Physics
Scientific paper
Sep 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997spie.3112..181t&link_type=abstract
Proc. SPIE Vol. 3112, p. 181-186, Telescope Control Systems II, Hilton Lewis; Ed.
Physics
Scientific paper
The Gemini telescope control system calculates the orientation of the Cassegrain rotator in a way that allows the desired angle to be specified relative to any of the four supported tracking coordinate systems (FK4, FK5, geocentric apparent place and topocentric az/el) independently of the tracking frame of the mount. In addition, the actual, rather than demanded, position of the rotator mechanism is used in the calculation of the mount position so that the telescope is able to track a target with an off-axis pointing origin (By pointing origin we mean the nominated point in the focal plane to which the pointing refers; i.e. where the image of the celestial object being tracked will appear.) even when the rotator is not in position. The actual rotator position is also used in the calculation of guide star coordinates so that guide stars can be acquired before the rotator is in position. This feedback of the actual rotator position to the mount calculation results in some unexpected behavior, particularly near the zenith blind-spot where the mount may track either north or south of the zenith depending on the positions of the rotator and the nominated pointing origin. The algorithm used to calculate the desired rotator position and the measures taken to avoid the unpredictable behavior near the zenith are described.
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