Procedure for escaping from Primary Mirror Collimation Limits

If either of the primary mirrors is moved by active optics so that it encounters one of the travel limits in the mirror cell, this is the procedure to recover.

How to recognize Collimation Limits

See the "Total Collimation" line on the PSFGUI. Limits will be highlighted in Red.

With the implementation of range-balanced collimation models, encountering collimation limits should be a less common occurrence.

Check for Bad Corrections

First, check that somebody hasn't sent "bad" wavefront corrections. For example, this can happen in clouds, or if the LUCIFER calibration unit is inserted with Active Optics still running. If there was a bad correction, then you can clean up by Clearing Active Optics as at the start of the night.

Second, check that the primary mirror and secondary mirror haven't stopped moving for some other reason (air, power, panic, etc.).

If the wavefront correction seems OK, and the mirrors seem OK, then the following sections describe how to adjust the optics to escape from the limits.

Prime Focus with LBC

Focus Limit

If the primary mirror hits a Z-limit, there is no software/operator adjustment you can make to escape. Either the position of the hub needs to be moved by the Instrument Specialists, or the observer needs to select a new target at a different elevation. Also note that the RX/RY limits for the tip-tilt of the primary mirror are automatically tightened as the mirror approaches the Z-limit.

X-Y Limits

If the primary mirror hits an X-Y limit with LBC (where there is no secondary mirror to move), then you can only escape the limit by making a Coma-free Pointing of the primary mirror (OPE M1). Use the Arbitrator subGUI of PSF to issue coma-free repointing commands to change either X&RY or Y&RX. View the Pointing Corrections and Total Collimation on the Primary Mirror subGUI. The repointing commands are absolute rather than incremental. Beware that this changes the telescope pointing by tipping the primary mirror, so you need to reset IE/CA accordingly to compensate with the mount.

Gregorian or Bent Gregorian Operation

Focus Limit

I don't think it is possible to get a focus limit at the Gregorian Focus. (A statement sure to get me woken during the night.)

X-Y Limits

If the Primary Mirror hits an X-Y limit at Gregorian or Bent Gregorian focal stations, the escape is to translate BOTH the Primary and Secondary Mirrors in the SAME direction. So if you hit a -X limit, then you want to increase the X-collimation values on both primary and secondary by +1 mm. This is accomplished by adding 1.0 mm to the X Global Offset of each mirror. The global offsets are absolute, so you must add to the existing global offset and not simply overwrite it. Moving both mirrors does not change the overall pointing, but the pointing will change briefly as you move each individual mirror.

If you are still guiding and taking an exposure, you'll want to uncheck 'report centroids to PCS' and 'report Zernikes to PSF' until the move is complete, or take baby steps so as to not lose guiding. (SAllanson 06/08/2010)

-- JohnHill - 25 Feb 2010
Topic revision: r5 - 14 Nov 2011, JohnHill
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