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.
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.