GHRIL Optical and Infra Red De Rotator TCS Alignment Instructions

GHRIL Infra Red and Optical De-Rotator TCS Alignment Instructions


This page contains notes on how to align the de rotation optics for the GHRIL IR and Optical De-rotators. This will be of interest mainly to Duty Engineers.


Overall, with auto guider logging turned on, a star is chosen on the autoguider and the de-rotator is ordered to perform one complete revolution (producing a revolution of 720 degrees in the image). See for some details on de-rotators also A description of an optical rotator

The data is then analysed using the PLOT utility, with the FIT2D RH command. This produces six constants, two of which are used to modify constants within the running TCS. The normal way to do this is with the VMS GSEXAM utility. This procedure is repeated several times until no further improvement can be obtained. The modified final constants are then saved for inclusion into a compiled version of the TCS to be released within a few days.

Contants C3 and C4 produced by the plot utility are used to adjust variables in the TCS, using an interative process where the de-rotation should get progressively better.

The ING documentation for the PLOT utility (available here as a a postscript file , and was extracted from LPAS3 under [DOCS.TCS.PLOT] with the file names PLOT.TEX and PLOT.PS and the updated version PLOT manual rev 2 also postscript ).

The notes here are intended as a practical guide to performing this task to supplement the PLOT manual.


Ask the WHT Telescope manager to enter a request in the engineering schedule. Check the week before that the OPS team have the installation in hand. In order for the equipment to be set up the OPS team must be informed as early as possible ( a week would be ideal ) that this test is to take place. The GHRIL bench may have other equipment scheduled for it and requires a connected working AUTO GUIDER HEAD, CONTROLLER and nitrogen flow system.
On the day of the test it is best to go up to the mountain in the morning and help with the set up and testing of the auto guider CCD etc. Also to check that the de-rotator is in place, along with the other equipment.
Try to ensure that the optical centre is near the centre of the CCD. If this is not done the star can run off the edge of the image and stop the autoguiding.
Make sure that the autoguider is set at nominal focus. This used to be marked on the bench as whircam sub chasis , now the NAOMI system is here, the other optical bench may be present and the autoguider head will have to be focused.
Or the Naomi equipment may be able to perform this test, this would be ideal as the light path is probably longer in NAOMI than in the whircam. More needs to be known about the NAOMI A/G system.
Obtain a copy of the PLOT manual and print these instructions out. Examine OWNINIT.FOR and determine which de-rotator you will be modifying.

Alignment Preparation

Start a PLOT utility with the GENUSER account.
Issue the command DEV XWINDOW from PLOT.
Open a window on the ALPHA and set up the GSEXAM (TCS VAX DBG) utility. Log in as ENGINEER.
The rotator centre must be determined. Note that the uncorrected rotator can have a pronounced loop within a loop shape. It is essential therefore that the rotator is is moved through 360 degrees for this, causing the image to rotate 720.

Alignment Procedure

Choose a suitable star, preferaably at an elevation between 40 and 60 degrees and start TRACKING it.
Issue the command ROT SKY 0
Offset the star on the autoguider so that it is approx 2"-3" arc seconds off centre. Use the handset to do this.
Start Auto Guiding.
Set autoguiding logging on with LOG AUTOGUIDER ON
Increment the demanded sky rotation positon by 50 degrees at a time, and when it reaches the demanded position, repeat until 720 degrees has been reached. Use the ROT SKY command to do this. Do not use the handset to move the de-rotator, use the ROT SKY command (as this can cause the rotator to reverse to a SKY PA position).
Turn auto logging off with LOG AUTO OFF .
Read the LOG file into the PLOT utility with the DATA command.
in PLOT PLOT> FIT2D RH this will print six co-effiecients. Note you MUST do this before trying to PLOT the data.
Enter the command PLOT > SQUARE (PLOT will not remember that it is in square node; you must type it before trying to view each graph).
in PLOT PLOT> PLOT and PLOT> PLOT FIT , decide whether the graphs contain sensible data. If they do update the GHRIL.COM file and run it into GSEXAM bearing in mind that....
The variables in the TCS may be changed while the TCS is running but the telescope MUST NOT BE TRACKING OR MOVING while this is being done. It is best to put the telescope into ENG mode while the variables are being changed.
Note carefully the signs of the values to adjust the TCS variables, and use the RADIAN values, NOT the ARCSECOND ones.
The variables in the TCS should be changed using GSEXAM . Care should be taken to get the correct index number for the GHRIL IR variables. Currently (21JUL1999) this is set to 4 for IR and 5 for Optical, but can change; be careful ! The following VMS DBG script prints the original variables, makes changes and then prints them again. It is intended to be modified with the constants from the FIT2D RH command.
To run the script (after editing and checking the values and signs/polarities carefully) enter it thus at the DBG> command line.
repeat this procedure until there is no further improvement.

Final Test : Rotator Centre.

Place the test star on the rotator centre and repeat the alignment procedure for best fit. This should adjust the first harmonic.

End of Test : Data to be saved for inclusion to next TCS version

Each time the GHRIL.COM file is run into GSEXAM, the last four co-efficients displayed represent the current best model for the de-rotation corrections. These must be saved, as the final values will be entered into the the TCS OWNINIT.FOR file, and compiled to produce a new version of the TCS.

Create New TCS version.

Step the final digit in the version number if the alignment is the only change made to the TCS. I.e. W8-0-5 would go to W8-0-6.
Compile and release the new version in the usual way.

Example alignment session with graphs and data changes .

Graph of a Good optical de rotation taken 03:20 am 28DEC1999 .

R.P. Clark. E-mail: Last Updated 1st of August 2000