B.4 FGS1r: Second AMA Adjustment

The second adjustment to FGS1r’s AMA was performed on October 12, 1998. The results included near-ideal S-curves, essentially similar to those displayed in Figure B.4. Since then, STScI has continued to monitor the temporal stability of these interferograms. Significant but tolerable changes in the amplitude and morphology of the y-axis S-curve have been seen. These appear to be due to a continuing, but very slow, shift of the y-axis Koester prism w.r.t. HST’s OTA. To assure the reliability of measurements made by FGS1r on close (< 25mas) binary systems, standard calibration reference stars have been observed once per cycle (if that spectral type is needed by GO programs during the cycle).