fibers which have excellent transmission at all wavelengths longward of 450 mm, declining towards the blue and UV.

The DQE of Argus has been measured to be 4% in the red under median seeing conditions (1.5 arcsec). Transmission efficiency of the fiber system relative to the R/C spectrograph including fiber absorption and other losses is approximately 60% at wavelengths longward of 450 nm, 50% at 400 nm and 30% at 350 nm. Overall spectrograph efficiency is about half of these figures when seeing is average because not all the light coming from the star can be captured by the fiber. There is about a 10% difference in throughput between individual fibers. Variation of fiber throughput with time or telescope position is less than 1%.

d) Using Argus

Observing begins by directing the center of the telescope field to a specified position and the probes to positions within this field. There does not need to be an object at the field center, although it is convenient to have one there. The periscope is then sent to a specific fiber which is pointed at an identifiable object. This object is then centered by moving the telescope. This corrects for telescope pointing errors. If the coordinates used are accurate, all target objects should then be sending their light into their respective fibers. The periscope can be used to check to see if this is so, but it wastes observing time to check all objects so the periscope should not be used to avoid the need to have accurate coordinates.

Ideally, visitors should arrive with coordinates having relative accuracy of a fraction of an arc second. If one's objects are star-like and of 18th magnitude or brighter, Argus can "autocenter" on all targets simultaneously in about two minutes and tweak fiber position for maximum throughput. Autocentering corrects for errors of up to 1.5 arcsec. There is also an autocentering offset mode, which allows centering on appropriate nearby objects followed by blind offsets to dim or non-stellar targets or objects located in extremely crowded fields.

e) Preparing for an observing run

Argus is presently controlled by a stand alone IBM PC/AT computer. The observer manipulates it from the PC while controlling the detector with a second terminal. The interface is easy to learn and use. Potential probe collisions are detected in software and aborted in hardware should they occur. Software which emulates Argus on a 80386 PC with mouse and VGA color graphics is available on request. This software permits observers to practice using Argus before coming as well as to have access to software for target selection, field assignment and collision avoidance.

Observers should come prepared with coordinate lists in standard format on PC compatible diskettes. The format used is quite simple. Sample files and format definition are provided with the emulator software.

4. THE 1.5-METER CASSEGRAIN SPECTROGRAPH

a) Introduction

The 1.5-meter cassegrain spectrograph is an ultraviolet transmitting grating spectrograph which is used at the f/7.5 focus where the scale is 18.1 arcsec/mm. It is identical to the 2.1-meter telescope White Cassegrain spectrograph at Kitt Peak National Observatory and therefore allows one to obtain similar spectrograms in both hemispheres. The CCD detector system used with this spectrograph is fully described in Section V.

b) Acquisition and guiding

The spectrograph is mounted to the telescope by means of the 1.5-meter guider/acquire module (GAM). This unit contains one TV camera on a motorized stage which can be used for field acquisition and offset guiding and a second fixed TV camera to view light reflected from the spectrograph slit jaws. A leaky guider module is connected the first of these cameras to facilitate automatic guiding. Full details of the GAM and auto-guider can be found in Section III.

c) Spectrograph rotation

The spectrograph is normally operated with its slit E-W. In principle the spectrograph can be rotated to any position angle. However, this operation is performed manually and requires bringing the telescope to the zenith and raising the observing floor. Consequently frequent changes of the position angle should be avoided, and ten to fifteen minutes of overhead time should be allowed for each such operation. It should be further noted that the leaky guider only functions if the spectrograph slit is close to the E-W or N-S directions.

d) Control of functions

The slit width and the comparison illumination system can be controlled remotely from the control room. All other controls are set manually at the spectrograph.

e) Optical layout

The basic optical layout is shown in Figure IV-5.

f) Decker

The dimensions of the decker plate which defines the slit length are given in Table IV-11.

g) Slit

The slit has a maximum length of 25.4 mm (460 arcsec) and a width which can be set from closure to a maximum of 25.0 mm. The width of the slit projected on the detector can be calculated from the information given in Figure IV-6. One arcsecond corresponds to about 55 microns.

h) Filters

There are two filter wheels each with a clear position and space for four 38.1 mm (1.5 inch) diameter filters. The pre-slit filter wheel contains neutral density filters with nominal attenuations of 1.5, 5.0 and 7.5 magnitudes. Note that the light from the comparison lamps does not pass through these filters. Standard order-sorting filters are normally available in the post-slit wheel. These filters can be changed with some

difficulty while the spectrograph is on the telescope. Focusing of the spectrograph must be performed with the appropriate order-sorting filter in place.

i) Shutter-Newall masks

Accurate timing of exposures is obtained by means of a fast computer controlled shutter. A pair of Newall masks are available for quantitative focusing of the spectrograph.

j) Collimator

The focal length of the collimator is 766.1 mm (30.16 inches). The pyrex mirror is figured as a 10 off-axis paraboloid, with a point-source beam diameter of 101.6 mm (4.0 inches).

k) Camera

The 1.5-meter spectrograph uses a semi-solid Schmidt camera with 160mm focal length. The optics are entirely of quartz so that is has usable transmission to the atmospheric cutoff. However, this camera requires that the front window of the CCD dewar be a field flattening lens. Thus, not all CCDs may be used with this camera. Currently it is only used with a GEC CCD.

l) Gratings

Table IV-12 lists the 127 by 152 mm Bausch & Lomb plane reflection gratings available with the spectrograph. Grating tilts and efficiencies for the 1.5-meter Cassegrain spectrograph are listed in Table IV-13.

m) Comparison sources

A neon discharge tube, an iron arc and an iron-argon hollow cathode discharge tube are available. A selector switch is at the junction of the sources.

5. THE 1.5-METER BENCH-MOUNTED ECHELLE SPECTROGRAPH

a) Introduction

An echelle spectrograph is available at the 1.5-meter telescope. It consists of a head attached to

the telescope containing the tips of two optical fibers 15 meters in length. These fibers run via the coud path to a bench-mounted spectrograph. The fibers are 100 and 200 microns in diameter (1.8 and 3.6 arcseconds on the sky). From 4000 to 8500 in good seeing this spectrograph is about 2.6 magnitudes slower than the 4.0-meter echelle spectrograph. Throughput drops in the ultraviolet and to the red of 8500 .

b) Acquisition and guiding

A pellicle arrangement permits the tips of the fibers to be seen superimposed on the field. Normally a star is acquired visually and its light directed into the fiber. Guiding is done manually by minimizing the light spilling out of the fiber.

c) Optical layout

There are two folded Schmidt cameras used with this spectrograph, with focal lengths of 300 and 700 mm. Either camera may be used with a GEC or TI CCD. By means of a simple lens arrangement, equivalent collimator focal lengths of 600, 900, 1200 and 1800 mm can be selected. Shorter collimator equivalent focal length produces larger projected fiber diameter with greater efficiency but lower resolution. Adjusting fiber size, collimator focal length and pixel size allows one to optimize resolution versus throughput in much the same way as by varying slit width in a regular spectrograph.

d) Cross dispersion

A 60 prism cross disperser is normally used. The prism produces evenly spaced echelle orders. A TI CCD with the 300 mm camera gives about 4500 of coverage with orders about 10 pixels apart. Coverage is about 2000 with the 700 mm camera. The 300 mm camera just barely separates orders with and the 31.6 l/mm grating. Coverage in successive orders overlaps to 6500 with a TI CCD. For greater order separation, the 79 l/mm grating can be used. Filters are not needed with a prism cross disperser.

e) Spectrograph configurations

The most efficient configuration is with a 200 micron fiber, 300 mm camera, 600 mm collimator equivalent focal length and TI CCD. This will give an echelle dispersion of 4.8 /mm or .07 /pixel at 5000 . The projected fiber diameter will be 100 microns (6 pixels). This will give a resolution of about 12,000 with a system DQE of several percent. In good seeing, the 100 micron fiber will be almost as efficient as the 200 micron fiber and the resolution will be about 25,000. Using the 100 micron fiber with a 900 mm collimator will decrease the efficiency by about 25% while increasing the resolution to approximately 35,000.

The highest resolution currently attainable with this spectrograph is 80,000, with the 700 mm camera, TI CCD and collimator equivalent focal length of 1800 mm. This extreme resolution decreases efficiency by at least a factor of 4.

f) Comparison lamps

The head contains Th-Ar and quartz reference lamps. Flat fielding must be done through the fiber. Quartz lamps do not have enough output in the blue and near UV to flat field well. In the blue, the best flat field attainable is the spectrum of a featureless blue object such a bright, rapidly rotating O star.

6. THE 1.0-METER TELESCOPE CASSEGRAIN IMAGE-TUBE SPECTROGRAPH

a) Introduction

The Boller and Chivens fast uv-transmitting, image-tube spectrograph is used at the f/10 focus of the 1.0-meter telescope where the scale is 19.5 arcsec/mm. Full details of the 2D-Frutti photon counting system available for use with the spectrograph can be found in Section V.

b) Acquisition and guiding

The spectrograph, together with a single channel photometer (ASCAP) is permanently mounted on the telescope by means of a special guider module. The change between spectroscopic mode and photometric mode is simply accomplished by inserting or removing a flat mirror. When the spectrograph is in use, a TV camera views the light reflected from the slit. A leaky guider module connected to this camera facilitates auto guiding. Full details of the acquisition TV and auto-guider can be found in Section III.

c) Spectrograph rotation

For convenience, the spectrograph is normally operated with its slit E-W. In principle the spectrograph can be rotated to any position angle. However, this operation is performed manually and requires bringing the telescope to the zenith and raising the observing floor. Consequently frequent changes of the position angle should be avoided, and ten to fifteen minutes of overhead time should be allowed for each such operation. It should be further noted that the leaky guider only functions if the spectrograph slit is close to the E-W or N-S directions.

d) Control of functions

The comparison lamp illumination system and slit width can be controlled remotely from the console room as can the filter wheels in the guider box. All other controls are set manually at the spectrograph.

e) Optical layout

An optical diagram of the spectrograph is shown as Figure IV-7.

f) Decker

The decker plate which defines the length of the slit contains six stellar windows, an open position, a pointer for locating the center of the slit and single and multiple apertures used for calibrating the geometric distortions introduced by the camera and detector system. Its dimensions are listed in Table IV-14.

g) Slit

The maximum slit length is 19.0 mm (.75 inches). Its width can be set from closed (5 ) to 12.0 mm. The nominal slit to detector demagnification factor is Fcam/Fcoll = 0.155. Four arcseconds on the sky (210 ) projects to 32 at the detector.

h) Filters

Two remotely controlled filter wheels each with space for five 50 mm by 50 mm (2 by 2 inch) filters and a clear position, are located in the guider box. A wide range of colored glass order sorting filters can be placed in the upper wheel. Neutral density filters with nominal attenuations of 1.0, 1.25, 2.5, 5.0 and 7.5 magnitudes are normally located in the lower wheel. The spectrograph itself contains a further manually controlled above slit filter wheel which normally contains neutral density filters with nominal attenuations of 2.5, 3.75, 5.0, 6.25 and 7.5 magnitudes plus a clear position. A single 12 mm by 41 mm order sorting filter can be placed in a special holder below the spectrograph slit. If this is used the spectrograph must be focussed with the filter in place.

i) Shutter-Newall mask

A fast computer-controlled shutter is available for accurate timing of exposures. A pair of Newall masks are also available for quantitative focussing of the spectrograph; when both masks are closed they also serve as an adequate dark slide to protect the detector.

j) Collimator

The collimator is an off-axis paraboloid of 10 cm clear aperture, 90 cm focal length and 9 cm beam size.

k) Gratings

Table IV-15 lists the 128 by 102 mm (ruled area) Bausch and Lomb plane reflection gratings which are available.

l) Camera

The camera for the 1.0-meter cassegrain spectrograph is a 140 mm focal length f/1.4 Cassegrain- Schmidt with a 110 mm clear aperture, which was designed to work with a nominal beam size of 100 mm. The camera mount is an integral part of the housing for the RCA 6303 "advanced Carnegie" image tube which forms the first intensification stage of the 2D-Frutti photon counting system.

7. THE RUTGERS FABRY-PEROT (RFP)

a) Introduction

The Rutgers Fabry-Perot is available for use at the Cassegrain foci of the 4.0- and the 1.5-meter telescopes. The RFP is used in combination with a TI or Tek CCD and is operated using the same basic instrument control program used for direct CCD imaging at CTIO and KPNO. This software runs on a DEC LST-11/73 computer in a multi-tasking FORTH-11 environment and includes facilities for the real-time display of data and for on-line image processing. A Rutgers support person will set up the instrument and provide detailed instructions as to its operation. Normally the Rutgers support person will remain on the mountain during the observer's entire run.

b) Spectrograph

The RFP consists of two major subsections: the spectrograph and the detector. The spectrograph contains the Fabry-Perot etalon, the collimating and re-imaging optics, the order-selecting interference filter and auxiliary optics for flat field and line source calibrations. The spectrograph is designed to image a 25 mm circular aperture without vignetting for incoming beams of f/7.5 or slower. The beam is collimated with a 300 mm f/4 achromatic lens, and is then folded 60 degrees to minimize the overall length of the instrument and decrease flexure. The filter and etalon are located near the exit pupil of the collimator (40 mm diameter at f/7.5). Finally, an 85 mm f/1.4 camera lens focuses the beam onto the CCD detector with a resulting 3.5x demagnification. Pixel scale with a TI CCD is 0.95 /pixel and 0.35 /pixel at the 1.5-meter (f/7.5) and 4.0-meter, respectively.

Two etalons are currently offered for used with the RFP. These have a clear aperture of 50 mm and are manufactured by Queensgate Instruments. Resolution and free spectral range figures for both at H are given in Table IV-16. The gap and parallelism are set by three piezoelectric crystals, and are monitored by five capacitive sensors deposited on the etalons. The monitoring signals are fed to a servo control system which continuously corrects the gap and parallelism to the desired values. The etalons are highly stable with immeasurable changes in parallelism and gap changes corresponding to wavelength drifts of 0.5 over a night and 1 over a month. Both etalons have broad band reflective coatings, devised and applied by Dr. Fred Roesler of the University of Wisconsin, which are usable from 4500-9000 . Order-selecting interference filters are available for only a subset of this wavelength range.

The calibration optics provide neon, argon or hydrogen line spectra or white light uniformly illuminating the entrance aperture. These, like most of the other major spectrograph functions can be controlled locally or by the LSI-11 computer.

c) Detectors

Please refer to Section V of this manual which describes in detail the properties of the TI and Tek CCDs which are available for use with the RFP.


B. OPTICAL PHOTOELECTRIC PHOTOMETERS

Currently, optical photoelectric photometry is supported at the 4.0-meter, 1.5-meter, 1.0-meter and 0.6- meter telescopes. Most standard photometric systems are supported, for which the observatory supplies all the necessary equipment. In addition, observers wishing to use another system can bring their own filters and/or detectors after proper notification via their observing proposal and observer support questionnaire.

Computer-controlled photometer operation and data acquisition are standard. The computer terminal is located in a console room and viewing is via a TV acquisition camera except at the 0.6-meter telescope. Offset guiding is possible at the 4.0-meter and 1.5-meter telescopes. Quick-look reductions are easily obtained except at the 0.6-meter telescope and full reduction programs are available on the VAX computer in La Serena. Data can be taken home on magnetic tape (800, 1600 or V6200 bpi) or floppy disk. Please advise Observer's Support Personnel of your choice.

The ASCAP (Automated single channel aperture photometer) and the 0.6-meter telescope photometer are described below.

1. AUTOMATED SINGLE CHANNEL APERTURE PHOTOMETER (ASCAP)

a) Introduction

Figure IV-7 presents a schematic diagram showing the basic components of an ASCAP. The aperture wheel, filter wheel and a movable mirror for field or through-the-diaphragm viewing are computer controlled. One ASCAP is available for use at either the 4.0-meter or 1.5-meter telescopes. The other ASCAP is permanently mounted on a side port at the 1.0-meter telescope.

b) Apertures

There are seven diaphragm sizes available with the ASCAP. Their diameters, in both mm and arcseconds at each telescope, are shown in Table IV-17. In addition, there is a field position of diameter 19 mm, which is larger than the field of view of the TV in each case (about 2 arcmin by 2 arcmin).

c) Filters

The ASCAP filter wheel has 8 filter positions, each with dimensions of 1 in. by 1 in. by 9 mm maximum thickness. Photometric systems for which CTIO maintains the necessary photocells and filters are listed in Table IV-18. If required, blocking filters are available and listed in Table IV-19.

d) Cold boxes

At present, S-1 (FW118), S-4 (1P21), S-11 (FW129), S-20 (FW130), RCA 31034A (III-V), Hamamatsu R943-02 (III-V) and Varian VPM159A (InGaAsP) photomultipliers are available in cold boxes. The relative spectral response of most of these cathode materials is shown in Figure IV-8.

Table IV-20 indicates which photomultiplier types are housed in the various cold boxes and can be used as a guide for choosing a cold box appropriate to your program. However, since the photomul- tiplier type inside a cold box may be changed, it is best to specify the desired photomultiplier type on your observing time request form. The table includes a suggested operating voltage, the measured dark count at this voltage, an indication of the relative sensitivity as determined from a Beta-light source and a B filter, the maximum count rate and whether or not the cold box can be used at the 4.0-meter or the 1.0-meter telescopes.

The count rate as a function of voltage for both the Betalight source and for dark are affixed to each cold box. It is a good idea to verify that the preamplifier actually on the cold box is the same one which has been used to derive this graph before choosing an operating voltage. The dead time is noted on each preamplifier.

Transmission curves for all filters are available from Observers Support personnel.

e) Data acquisition and reduction

The output signal from the cold box is first fed through a pulse- counting preamplifier-discriminator and then via CAMAC electronics to the computer. A versatile photometry program allows: integration times as short as 1 msec; automatic termination after reaching a desired precision; detailed monitoring of current data status; quick-look reductions, etc. All data, including star identification, diaphragm size, reject, star/sky and standard/program flags, filter number, integration time, counts, UT, hour angle and declination arestored on magnetic tape. Currently, the data acquisition program is People's Photometry (standard photometry) or Time-Series Photometry (fast, multiple observations of variable stars). A separate manual detailing the current data acquisition program(s) is available. See Appendix A of this manual for details if you wish to request a copy.

The quick-look reduction is available immediately following an observation. With some experimentation, it is usually possible to obtain quick-look results that are within about 0.02 magnitudes of the final values. Full reduction programs for People's Photometry data are available using the VAX computer in La Serena.

As a rough guide to detector performance, Table IV-21 lists approximate magnitude levels for the various filter/telescope combinations at which a 1% precision is attainable in one minute of integration assuming negligible sky and low airmass. Variations from tube to tube may reach 0.5 magnitude. The stellar color is also an important factor. Only the recommended photocathodes for each photometric system are considered.

2. THE 0.6-METER TELESCOPE PHOTOMETER

The 0.6-meter photometer allows a 5-position filter slide to be operated from a control box at the observing desk. An optional second (manual) 7-position filter slide can be used alone or in conjunction with the other slide. This allows the use of additional filters, blocking filters or neutral density filters. Both slides have filter dimensions of 1 inch by 1 inch by 9.5 mm maximum thickness. A variety of manually-operated diaphragm slides are available, each with 4 different diaphragm sizes as well as a 15 mm diameter hole for field viewing. Diaphragm diameters range from 0.22 mm (5.5 arcsec) to 8 mm (200 arcsec). A mirror enables viewing through the diaphragm. Any cold box can be used. TV viewing is not implemented.

A simple data acquisition program is operated from a terminal at the observing desk. Integrations as short as 30 msec are possible. All data, including identification, filter number, integration time, counts, UT and any comments are recorded via computer. On-line reduction is currently unavailable.

3. MISCELLANEOUS INFORMATION

a) Sky conditions

Please refer to Section VII of this manual for weather statistics. The brightness of the dark sky varies with distance from the zenith, ecliptic and galactic plane as well as with other factors. Table IV-22 gives the sky brightness values (in mag/sq arcsec) in the UBV (Johnson) and RI (Kron-Cousins) passbands as a

function of lunar phase, from observations of starless fields near the south galactic pole. The values for 10 and 14 days after new moon apply to sky more than 90 from the moon and should be regarded as approximate since the sky brightness can be much greater if transparency is poor.

b) Extinction coefficients

Typical extinction coefficients for CTIO are listed in Table IV-23.


C. CAMERAS AND AUXILIARY OPTICS

1. THE 4.0-METER TELESCOPE PRIME-FOCUS CAMERA

a) Introduction

The 4.0-meter prime-focus camera is illustrated by the cross-section drawing as Figure IV-9. It is visually guided and manually operated. The "R, theta" positioned guide probe swings through an arc slightly more than 90 but the camera head as a whole can be rotated through 360 and locked in cardinal positions so that at any one time the observer can be seated in front of the camera eyepiece in the prime-focus cage.

The corrected field at prime focus is 50 arcmin in diameter. It is flat but has some pincushion distortion. (See Chiu, P.A.S.P., 88, 803, 1976). The photometric quality of the field has not been thoroughly investigated and users should not depend on its uniformity except close to the center of the field. All optics and most filters are anti-reflection coated. However, deep limit photographs will sometimes show non-uniformities due to residual internal reflection, particularly if bright stars are nearby. Exposure times depend on seeing, emulsion batch, sensitization techniques, mirror reflectivity, etc. Five and ten minute exposures on unbaked IIa-O with a GG 385 filter will reach to B about 20.5 to 21.0 and 21.5 to 22.0 0

respectively, under conditions of superb seeing; poor seeing decreases these limits by 1.0 or more magnitudes. As a general rule, large telescopes are more seeing dependent than smaller ones, and one should plan programs for superb, good, and poor seeing. About a 5 second exposure on unbaked 098-04 of the Orion Nebula through a 100 wide interference filter centered on H will give a good exposure, while an exposure of about 3 hours on baked 098-04 through a 100 H interference filter will reach sky.

Exposure times for sky limited exposures depend on sky brightness and filter-plate combinations are approximately given for dark skies away from the Milky Way by:

exposure time in minutes = A f2

where f is the focal ratio which is 2.66 at the 4.0-meter telescope prime focus. Approximate values of A for commonly used filter-plate combinations are:

        A =  6.5 for IIIaJ baked in forming gas + GG385
          =  3.5 for IIaO baked in forming gas + GG385
          =  6.0 for IIIaF baked in forming gas + RG610
          =  5.6 for IV N treated with Ag NO3 + RG695.

The limiting magnitude obtained in such sky limited exposures may be estimated from:

        m = K -2.5 loga   + 2.5 log F (meters),

where a is the seeing in arc seconds and F is the focal length which at the prime focus of the 4.0-meter telescope equals 10.6 meters. For dark skies, approximate values of K are:

        K =  21.5 for IIIaJ baked in forming gas + GG385
          =  21.0 for IIaO baked in forming gas + GG385
          =  19.7 for IIIaF baked in forming gas + RG610
          =  14.4 for IV N treated with Ag NO3 + RG 695.

Further useful information about phtomgraphic exposures is published by M.S. Bessell, P.A.S.P., 91, 589, 1979. (See VII, Section C.)

b) Correctors and filters

The camera is used with either the UBK-7 ( 3200 - 5000) or the BK7 ( 5000 - 10000) Wynne triplet corrector. The spectral ranges are fixed by design and by multi-layer anti-reflection coatings. The performance is good outside the design ranges; for instance, we find the blue correctors to be quite adequate to 8800 and hence usable for UBVRI photography. The plate scale with the blue corrector is 18.56 ( 0.02) arcsec/mm at the center for a blue pass band. However, it varies between 18.5 and 18.8 arcsec/mm depending on the corrector, pass band, and temperature at the time of exposure. Standard Schott filters (UG2, UG5, GG385, GG495, OG530, OG570, RG610, RG645, RG665, and RG695) mostly 2 mm (some 3 mm) thick are available. Two special glass filters have been assembled to enable the standard B and V transmission curves to be reproduced with IIIa-J and IIIa-F plates. Also available is a set of 12.7 cm (5 inches) square interference filters with transmission bands approximately 100 wide centered at the following wavelengths of astrophysical interest: 4200, 4341, 4680, 4768, 4856, 5000, 5892, 6320, 6485, 6565, 6650, 6725. A polaroid filter is also available. Tests of this filter show that it is of high effectiveness between 5250 and 7250 with degree of polarization and transmission averaging 99% and 40% respectively over this range. The filter can be used down to 4000 , below which the transmission rapidly falls, while at wavelengths longer than 8000 the degree of polarization becomes very low. Detailed characteristics of the filter can be supplied on request.

c) Eyepieces and plate holders

Accessories include a knife-edge focusing assembly (field about one arcmin), eight 20.3 by 25.4 cm (8 by 10 inches) interchangeable (i.e. no focal differentials) plate holders and a sensitometer projector that impresses a graded calibration exposure while the astronomical exposure is in progress. In addition, an adaptor for 12.7 by 17.8 cm (5 by 7 inches) plate holders is available. Calibration data are available from Observers Support personnel.

d) Focusing

Focusing of the camera is critical and should be done with care. If you lack experience at making knife-edge tests, a member of the Observers Support Personnel can give you advice. Our experience is that it is good practice to average four independent focus readings, two cutting the image with the knife edge in the E-W direction, and two in the N-S direction. At zenith distances, more than 40 it is usually better to use the knife-edge in the vertical direction because of differential refraction. Focus readout is available on two counters placed 180 apart on the base of the prime-focus pedestal; they read identically and may be used interchangeably. We find that the focus changes more as a function of time (i.e., temperature) than as a function of position in the sky. With a GG385 or GG495 filter a star of magnitude seven gives a very sensitive focus determination. With denser filters there are three options for obtaining a good focus:

(1) (Best) Focus on a bright star away from the object.

(2) Focus on nearby stars with GG495 filter and use differential focus corrections from the table.

(3) (Worst) Offset from last used focus by using the table to determine the focus correction between the old and new filter.

The 4.0-meter prime-focus camera filter corrections are given in Table IV-24.

e) Guiding

Guide-probe optics include a negative lens, a cross-hair which can be illuminated by a red light and an eyepiece. The magnification is about 1200. With the present guide probe, the reading on the radial scale must be greater than 10 cm in order to avoid the probe intruding into the photographic field. At this distance, the image quality begins to fall off and under conditions of excellent seeing some observers may prefer to guide inside the 10 cm limit. Normally, suitable guide stars can be located quickly and pre-selection is not necessary.

f) Plates and processing

The darkroom facilities for each telescope are described in Section III. A list of normally stocked plates is described in Section V of this manual.

2. THE 1.5-METER RITCHEY-CHRETIEN FOCUS CAMERA

a) Introduction

A camera is available for use at the f/7.5 Ritchey-Chretien focus (18.06 arcsec/mm) of the 1.5-meter telescope. Occasionally the camera is used at the f/13.5 in order to take advantage of the improved plate scale (10 arcsec/mm). A 30-minute exposure at f/7.5 with 103a-O + GG 385 (B on the UBV system) reaches to B about 20 magnitudes. The camera is mounted on an instrument rotator that can be turned to any position angle.

The camera uses both an aspheric corrector plate to minimize astigmatism and a field flattener; the former is mounted on spacer 8.9 cm above the main camera frame, while the latter is mounted just above the focal surface. Both lenses are made of fused silica and are coated with a single layer of MgF. Filters are inserted between the field flattener and the plate holder.

Please note that for the standard configuration used in wide-field photography it is very important to check that the f/7.5 secondary with its attached baffle is facing the primary and that the symmetrical light baffle is projecting from the central perforation of the primary mirror. The asymmetrical Coud a tertiary-mirror support that is interchangeable with the simple light baffle causes some vignetting in wide-field photography.

b) Focus and guiding

A knife-edge focus apparatus is available. Under normal temperature conditions it is recommended that the focus be checked about every hour in the same field and every time the telescope is moved a substantial distance. Generally focus plates are not necessary, as the average of knife-edge readings taken in the orthogonal directions are quite reliable; however, a focus plate taken with 2.0 division increments about nominal focus may be helpful the first time the telescope is used. Always approach final focus by moving the secondary mirror against gravity. If you lack experience making knife-edge tests, please tell a member of the Observer's Support Personnel.

Guiding is entirely manual and is accomplished via the telescope slow motions. Guiding microscopes with illuminated reticles are insertable on either side of the long axis of the plate holder. By a motion in the z-coordinate the microscopes may be focused to the taste of the observer. Acquisition of guide stars is facili- tated by releasing one or both of two retaining screws so that the eyepiece assembly can be quickly moved around the outer edge (about 3 by 30 cm) of the field. Probe positions may be read from mechanical scales for ease in re-acquiring a particular guide star. Please note that the 1.5-meter camera is not light-tight. During exposure, extreme care must be taken not to veil the plates with any lights within the dome.

c) Operating configurations for standard 20.3 by 25.4 cm (8 x 10 inches) plates

For wide field, f/7.5 photograph, both the corrector and field flattener are used with 20.3 by 25.4 cm plate holders. Guide stars are in focus at all positions of the guider probes. Four plate holders are available, three of which are interchangeable. Good images are obtained over all of the plate (1 by 1.3 degrees).

d) Filters

Schott filters suitable for use with the 20 by 25 cm plate holders are: UG2, GG385, GG495, RG610, 630 and 695. The following filters are available for use with the 28 by 36 cm plate holder: UG1, GG385, GG495, and RG610. These larger plates however, are not inventoried at CTIO.

e) Processing

Processing equipment consists of a rocker and trays or vertical tanks for hand development and a large sink. The darkroom is located conveniently at the base of the stairs leading to the rising platform and a plate loading room is available. With prior consultation with the Observers Support Personnel and the astronomer assigned the telescope, it might be possible to use one of the batch processing nitrogen-burst developing systems located in the 4.0-meter, 1.0-meter or 0.6/0.9-meter telescope buildings. Processing procedures and plates kept in stock are described in Section V.

3. AUXILIARY OPTICS

a) Objective grating

For the direct camera of the 1.5-meter telescope, a 13 by 10 cm plane grating (150 lines/mm) is available. When mounted, it produces slitless spectra with dispersion about 1200 /mm on the photographic plate. The grating is blazed to concentrate the light in the first-order blue part of the spectrum. Spectral widened by 0.3 mm are recorded in 30 minutes on 103a-O emulsion to a limiting magnitude of 16. Some of its astronomical applications are described in PASP, 86, 829, 1974.

b) Grism

CTIO has copies of the grating-prism assemblies developed by Hoag at KPNO for use at the prime focus of the 4.0-meter telescope. Each assembly consists of a plane transmission grating and a thin prism to shift the position of minimum aberration from the zero order to the desired spectral region. The gratings have ruled areas of 120 by 120 mm and groove spacings of 150/mm. One is blazed at 3550 in the first order and is normally used over the range 3200-5000. The other is blazed at 5750 in the first order and is for the range 5000-7000. Field coverage at the prime focus is 34 arcmin. In their normal position the grating-prisms give a dispersion of about 1600 /mm. In searches for faint emission-line quasars, Hoag and Smith detected candidates down to B = 20-21 on untrailed exposures of 30 minutes with baked 127-04 plates. The grism is mounted in special grooves.