Testing the Encoders

 

Problems with the telescope pointing are nowadays more often than not due to problems with the encoders. If encoder problems are suspected, e.g. because even after a repeated 1-Star SNAFU the pointing remains poor, test the encoder readings, as follows:

0.3cm
1. Be sure that the slow-motion drive is engaged, e.g. the telescope is tracking (to ensure that the Baldwin encoders are engaged).
2. Stop the telescope and switch to Engineering mode (for safety).
3. Run the task ENCODERS from the Telescope Control System terminal. Type:

   ENCODERS

   This produces a status report of 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 is not already disabled, then that individual encoder is baulking. Confirm this by looking at the counters in the CLIP Centre. Correct any obvious problems (such as disconnected electronics).

   If the problem persists run the INPSCF task (see section ) and disable the faulty encoder by resetting 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. A long-term fix is to ask software support staff to update the initialization files.

4. 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). However, 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.