Control of the Hobby-Eberly Telescope primary mirror array with the segment alignment maintenance system

The Segment Alignment Maintenance System (SAMS) is a control system to maintain the alignment of the 91 segment Hobby-Eberly Telescope (BET) primary mirror array. The system was developed by Blue-Line Engineering (Colorado Springs, CO) and NASA-Marshall Space Flight Center (Huntsville-Al). The core of the system is a set of 480 inductive edge sensors which measure relative shear between adjacent segments. The relative shear is used to calculate segment tip/tilt and piston corrections. Although the system has dramatically improved the performance of the BET it does not meet its error budget due to thermal drifts in the sensors. The system is now sufficiently stable that it routinely requires only one primary mirror alignment at the beginning of the night. We describe methods to calibrate this sensor drift.