Fine Guidance Sensor (FGS) for the Hubble Space Telescope is a system of three instruments used for pointing the telescope in space, and also for astrometry and its related sciences.[1] To enable aiming the telescope at a specific spot in the sky, each FGS combines optics and electronics.[1] There are three Hubble FGS, and they have been upgraded over the lifetime of the telescope by crewed Space Shuttle missions.[2] The instruments can support pointing of 2 milli-arc seconds (units of degree).[3] The three FGS are part of the Hubble Space Telescope's Pointing Control System, aka PCS.[4] The FGS function in combination with the Hubble main computer and gyroscopes, with the FGS providing data to the computer as sensors which enables the HST to track astronomical targets.[5]
The FGS can be used to locate something in space, and then lock-on to it.[6] It can provide the movements the telescope must make to keep the object in view, for the main instruments to record data on.[7]
The FGS were originally made by the optics company Perkin-Elmer, and as removable and repairable instruments it has been possible to refurbish them over the lifetime of the telescope.[7] The first replacement FGS was installed in 1997, swapping out FGS1.[4]
In May 2009, on STS-125 a FGS was replaced during the mission to the Hubble telescope by the Space Shuttle.[8] The astronaut crew performed an EVA (spacewalk) to service the FGS and other components on the telescope in Earth orbit.[8] This was the SM4 mission.[5]
An example of astrometry science with the Hubble FGS system is observations of the Low-Mass Binary star system L722-22.[9] Observations were taken of the system in 1990s, and the data helped determine the mass of each of the components of L722-22, which is also known as LHS 1047 and GJ 1005.[9]
The FGS are white-light shearing interferometers.[4] The FGS weigh 220 kg (485 lb) and have dimensions of roughly 0.5 m × 1.0 m × 1.6 meters.[10]
The smallest Kuiper Belt Object (KBO) yet detected at that time was discovered in 2009 by poring over data from the Hubble Space Telescope's fine guidance sensors.[11] They detected a transit of an object against a distant star, which, based on the duration and amount of dimming, was calculated to be a KBO about 3200feet in diameter.[11] It has been suggested that the Kepler observatory may be able to detect objects in the Oort cloud by their occultation of background stars,[12] and the Whipple proposal would also try to use this concept.
A Hubble FGS has also been used for astrometry, tracking the movement of different stars.[13] This ability was used for exoplanet research, where the motion of the star caused by the movement of planets around it was detected.[13] Hubble was used via the FGS sensors to detect the motion of star caused by an exoplanet orbiting it.[14] The effect on the red dwarf Gliese 876's by companion Gliese 876b was measured.[14]
FGS was used to study Double-star systems (aka Binary star systems) and to measure distances to astronomical bodies.[14]
FGS has also been used to observe asteroids and calculate their size.[15] Asteroids studied include (63) Ausonia, (15) Eunomia, (43) Ariadne, (44) Nysa, and (624) Hektor.[15]