8.2 Definitions

8.2.3 Rotator control strategy


When objects in the focal plane are to be kept at a fixed orientation relative to the sky ("position angle mode"), the rotator angle is continuously adjusted so as to maintain the (XIM,YIM) axes at a fixed orientation relative to the (East,North) axes. The relative orientation is defined by a "user angle" which is the angle between North and the YIM axis. It is thus the position angle of the YIM axis and is referred to as "PA" on the diagram.

For all Keck I instruments, the rotator angle (r) is calculated by the DCS and supplied as a time-stamped demand to the system which is actually controlling the rotator. This is because it is necessary to take various telescope pointing terms into account in order to calculate a rotator angle which is correct even near the zenith.

The AO rotator can work in the same way, but there are other AO mechanisms whose positions are a function of the parallactic and elevation angles. The full set of such mechanisms is:

It is intended that the DCS will provide the rotator angle and the elevation angle. The rotator control code will use the supplied rotator angle and the ADC control code will use the rotator angle and the elevation angle.


AO Software Design Book - 25 FEB 1997

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