Measurement program for the Green Bank Telescope

The development of metrology instrumentation needed for realization of the active surface and precision pointing for the Green Bank Telescope (GBT) presents the opportunity for a complete characterization of this complex structure prior to the first astronomical observations. It is our intention to use the metrology program to evaluate the completed telescope and to point the way to improvements in performances. We also anticipate using the instrumentation for early detection of possible developing faults in the structure and the foundation. For the first time, it should be possible to derive parameters that determine the repeatable pointing variations as a function of elevation and azimuth position prior to astronomical observations. The measurement program will also give an estimate of the non-repeatable pointing errors resulting from thermal gradients within the structure. Laser rangefinders surrounding the telescope permit the non-invasive measurement of thermal and gravitationally-induced deformations within the structure at the 100 micron accuracy level. These measurements, and others, will be used to confirm the finite element analysis of the structure and to predict the performance of the completed telescope. This paper outlines the instrumentation used in the measurement program and gives the results obtained to date. These include dominant modal resonances in the structure, along with the damping coefficients associated with these resonances.