Fine Guidance Sensor Instrument Handbook for Cycle 17

1.7 FGS Replacement in SM4

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A total of four FGS units were built, an "engineering test unit" (ETU) and three for installation on HST. Prior to the second servicing mission the mechanical health of both FGS1 and FGS2 had degraded to the point that the reliability of each unit became questionable. Moreover, it was recognized that the deleterious affect of HST's spherical aberration, to which the FGSs are still subject, could be partially mitigated by replacing the fixed mounted "fold-flat mirror #3" with an articulating mirror assembly (AMA) that can be commanded from the ground to provide the means to align the FGS interferometric elements with HST's optical axis. Therefore the ETU was refurbished with the AMA and made flight-ready for insertion into HST in SM2.

FGS2 had been showing chronic trends of mechanical wear, more so than FGS1, and thus had been slated for replacement in SM2. However, a few months before the servicing mission, FGS1 displayed acute mechanical failure symptoms and appeared to be at greater risk than FGS2. Therefore, the refurbished ETU replaced FGS1, and has since been referred to as FGS1r. With the advantage of the AMA, FGS1r has proved itself superior to FGS3 as an astrometric instrument, and has been used as such since 1999.

Meanwhile, the original FGS1, which was returned to Earth at the completion of SM2, was refurbished with new mechanical components and an AMA. Designated FGS2r, it was install in HST during SM3A. The original FGS2 was returned to Earth and refurbished with the expectation that it would replace the venerable FGS3 in a future servicing mission. However, since mid 2006 FGS2r has been showing problems with the LED on one of it star selector servos. With a failing LED, which is sensed by the FGS firmware to provide closed-loop control of the instrument, FGS2r is experiencing an increased guide star acquisition failure rate. Therefore, it is currently planned to replace FGS2r in SM4 with the refurbished unit. However, NASA will continue to monitor the relative health of all three FGSs, with the expectation that the unit showing the most risk will be the one that is actually replaced. It should be noted that FGS1r appears to be in good health and is expected to remain on HST, providing reliable service as both a guider and an astrometer.

The refurbished FGS to be installed in SM4 may very well become HST's best fine guidance sensor. Unlike FGS1r and FGS2r, its optics has been realigned using the specially developed full field of view test set. This enabled the Goodyear optical engineers to eliminate "beam walk", an effect that causes the interferometer's relative alignment with the HST optical axis to change as the star selectors assembly is rotated to observe stars across the FGS FOV, which in turn degrades the benefit of the AMA for mitigating HST's spherical aberration. (The AMA can be used to optimize the FGS performance at any one place, but only one place, in the FOV.) Thus, the new FGS, after AMA adjustment, is expected to have optimal interferometric performance across its entire FOV. If so, this will permit the use of the F583W filter for guiding, which provides access to guide stars that are 0.7 magnitudes fainter than what can be achieved with the 2/3 PUPIL. The question of the new FGS replacing FGS1r as HST's astrometer will have to await an assessment of its relative performance, but this can only be considered after the new instrument stabilizes on orbit (i.e., out gasses at an asymptotic rate). Experience with FGS1r showed that this can take up to two years.