The Fine Guidance Sensors (FGS), originally designed and built by the Perkin-Elmer Corporation in Danbury, CT (now Goodrich Corporation's Optical and Space Systems), comprise a set of three radial-bay instruments on board the Hubble Space Telescope (HST). The main purpose of the FGS is to provide an absolute pointing reference to enable the telescope's Pointing Control System (PCS) to maintain the pointing stability of the telescope at the milliarcsecond level, often over exposure times as long as tens of minutes. The HST pointing requirements necessitated a design with a large observable field of view (FOV) with a high dynamic range in order to take advantage of the variety of observing scenarios HST was expected to encounter.
However, the potential of the Fine Guidance Sensors to provide high-precision astrometric measurements was identified early in the FGS design process, and the prospect of the FGS becoming in effect another scientific instrument.
The Hubble Space Telescope was deployed in low-Earth orbit (600 kilometers) by the crew of the space shuttle Discovery (STS-31) on 25 April 1990 with a complement of three Fine Guidance Sensors. Due both to its position in the HST FOV and its relatively good interferometric response and low optical field angle distortion (OFAD) in the spherical aberrated beam, FGS3 was designated the astrometric science instrument, and remained so until the installation of FGS 1R.
In February 1997 FGS 1 was replaced during Servicing Mission 2 (SM2) with FGS 1R. FGS 1R had been improved over the original FGS design by the insertion of the articulating mirror assembly (AMA) designed and built by Raytheon (currently Goodrich Corporation's Optical and Space Systems). A static fold flat mirror in FGS1r was mounted on a mechanism capable of tip/tilt articulation. This Articulating Mirror Assembly (AMA) allowed for in-orbit re-alignment of the wavefront at the face of the Koesters prism. An adjustable AMA has proven to be an important capability since, given HST's spherical aberration, even a small misalignment degrades the interferometric performance of the FGS. On orbit testing and adjustment of the AMA were completed during FGS 1R's first year in orbit. A high angular resolution performance test executed in May 1998 demonstrated the superiority of FGS 1R over FGS3 as a science instrument, and FGS 1R was designated the FGS Science Instrument for Cycle 8 and beyond.
FGS2 has been replaced twice. The first time was during Servicing Mission 3A (SM3A) when the original FGS2 was replaced with a refurbished FGS1 and was designated FGS2R. Then, starting in mid 2006, FGS2R started to experience an increased guide star acquisition failure rate because of a failing LED on one of its selector servos. Consequently, in 2009 FGS2R was replaced during Servicing Mission 4 (SM4) with the refurbished FGS2 and is designated FGS2R2.