1.2m Collimation Procedure

Updated: 08/20/14 by EF

Put the guide TV mirror on axis.

Find a star near zenith, not too bright. Put it at the center of the guide TV.

First find the rough correction for Coma by using the hexapod tilt command (in the collimate menu, NOT for novices), moving the star on the sky in the opposite direction of the narrow or bright part. Change the focus occasionally to check that the secondary obstruction is centered on the pupil.

Next, refine the coma and correct for astigmatism by bringing the secondary axis on the same axis as the primary by tilting about the neutral point, using the "tilt" and "neutral tilt" commands. This is done with Brian McLeod's IRAF script "aberfit", which uses his defocfit.

You should be logged in as observer, which should have these set:



loginuser.cl (or login.cl):

reset collimate  = /home/falco/collimate/Fit/
task $collimate.pkg = collimate$collimate.cl

As observer, start up iraf by typing cl at the top-level observer directory and cd to the data directory. Load "collimate". Take a focus frame (use telshell command kfindfwhm) in the r filter and determine the best focus, F. Now in the telshell type kdodefoc. That runs a script that brings the telescope near zenith and takes two 30-sec exposures using all 4 chips with focus F-200 and F+200 (200 harks back to the old pod units; it corresponds to 0.254 mm in hexapod speak). These are called 0xxx.m200 and 0yyy.p200, respectively (xxx and yyy are consecutive numbers).

Make sure you have ds9 running. Then to run,

cl> collimate
co> aberfit 0xxx.m200 0yyy.p200 200

This program will display each extension in ds9 in the order 1,2,3,4 (top right, left, bottom right, left). In each of these, mark 5 stars by plopping the cursor on each star and hitting the space bar. The stars should all be about the same distance from the focal plane center (bottom left, right and top left, right), fairly far away from that center, far away from each other and away from chip defects. All the stars should be well-exposed. After each set of 5 stars, type "n" to move on to the next FITS extension. After the last star in the 4th extension, type "q".

The program will then churn for about 5 minutes, and eventually print out the corrections to be applied in a format that can be cut and pasted into the telshell window (all the "tele colmot" commands). If you have collimated before, the tele colmot commands will look familiar to you.

If you started well away from collimation, you may have to repeat this recipe a couple of times, or until the aberfit estimates for the corrections are all 0.07 deg or less.