Measurement system for the Green Bank Telescope

The Green Bank Telescope (GBT) at the National Radio Astronomy Observatory (NRAO) site in Green Bank, WV, was completed in June 2000. At the time of writing, the GBT is undergoing testing of the various systems and characterization of the actual structural modes. It is the largest, fully-steerable radio telescope in the world. The main reflector which is a solid-surface paraboloid has an unblocked 100-meter projected aperture. The telescope will operate at prime focus from 25 MHz to 1200 MHz and at Gregorian focus from 1.15 GHz to /spl ges/ 100 GHz. It is a state-of-the-art instrument with an active surface to compensate for deformations of the primary reflector backup structure due to external loads. The GBT has a unique six-degree of freedom subreflector and a three-degree of freedom prime focus mount that allow realigning of the optics which is required as the telescope moves in elevation. A receiver turret that holds 8 secondary focus receivers will provide rapid frequency agility for the telescope. This paper describes the metrology system, based on laser rangefinders, that has been developed at NRAO for the realization of the active surface and precision pointing of the GBT.