Simple Instructions for How to Make a Collimation Model to be used by the PSF Subsystem at LBT

These steps were extracted from a 20-Nov-2007 email by J. Hill. (J. Hill, O. Kuhn, and C. Biddick are the principal experts on doing this.) This is to make a primary mirror collimation model. The secondary collimation model works the same way, but the details of how to measure the field aberrations in practice are still being worked out.

a) collimate SX/DX telescope at a variety of elevations with LBCFPIA/EF/wGCS (using whatever combination of active optics, old collimation table, global offsets are needed). Beware of elevations above 80 degrees which may have false astigmatism in the pupils from open-loop tracking errors.

b) Be sure that PSF is set to apply active optics corrections to M1 only (not split mode), and that no residual active optics corrections are M2.

c) extract MIRRORX, MIRRORY, etc.,TELALT from the headers of the images taken AFTER good collimation is achieved at each elevation. (These are the "sum"="Total Collimation" terms from the top of the PSF(R/L) GUI Primary Mirror display.)

in IRAF: hselect irtc2*.fits LM1X,LM1Y,LM1Z,LM1RX,LM1RY,TELALT,LM1GLASS,WSTEMP,$I yes > collimation.txt

or hselect luci*.fits M1-X,M1-Y,M1-Z,M1RX,M1RY,TELALT,M1CTEMP,$I yes > collimation.txt

d) use these X,Y,Z,RX,RY values versus elevation and glass temperature to build the new table with the IDL task mkcoll.pro (or the appropriate mkcoll_lucifer, mkcoll_lbc depending on the instrument you are using). (Example data files and the program live in /home/lbcobs/Collimation/new/) Note that mkcoll_irtc.pro seems to be the version that is updated with the most features.

Attention - starting in mid-Jul-2008, modified IDL programs to handle the new polynomial-based collimation tables will live in /home/lbcobs/Collimation/new/.

Attention - As of April 2009, John has enhanced mkcoll_irtc.pro so that it will self-optimize collimation data taken over a range of temperatures.

  • save a GIF file of the plot with write_gif,"first.gif",tvrd() (courtesy of PH703 at SFSU) This only produces monochrome images since we have a 16-bit display. Equivalent commands exist for JPEG and TIFF images.

  • if you want a rainbow-colored plot say loadct, 39 before the above command.

e) edit the output file "Collimation.dat" of mkcoll for temperature, and Z linear temperature coefficient (+0.1 mm/degC). mkcoll_lucifer and mkcoll_lbc should do this automatically, so this step is mostly obsolete.

f) move the new table into the right place/name where PSF will find it. As of 16-Apr-2008, the file with an appropriately coded name goes in /home/telescope/TCS/Configuration/PSF/.

Attention - starting in mid-Jul-2008 new polynomial-based collimation tables will live in /home/telescope/TCS/Configuration/PSF/new/. Names look like SXPMLUCIFERCollimation.dat.

Attention - starting on Dec 14, 2010 the collimation tables will once again live in /home/telescope/TCS/Configuration/PSF/.

g) Press the initialize button on PSF to load the new table. Be sure that Active Optics corrections are cleared , Pointing Corrections are cleared, and Global Offsets are cleared (green numbers are either zero or what you put in the collimation model).

h) Then it should work. Note that a small change in temperature can result in a different initial focus. The new table should be good enough for LBCFPIA to run without manual intervention. Verify that the new table isn't causing the mirror cell to "saturate" its position (X,Y,Z more than +-2.6 mm, RX,RY more than +-100 arcsec) at high or low elevations.

Adjusting the Collimation Model after M2 has been remounted - See BGTechProcedures

These are the instructions from Andrew Rakich(08-Dec-2011) for adjusting the Collimation Model after an M2 unit has been remounted. This procedure is based on the observed premise that the secondary mirror shifts slightly in translation, but not much in tilt, when changing from the rigid mirror to the adaptive mirror. Andrew's instructions have been interpreted here by JohnHill.

  • Load the existing (default) collimation models and pointing models.

  • Go to a pointing star in ACQUIRE mode and roughly center it with adjustments to IE/CA.

  • On the same acquisition preset, defocus the star until you can see any coma that might be there, adjust out the coma manually on the defocussed image using M2 global offsets for X and Y, stepping IE/CA to compensate pointing as you go.

  • Once complete, record the global offsets, and also the total change in IE/CA from the pointing model. Edit the collimation model, inserting the global offsets for SXM2 that you measured. Also edit the pointing model to insert the total change in IE and CA that you ended up with.

-- JohnHill - 15 Apr 2008

This topic: Commissioning > Main > WebHome > CollimationModel
Topic revision: 30 Jul 2018, JohnHill
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