This document is meant for the technical staff. It is provided to the users in case they want more information on the general (xy) calibration of Argus.
The positioners are rotated about their pivot point by similar linear actuator arms, moving in 10 micron steps. The line of force is nominally 135 mm behind the respective pivot point. Thus, the physical distance a fiber moves in a step is approximately d = 10*R/135, where R is the distance the fiber is from the pivot point at that instant.
In order to position Argus properly, a very accurate coordinate transformation must be made between astronomical space (R.A. and Dec.) and positioner space (radial and angular steps). To simplify this transformation, it is done as two completely independent processes. Astronomical and mechanical positions are transformed to and from X Y space in mm in the focal plane, i.e.
Transformation 1: Astronomical (arcsec on the sky) <--> X,Y (mm)
Transformation 2: Mechanical (radial and angular pulses) <--> X,Y (mm)
All of the information which Argus uses to do this is contained within the file argus.par, which is presented in an Appendix, for reference. When using Argus, it is important to know that argus.par is read only when the Argus program is started, so to change these values permanently, the program must be exited, new values entered in argus.par and the Argus control program re-entered to load the new values.
Whenever a positioner is mechanically adjusted, the mechanical model MUST be checked. Any adjustment of the arm is likely to change the parameters and affect the positioning. This is done with a special reference reticle. The reticle is made with a very precise xy grid of lines etched on it at 5 mm intervals, with a precision of one micron.
Essentally, an Argus positioner is calibrated by installing the reticle and sending the positioner until it is over one of the intersections, noting the number of steps which were required to get there. This process is repeated a number of times. These data are then processed by a computer program and new coefficients for the mechanical model are calculated. The coefficients are then entered manually in the argus.par file.
The calibration process is time consuming if it has to be repeated for all 24 positioners, but it is not difficult.
1. Install the reference reticle. This is done with Argus on its stand. The periscope and fibers are all withdrawn to the home position to get them out of the way. The reticle is installed in the Argus focal plane from underneath. The reticle is mounted on an aluminum plate with captive mounting screws and locating pins. Installation is straightforward and obvious.
2. Next, a special viewing device must be installed to permit viewing the lines on the reticle on a TV screen. The viewing system is installed in place of the periscope head. The viewer consists of the WATEC (ice cube) TV camera and a transfer lens which mounts on top of it. Once installed, it can roam around the field using the periscope motion control system and position itself under all the parts of the reticle grid.
3. Once the reticle and the WATEC have been installed, focus the WATEC on the grid of lines in the reticle and move it until it is over the center intersection of the reticle (identifable by a small arrow next to the lines). This is the center of the field. Move the positioner until it is directly over this spot and define it this as the center using the command "define center position" . This is done in the normal way, as should be done when using the WATEC to center the positioners at the start of every night.
4. Now, go to the Argus "Command Mode" by pressing F3.
5. Issue the command "cal n" , where n is the positioner number you wish to calibrate.
6. the program will now move the positioner to put it where it thinks it has to go to be over one of the intersection points on the calibration grid and jump into keypad mode. If the model is perfect it will go exactly to the intersection. If there is any error, center the fiber on the intersection and hit ESC. This will record the correct position.
7. The program will now go sequentially to a number of grid intersections. Repeat the above procedure until the program has moved to all positions. At the end, a file named "newN.tst" will automatically be generated ( N corresponds to the positioner number, ie. new5.tst). This is a list of the number of steps which this positioner actually needed to get to all the intersections and is the information which it will use to make a new, more precise model for that positioner. DO NOT try to stop this program in the middle. It expects to go to a certain number of positions and then exit. It is not forgiving of an attempt to escape the sequence.
8. Repeat this procedure for all the positioners you intend to calibrate. Each cycle will generate a file newN.tst.
9. Copy the newN.tst file(s) to the SUN and run the program "tsm". It will ask you which positioner number you want to calibrate, take the information contained in newN.tst and calculate new coefficients for the model. It will display the new coefficients on the screen.
10. Copy down these coefficients and carefully edit them into the "argus.par" file. An example of what this file looks like is shown in the Appendix.
arcsec on the sky -> physical distance from the center
R = F*A*(1 + D3*A**3 + D5*A**5)where A is the angular distance in radians and r the physical distance in mm.
F = 11452.9 mm
D3 = 356.5 (mm^-2)
D5 = .7e6 (mm^-4)
version-5.0
* fiber_size krad kang krot d3 d5 focalen
0.10 0.000 0.000 -0.169 356.5 0.7e6 11452.9
* per_center_x,y per_calib_x,y safe_limit tv_nominal
4281 3827 4642 2890 50 1885
* mirror positions [1-5]
679 734 854 896 906
* xt df ap ad fk x_cen y_cen
1 135.50 176.30 0.00 0.50 0 7852 4452
2 135.60 176.50 0.00 0.20 0 7789 4502
3 135.60 176.70 0.10 0.55 0 7797 4440
4 135.10 176.00 0.00 0.65 0 7807 4517
5 135.70 176.60 0.05 0.60 0 7778 4462
6 135.10 175.90 -0.05 0.30 0 7813 4574
7 135.10 176.30 0.00 0.30 0 7712 4589
8 135.80 176.90 -0.05 0.20 0 7756 4623
9 135.40 176.40 -0.05 -0.10 0 7729 4460
10 135.60 176.00 -0.15 -0.30 0 7809 4555
11 135.30 175.90 -0.05 -0.20 0 7807 4550
12 135.40 175.80 -0.10 0.00 0 7788 4524
13 135.50 176.50 0.00 0.00 0 7810 4471
14 135.30 175.70 -0.10 -0.50 0 7803 4508
15 135.50 176.50 -0.05 -0.60 0 7761 4510
16 135.20 176.00 -0.05 -0.10 0 7792 4511
17 135.50 176.50 -0.05 -0.50 0 7717 4538
18 135.40 176.20 -0.10 -0.40 0 7817 4520
19 135.20 175.80 -0.05 0.10 0 7778 4488
20 135.20 176.20 -0.10 0.00 0 7786 4572
21 135.20 176.10 -0.05 -0.30 0 7834 4547
22 135.10 175.90 0.00 0.20 0 7838 4570
23 135.20 175.70 -0.05 -0.20 0 7845 4499
24 135.10 175.70 0.05 0.20 0 7748 4530
*
* Last updated by: T. Ingerson 31 March 1995
* Format changes only
*