The Fine Guidance Sensors (FGS), originally designed and built by the Perkin-Elmer Corporation in Danbury, CT (now Goodrich Corporation 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 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.
The FGSs are dual-axis white light shearing interferometers, each with a ~69 square arcminute FOV. Under nominal operating conditions, the FGS are routinely able point the spacecraft with a precision of ~2 mas or less. Unfortunately, the original design of the FGS did not compensate for the spherical aberration of the mis-formed HST primary mirror, and as a result the original FGSs suffered from degraded performance. In response to this, Goodrich re-engineered the spare FGS to include a commandable mechanism to mitigate the deleterious effects of the spherical aberration. This revised instrument replaced FGS1 during the second Hubble Servicing Mission in 1997. This device, now designated FGS1r, was joined by FGS2r during the HST Servicing Mission 3a. FGS2r was replaced in SM4 with another refurbished unit, designated FGS2r2.
The high precision pointing capabilities of the FGS coupled with a fourteen magnitude dynamic range enable the FGS to perform as a high-precision astrometer and a high angular resolution science instrument. The 40 Hz readout time and detector stability allow for milli-magnitude relative photometry over orbital timescales and 1-2% relative photometry over long baselines (i.e., months).
For further detailed information about the FGS, please see the FGS Instrument Handbook.