23 Mirror Position Sensor System Non-Linearity  and Thermal Error Correction   Non- Linearity Correction   The original MCE FPGA design had only gain and offset terms for converting DPSS voltage to mirror angle.  It was decided to expand this to a third- order polynomial correction to compensate for higher -than- expected  non-linearity in the DPSS output. The DPSS tuning process balances non- linearity, thermal stability, and  resolution;  the latter two were deemed more important to correct in hardware, though  non-linearity could  be corrected in firmware. At the mechanism level, calibration was performed with our autocollimator as a  reference.  At the instrument level, calibrations will be performed by scanning across a known star field prior  to each encounter. This correction is a polynomial which is computed on each 5 kHz position sample before  it is passed to the feedback input of the mechanism’s closed- loop controller.   Performance testing at each environment started with sensor recalibration, where static DPSS voltage  measurements were compared to autocollimator measurements at over 200 points across the range of  motion. The remaining scripted test s were divided into three parts;  stability, repeatability, and scan tests.  For the stability tests, the SMS  was commanded to dwell for several minutes each at various fixed positions  across the optical range of the mirror wh ile data such as pointing stability, pointing error, and standby power  were captured. R epeatability tests commanded the SMS to a series of test positions in a cyclic manner to  ascertain how repeatably  the mechanism was able to position the mirror. Finally,  the SMS  was commanded  to scan across several optical ranges at several scan rates  to obtain data such as optical  smear, torque,  and scan power. These tests were automated, and test results were trended  across the entire mechanism  test campaign to verify the performance of the mechanism against system requirements.   Mechanism performance testing showed all requirements were achieved except for one:  the p ointing  resolution requirement. DPSS gain and non- linearity vary with temperature, but because of recalibration  prior to each test, there was  no noticeable performance degradation across the range of qualification  temperatures. The pointing resolution issue appears to be a result of thermal gradients present during the  sensor calibration step, which ul timately led to large residuals in the calibration data and thus inaccurate  sensor parameters for the controller. To obtain valid calibration parameters, it was necessary wait until  mechanism temperature was stable to within ±1.5 °C of the desired temperature plateau. As a result, there  were consistent pointing errors, especially at the end- of-travel positions where non- linearity is higher, and  least -squares polynomial fitting has the highest residuals. Rate and smear are the driving requirements for  this mechanism, so relaxation of the pointing resolution specification was deemed acceptable by the project.     Thermal Error Correction   Initial tests of the ETU SMS  reveal ed a drift in mirror position with a time constant on the order of  20 seconds. In stability tests where the SMS was commanded to step and hold at a mirror position, the  DPSS would report the SMM mirror position as stable at the commanded location, while autocollimator  measurements would show the SMM mirror had overshot and was slowly settling into position. The inverse  was true in tests where a constant current was used to drive the SMM into one of its hard -stop locations:  the autocollimator would show the mirror angle to be static as expected, while DPS S measurements would  show the SMM  mirror had undershot and was slowly drifting to the expected location. Further testing  showed that the initial amplitude of this drift scaled linearly with the size of the step: a larger step meant a  larger initial error. Stepping from one end of travel to the other, a 70- mrad motion, led to approximately  200 µrad of initial error.   This drift turned out  to be a ther mal phenomenon correlated with self -heating of the sensor heads  themselves. An experiment was run by the vendor  where a single sensor head was heated approximately  1°C, which resulted in a voltage error equivalent to less than 1 mrad, decaying with a simil ar time constant  to what was seen in the SMS . The vendor  found  by measurement  under normal operation, power  dissipation of a single sensor head changed by a few milliwatts . Based upon the sensors head ’s  approximate thermal resistance, moving the SMM from one end of travel to the other would lead to a  fractional  °C change in temperature for a sensor pair. Based on the results of the sensor heating  experiment, that temperature delta would cause a voltage error consistent with what was obs erved in SM S