Fine Guidance Sensors: Calibration Status

Servicing Mission Four (SM4) include the installation of a refurbished FGS, designated FGS2r2. The commissioning and calibration of this FGS for guide duty is outlined in this paper. Prior to SM4, the FGS1r interference fringes were re-optimized for the first time since 1999 by an adjustment of the instrument’s articulated mirror assembly. While this adjustment improved the FGS1r angular resolution when operated as a science instrument in Transfer mode, it also changed the FGS1r geometric distortion by a sufficient amount to render the instrument inoperable as a astrometric science instrument. An interim calibration was performed that has restored the FGS to nearly its previously well calibrated configuration, but additional on orbit calibration observations may be required to fully restore the FGS1r astrometric performance.