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.
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
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> READ 1 AGX
in PLOT PLOT> READ 2 AGY
in PLOT PLOT> READ 3 GHRIL POS
in PLOT PLOT> FIT2D RH this will print six co-effiecients. Note you MUST do this before trying to PLOT
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
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.
Last Updated 1st of August 2000