Incremental encoders can be 'zeroset' by a variety of methods. The method of ...TO...

allows the current position of the mechanism to be input manually. For applying this method the telescope has to moved to a reference position first. In case of the WHT azimuth axis the position reference are arrow marks on two brass plates inside the bull ring. This method provides a  backup in case of failure of both the absolute encoder and the target electronics.

Referencing:

1. This is done from the bull ring with AZ approximately in park zenith position with AZ in the green zone. Referencing is a two man job.
2. Move the telescope first to 297 degrees and switch it into engineering mode.
3. Then move in '+' direction using the hand wheel on one of the motor in the bull ring. The 2nd person has to watch the marks on the brass plates in the bull ring and give notice when they exactly coincide in order to stop the motion. Once aligned detach the hand wheel.
4. In the control room enter the command: ZEROSET AZ TO 298.66

We tested this method on the 29/10/2009 and found that a positioning error of approximately 1 arcmin can be expected.
Note that after each manual zeroset, you need to do a 7 star calibrate in order to compensate for the positioning error, before you can continue observing.