The telescope is tracking but pointing badly

 

• Have you zeroset correctly and done a SNAFU?
• Check the coordinates (including proper motions) and the equinox. If you are observing a star with a high proper motion (several of the white dwarf spectrophotometric standards fall into this category), it is vital to include the proper motions and to beware of old finding charts. Has the proper motion in Right Ascension A been entered in /year (which is wrong) rather than seconds-of-time/year (which is correct)?
• Has the telescope control software been initialised with the correct focus file?
• Are the encoders working correctly? Do a systematic check as follows:
  1. Stop the telescope and switch to Engineering mode
  2. Check the position roughly on the synchros. Is there a big pointing error: more than 2 in RA or 30 in Dec? If so, shut down the Telescope Control System and start again. If the error persists call the Duty Technician.
  3. Test the performance of the encoders by running the task ENCODERS from the Telescope Control System terminal. Type:

     ENCODERS

     This runs a program which compares the readings from the 4 independent encoders (3 Ferranti, 1 Baldwin) on each axis. Firstly, look at the Ferranti readings. If one of these is grossly different from other two on the same axis (and from the the synchro), then that individual encoder is baulking. Confirm this by looking at the counters in the CLIP Centre. Correct any obvious problems (disconnected electronics). If the problem persists run the INPSCF task (see section ) and reset the appropriate encoder weight to zero with variables WHAFEn and WDCFEn for Hour Angle and Declination Ferranti head n, respectively. Thus, to disable Hour Angle Ferranti Encoder number 2, do:

     INPSCF

     WHAFE2 0.0

     END

     Note that this procedure only affects the running system - if, for any reason, the TCS is stopped and then restarted you will have to reset again the appropriate weights to zero.

  4. Now repeat the ZEROSET and SNAFU and try again.
  5. If all of the Ferranti readings appear to be sensible, compare the Baldwin+Ferranti values with those for the Ferrantis alone. If they are more than a few arcseconds different, then the Baldwin encoder has probably broken. It is easy to switch over to using Ferrantis only in Declination (just set Task Common parameter JDCBW to zero using INPSCF). If the problem is with the Hour Angle encoder, better call out the Duty Technician in order to check out the encoder and adjust the servo constants.