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Paper: GBT Active Optics Systems and Techniques
Volume: 216, Astronomical Data Analysis Software and Systems IX
Page: 355
Authors: Wells, D. C.
Abstract: The GBT is a 100-meter radio telescope whose parabolic primary mirror is composed of 2004 rectangular panels, with 2209 actuators at their corners. Retroreflector prisms are installed on the panels. Six laser rangefinders are used to trilaterate to these prisms so that departures from paraboloidal shape due to wind pressure and thermal gradients in the backup structure can be measured. The actuators can adjust the panels to correct these wavefront errors. Deflections of the secondary optics from their design positions (focus tracking errors) will also be measured by the six rangefinders, and will be corrected by moving the ellipsoidal subreflector. Twelve rangefinders on ground monuments will measure to retroreflector spheres and prisms attached to the elevation bearings and to the backup structure so that pointing errors due to wind pressure and thermal gradients can be measured; pointing corrections will be supplied to the telescope control system.
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