The Green Bank Telescope

The National Radio Astronomy Observatory Green Bank Telescope (GBT) is the world's largest fully steerable telescope. The GBT has now been in routine operation for over two years, observing at frequencies up to 50 GHz. In order to deliver the tracking accuracies required at 50 GHz, we solve simultaneously for gravitational and thermal effects in the development of the static pointing and focus tracking models. A precision temperature sensor system then generates additional real-time corrections to compensate for varying thermal gradients in the antenna. Collimation and surface accuracy requirements are met by an active surface control system which combines initial corrections derived from a finite element model of the antenna with additional terms derived from astronomical phase-retrieval holography measurements. The GBT has a rich suite of instrumentation including receivers which cover almost the complete frequency range from similar to 290 MHz to 50 GHz, and backends for spectroscopy, pulsar observing, broadband continuum, very long baseline interferometry and planetary radar reception. A 64-pixel bolometer camera is under development by a consortium including UPenn, NASA-GSFC, NIST, UCardiff and NRAO. Recent software developments include an extremely flexible application which combines traditional interactive observing, scheduling-block based observing and real-time monitoring and data display in a single, convenient interface. In this paper I will summarize the current performance of the GBT, and review some recent science results. I will also describe how plans changed with time, and review some of the lessons learned in the development of the telescope.