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The TCS is set up so that if the UES focal station is selected with the derotator option enabled (type UES DEROT at the TCS), and the slit angle set to zero, the (X,Y) coordinate system used by the handset will always be aligned with the spectrograph slit, regardless of the sky PA. Offsetting a star in Y will move it along the slit, and offsetting it in X should move it perpendicular to the slit.
If there is any evidence that the derotator is not being positioned correctly (if for example attempting to close the autoguider loop results in the telescope driving off the object), the positioning of the derotator can easily be checked by viewing a star on the TV, and checking that the (X,Y) handset works as advertised.
If it doesn't, the first thing to check is that the absolute and incremental encoders are reading the same. Check this on the TCS encoders page (type PAGE ENC on the TCS). If they appear different, try zerosetting the incremental encoder by typing ZERO UES ROT on the TCS.
If the absolute encoder has been moved, it will be necessary to redetermine the encoder zeropoint. The simplest way to do this is to use the TCS ZEROSET command to zeroset the UES derotator to a specified position (i.e. type ZERO UES ROT TO <position>). Try different positions until the behaviour of the TCS handset is as advertised above. You will now be able to observe normally. Stop the rotator, and write down the value of both absolute and incremental encoders as displayed on the TCS encoders page, so that the newly determined zeropoint can be set up in the telescope parameter file. You will be able to observe, but until the the zeropoint has been set up in the parameter file, it will be lost each time you shut down and start up the TCS.
N.B. If it is neccessary to redetermine the postion of the absolute encoder, please ensure that Telescope Operator notes this in the WHT Log Book and also enters a fault in the Fault Database.