Pointing Tests using POINT and AUTOPNT

 

The pointing model for the INT has 17 terms, comprising the standard errors of an equatorially-mounted telescope (offsets in HA and Dec, collimation error in HA, misalignment of the polar axis and non-perpendicularity of axes), together with an empirical (Fourier series and polynomial) model of flexure in the telescope structure and large-scale errors in the Ferranti encoder readings. If pointing tests are made sufficiently frequently in order to update the pointing model, the rms error in the absolute pointing of the INT should always be better than 5 arcsec: values between 2.8 and 4.5 arcsec have been measured; a small residual oscillation at about 1 Hz persists in Hour Angle.

The data necessary to produce pointing models for the telescope are logged by the PE3220 running the task POINT. Data are logged by this task when the LOG button on the control console is pressed. Note that this button lights up while the logging task is active and the telescope should not be moved until the light goes out. The pointing program creates four data files on the PE3220 system disc. The first one is called POINT.DAT and must not exist when the program is started. Either delete them or rename them if they are to be kept. The extra three files created at the INT are called POINT1, POINT2 and POINT3 (all .DAT file extension). They contain details of the individual Ferranti encoder readings alone so that checks can be made for any systematic drifts in the Baldwin encoders and also to provide data from which it may be possible at a later stage to model the individual Ferranti encoder errors and separate them from global pointing effects such as a polar axis misalignment. The main data file, POINT.DAT, contains the name of the observed source, the Baldwin and Ferranti encoder readings of RA and Dec and the Local Siderial Time of the observation. A grid of objects suitable for use in the pointing test is contained in the file PTGRID.CAT (see Appendix ), which resides permanently on the system disc.

To run a pointing test with POINT:

1. Load the PTGRID Catalogue by typing at the User Interface:

   I(NPUT) ptgrid

2. Run POINT by typing at the TCS terminal:

   POINT INT

   If the computer responds with a message to the effect that file POINT.DAT already exists, and that therefore the task can not be started, rename that file with:

   RENAME POINT.DAT, POLD.DAT

   and similarly for the other three data files:

   RENAME POINT1.DAT, P1OLD.DAT , etc.

   Do not delete the files, since they may still be of interest. Further, report this action to the Software Manager.

3. Select a grid star from the catalogue with:

   NEXT sourcename

4. Move the telescope to the grid star by pressing the SRCE button. Centre the star on the SNAFU reference point (see section ).
5. Press the LOG button on the Control Desk - wait until the light goes out (it should light up for a brief moment).
6. Repeat steps 3 through 5 until sufficient stars have been observed. Choose your objects all over the sky.
7. Stop the POINT program by typing at the TCS terminal:

   ENDPOINT

   This will stop the program and print out the data files. Subsequently, press the on-line button in order to re-initialise the terminal.

8. Ask the software group to transfer the file POINT.DAT to the POINTING account on the VAX 8300.

Having transferred the POINT.DAT file to the VAX 8300 it is necessary to add two lines (with the editor) at the beginning with the date and observer's name, and the temperature and atmospheric pressure at the time the measurements were made. The data can then be analysed with the TPOINT software (SUN 100.7, a Starlink User Note by P. Wallace, 1989) in order to determine the pointing model coefficients. These can then be entered in the relevant .STD file (by qualified La Palma staff!).

AUTOPNT

The pointing procedure has now been completely automated and the procedure AUTOPNT should be used at Cassegrain. The acquisition of pointing stars is made with the autoguider probe; the stars are put at the SNAFU reference position. Therefore, the guide probe has to be told where the reference position is by placing a star on the reference point and acquiring it with the autoguider in `probe' mode. The procedure is now as follows:

1. Place a star on the SNAFU reference position, i.e. the slit or the Grinnell crosswire (press crosshair).
2. Acquire this star with the autoguider using the ACQUIRE/PROBE option.

   (Alternative to steps 1 and 2: drive the probe to the centre position. The idea is just to put the probe at the reference point.)

3. At the user interface select ACQUIRE/TELESCOPE.
4. At the Telescope Control System Console type:

   AUTOPNT INT

The telescope will work its way through every available star from PTGRID.CAT from North to South and then back again. When it has acquired each star it will guide for a few seconds and then log that position before moving on to the next star.

At Prime focus the situation is more complicated because it is impossible to move the autoguider to the rotator axis of the telescope. The Prime Focus geometry is set up such that if a star is on the optical axis of the telescope pressing BEAMSWITCH B will move the telescope so that the star is centred on the autoguider in its parked position. Thus, when using AUTOPNT at Prime you have to press BEAMSWITCH B so that the stars fall within the field of the autoguider. Unfortunately the Telescope Drive Software has a feature that at the beginning of every slew the BEAMSWITCH automatically reverts to the NOMINAL aperture. Therefore, when running AUTOPNT at Prime it would be necessary to press BEAMSWITCH B every time the telescope begins a slew to the next star! This would be a cumbersome exercise, but a simple way around it is to change the NOMINAL aperture position before starting AUTOPNT as follows:

1. Inquire the current BEAMSWITCH B aperture co-ordinates at the User Interface console:

   ADV/TEL

   The aperture position is given in arcseconds.

2. Transform the co-ordinates to radians through division by 206265.
3. Inquire the current NOMINAL aperture co-ordinates at the TCS console:

   INPSCF
   @AOXNOM
   @AOYNOM

   Important: make a note of the current values.

4. Replace the NOMINAL offset values by adding the beamswitch offsets to their current values:

   AOXNOM new-aoxnom
   AOYNOM new-aoynom

    where: Tnew-aoxnom = current aoxnom + beamswitch B.x
   
                		 new-aoynom = current aoynom + beamswitch B.y
   
   

Important: do not forget to reset AOXNOM and AOYNOM to their original values after AUTOPNT has finished! Some NOTES:

• At any time the contents of the data file(s) being created can be inspected by typing at the TCS terminal:

  PRINT filename

  It is worth checking that the program is working by printing the POINT.DAT file after the first 5 observations have been made with:

  PRINT POINT.DAT

  A listing should then be output to the lineprinter standing in the Instrument Computer Room, adjacent to the Control Room. Check that it is switched on-line. If there does not appear any output, go to the TCS console and try:

  TASK SPOOLER

  CO(NTINUE)

  If that doesn't work, type

  STARTSPL

  If this still doesn't work write a `Software Error Report' (SER).

• It is not possible to run the task INPSCF or the ENCODERS test program simultaneously with POINT.
• If you have renamed an existing POINT.DAT file, report this action to the Software Manager so that he can archive or delete the file as he sees fit. Don't forget to do this, or the disk will get littered with old pointing files. Do NOT delete the file yourself unless you are positive that it is of no further interest.
• The POINT program can be used to keep a record of the directions to which the telescope was pointed throughout an observing night. This can be particularly useful for analysing the geometrical errors of the mounting (see for instance RGO/La Palma Technical Note No. 32). POINT is started at the beginning of the as for a normal run with PTGRID, but now your own objects are the test points. Thus, press the LOG button on the console every time you centre a star. At the end of the night type ENDPOINT at the TCS terminal, press the on-line button and issue a request to transfer the file to the VAX 8300.