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. [2] To enable aiming the telescope at a specific spot in the sky, each FGS combines optics and electronics. [2] There are three Hubble FGS, and they have been upgraded over the lifetime of the telescope by crewed Space Shuttle missions. [3] The instruments can support pointing of 2 milli-arc seconds (units of degree). [4] The three FGS are part of the Hubble Space Telescope's Pointing Control System, aka PCS. [5] 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. [6]
The FGS can be used to locate something in space, and then lock-on to it. [7] It can provide the movements the telescope must make to keep the object in view, for the main instruments to record data on. [8]
The FGS were originally made by the optics company Perkin-Elmer, and as removable and repairable instruments it has been possible to refurbish them over the lifetime of the telescope. [8] The first replacement FGS was installed in 1997, swapping out FGS1. [5]
In May 2009, on STS-125 a FGS was replaced during the mission to the Hubble telescope by the Space Shuttle. [9] The astronaut crew performed an EVA (spacewalk) to service the FGS and other components on the telescope in Earth orbit. [9] This was the SM4 mission. [6]
An example of astrometry science with the Hubble FGS system is observations of the Low-Mass Binary star system L722-22. [10] 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. [10]
The FGS are white-light shearing interferometers. [5] The FGS weigh 220 kg (485 lb) and have dimensions of roughly 0.5 m × 1.0 m × 1.6 meters. [11]
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. [12] 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 1,000 meters (3,200 ft) in diameter. [12] It has been suggested that the Kepler observatory may be able to detect objects in the Oort cloud by their occultation of background stars, [13] 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. [14] This ability was used for exoplanet research, where the motion of the star caused by the movement of planets around it was detected. [14] Hubble was used via the FGS sensors to detect the motion of star caused by an exoplanet orbiting it. [15] The effect on the red dwarf Gliese 876's by companion Gliese 876b was measured. [15]
FGS was used to study Double-star systems (aka Binary star systems) and to measure distances to astronomical bodies. [15]
FGS has also been used to observe asteroids and calculate their size. [16] Asteroids studied include (63) Ausonia, (15) Eunomia, (43) Ariadne, (44) Nysa, and (624) Hektor. [16]