Encoder Zeropoints: ZEROSET

  To be done daily, as part of the start-up procedure for the INT.

1. In engineering mode, position the telescope roughly at zenith, i.e. position Hour Angle and Declination 2846 on the synchros.
2. Now drive the telescope in slow motion a few degrees away from the zenith in both Hour Angle and Declination by pressing the positive direction buttons +.
3. Type ZEROSET on the User Interface and press RETURN, but only if you have successfully performed action (2).
4. Check that the two red encoder/zeroset-in-progress lights on the Control Desk are both lit. If they are not, it means that the telescope is not far enough away from the zeroset markers, so cancel zeroset by pressing the ACK button on the control console and start again.
5. Set the engineering-mode speed controls (on the Engineering Rack) to the position markers, and make sure the slow-motion drive is selected. It is worth to move the telescope in slow motion to see that the zeroset is triggered. The accuracy of the result is now independent of speed to good approximation.
6. Drive the telescope through the zeroset position (i.e. zenith) by pressing the negative direction buttons -. The indicator lights on the Control Desk should go out and a message:

   ``Zeroset completed''

   should appear at the User Interface.

If for any reason, the procedure does not work, or if you feel it is giving inconsistent results, report it on a Fault Report Card; in order to escape from the zeroset procedure at any stage, press the ACK button.

If the above described zeroset fails, it is still possible to observe by doing a rough zeroset, as follows:

1. Position the telescope at zenith, as before, and set the telescope as accurately as possible on the centre of the position range where the yellow-green lights are on.
2. Type at the User Interface terminal:

   ZEROSET/MANUAL

The zeropoint determined in this way is far less accurate than when using the regular procedure, and it is necessary to do a 1-star SNAFU in order to adjust the software zeropoint.