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The altazimuth design of the telescope means that the field of view rotates as the telescope tracks. It is necessary to compensate for this in order that:
At Cassegrain the approach taken is to mount instruments on a turntable, and physically rotate them so as to keep the final image at a fixed orientation. This approach is not possible with heavy instruments such as UES; instead an optical image derotator is used. This allows the instrument to remain stationary, at the cost of reduced field and throughput.
It is possible to observe either with or without the derotator in the beam. Inserting or removing the derotator is a fairly major change; it has to be carried out during the daytime, and advance notice is required. In normal operation UES is used with the derotator.
(N.B. Polarimetry, when implemented, must be done without the derotator and hence will be limited to point sources)