This photo shows a team of astronauts removing a Fine Guidance Sensor from its protective enclosure in the shuttle during a servicing mission. The Fine Guidance Sensors were built by Hughes Danbury Optical Systems in Danbury, Conn.

Hubble has three Fine Guidance Sensors. Two are normally used in observations to locate and lock onto a target star while observations are made with a science instrument. One of the three Fine Guidance Sensors is also used to precisely measure the positions and ratios of stars, and detect whether a star is single or has companions. This is the science of astrometry.
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