LASSI — Real Time Surface Adjustment of the Green Bank Telescope

The 100-meter diameter Green Bank Telescope (GBT) has a surface consisting of 2004 panels that can be adjusted in real time to compensate for any known deformation. While gravitational deformations are stable and well-characterized, thermal deformations can vary significantly on time scales of one hour, especially during the day when sunlight strikes the instrument in variable ways. We have received a grant from the National Science Foundation to develop techniques for using a commercial laser scanning instrument mounted near the prime focus of the GBT to characterize the surface shape and produce real-time corrections for all deviations from the ideal shape. The goal is to measure the surface to an accuracy of at least 100 micrometers in a matter of minutes. We will report on the status of this project. In preliminary tests, the system has been able to identify deformations represented by Zernike polynomials with coefficients of order 75 microns, thus meeting the goal for measurement accuracy.