After the second servicing mission (SM2) the original FGS1 was returned for refurbishment to its manufacturer, Raytheon Optical Systems Inc. (ROSI, currently BFGoodrich Space Flight Systems). Its (worn out) star selector shaft bearings were replaced and, like FGS1r, an AMA was installed. During the December 1999 servicing mission (SM3a) this repaired unit, redesignated as FGS2r, replaced the original, mechanically worn FGS2. As part of the orbital verification activities ROSI engineers, using observations of the standard star Upgren69, adjusted the AMA to align FGS2r's internal optics with the OTA to optimize the instrument's interferometric response.
With two AMA optimized FGSs now on board HST, the question of whether FGS1r or FGS2r should be designated as the science instrument might arise. The amplitude and morphology of both the FGS1r and FGS2r S-curves vary with location in the FOV due to "beam walk", for which the AMA can not compensate (discussed in chapter 2). But FGS1r demonstrates optimal fringe visibility and morphology along both its x and y axis simultaneously at the center of its FOV, while FGS2r's x and y axis S-curves are not simultaneously optimal anywhere in its FOV. Therefore, FGS1r has significantly better angular resolution and remains the clear choice as the science instrument (FGS1r's performance as an astrometer is not impaired by the FOV dependent fringes).
There is also the consideration of the orientation of an FGS's FOV in HST's focal plane. For efficient astrometric determination of parallaxes, targets and reference stars are observed at times of maximum parallax factor. Due to HST sun angle constraints, FGS1r's orientation at such times is more favorable than that of FGS2r.
STScI does not regard FGS2r as a science instrument and has no plans to calibrate it as such. It has been calibrated only to the level needed for reliable performance as a guider. It will be monitored over time to verify that these calibrations remain within tolerance as the instrument desorbs water from its graphite epoxy composites.
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