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Thermal effects on the collimation of the GBT

Intro

Daytime measurements of large scale aberrations present in the GBT optics using the OOF holography technique have shown that during these times (when thermal effects are known to be important), the dominant aberrations are similar to that which may be expected from misalignment of the secondary reflector (due to the off-axis design of the GBT, the expected aberrations are close to classical astigmatism). We are currently pursuing strategies to correct these aberrations, initially by focusing on using the correlation between sub-reflector misalignment and the resulting pointing shift.

Implementation of test routines

Current routines used for testing strategies for dealing with the thermal mis-collimation are in the file Subreflector.py in the standard directory /home/groups/ptcs/obs/turtle.

The SubLPC routine

This routine communicates directly with the telescope via a grail client to fire: use with caution

The purpose of this routine is to correct all of non-repeatable pointing errors by moving the subreflector rather than offsetting the entire telescope structure. It accomplishes this by

1. Moving the sub-reflector to compensate for the measured current pointing error. This current pointing error is retrieved from current LPC values: therefore it is recommended to run a Peak procedure before running this procedure.
2. Re-setting the local pointing correction so that it reverses the current dynamic pointing corrections. This should zero all pointing corrections for non-repeatable effects.

The routine takes following parameters:

usedynonly

(This is short for USEDYNamicONLY?) Only use the dynamic corrections to estimate how far the sub-reflector needs to moved. This parameter is default to False. Using this is not recommended since dynamic corrections are not accurate during daytime.

doturtle

Actually send commands to the telescope. Default is True. This parameter can be set to False to make debugging code easier since the procedure should not then interfere with ongoing observations.

The ZeroSubLPCs routine

The purpose of this routine is to re-set the sub-reflector to the default position and optionally restore the previous local pointing corrections (which need to be supplied).

The routine takes following parameters:

lpcaz: The azimuth local pointing correction to set. The default value is zero.

lpcel: The elevation local pointing correction to set. The default value is zero.

doturtle: As above: if False will not actually do anything.

The GetLPCs routine

The purpose of this routine is to retrieve the current local pointing corrections from the telescope. More precisely, it returns the values of samplers azOffset2 and elOffset in the units of minutes of arc.

This routine takes no parameters.

This routine returns a tuple: (azoffset, eloffsett)

Expected typical usage

A typical approach to correcting the pointing by moving the subreflector would run along the following lines.

1. Run a Peak procedure to determine the current LPC.
2. Call beforelpcs = GetLPCs?() to store these LPCs.
3. Call SubLPC() to take up the pointing in the sub-reflector. This will overwrite the LPCs in the system hence saving them in beforelpcs.
4. Do science......
5. Call ZeroSubLPCs(beforelpcs[0] , beforelpcs[1] ) to restore the LPCs in the system before the sub-reflector position was adjusted.

-- BojanNikolic - 08 Nov 2005