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Fine Guidance Sensor (HST)
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<p>Fine Guidance Sensor (FGS) for the <a href="/facts/Hubble_Space_Telescope/NQeIf0vw">Hubble Space Telescope</a> is a system of three instruments used for pointing the telescope in space, and also for <a href="/facts/Astrometry/9ycEEAvD">astrometry</a> and its related sciences. To enable aiming the telescope at a specific spot in the sky, each FGS combines optics and electronics. There are three Hubble FGS, and they have been upgraded over the lifetime of the telescope by crewed <a href="/facts/Space_Shuttle/7BoyetEv">Space Shuttle</a> missions. The instruments can support pointing of 2 milli-arc seconds (units of <a href="/facts/Degree_(angle)/f48Ns8SX">degree</a>). The three FGS are part of the Hubble Space Telescope's <i>Pointing Control System</i>, aka PCS. The FGS function in combination with the Hubble main computer and <a href="/facts/Gyroscope/UjXGsfMq">gyroscopes</a>, with the FGS providing data to the computer as sensors which enables the HST to track astronomical targets. </p><p>The FGS can be used to locate something in space, and then lock-on to it. It can provide the movements the telescope must make to keep the object in view, for the main instruments to record data on. </p><p>The FGS were originally made by the optics company <a href="/facts/Perkin-Elmer/ghJaeI3Q">Perkin-Elmer</a>, and as removable and repairable instruments it has been possible to refurbish them over the lifetime of the telescope. The first replacement FGS was installed in 1997, swapping out FGS1. </p><p>In May 2009, on <a href="/facts/STS-125/epWDLLKH">STS-125</a> a FGS was replaced during the mission to the Hubble telescope by the Space Shuttle. The astronaut crew performed an EVA (spacewalk) to service the FGS and other components on the telescope in Earth orbit. This was the SM4 mission. </p><p>An example of astrometry science with the Hubble FGS system is observations of the Low-Mass Binary star system <a href="/facts/L722-22/vkyI37LY">L722-22</a>. 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. </p><p>The FGS are white-light shearing <a href="/facts/Interferometer/X3E1SsQT">interferometers</a>. The FGS weigh 220 kg (485 lb) and have dimensions of roughly 0.5 m × 1.0 m × 1.6 meters. </p>