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Fine Guidance Sensors The Vault

This photo shows a team of astronauts replacing one of three Fine Guidance Sensors on Hubble during a servicing mission.

These sensors are located at 90-degree intervals around the circumference of the focal plane structure. Each Fine Guidance Sensor is just over five feet wide and three feet long and weighs 485 pounds. Each Fine Guidance Sensor enclosure houses a very precise optical system called an interferometer. The telescope's high pointing accuracy and stability is due largely to the Fine Guidance Sensors. Two Fine Guidance Sensors are normally used in observations to locate and lock onto a target star while observations are made with a science instrument. From time to time one of the Fine Guidance Sensors is also used to perform scientific measurements, determining highly precise positions and motions of stars, and detecting the positions of companion stars. The process of determining the positions of stars is called astrometry.

One re-certified Fine Guidance Sensor was installed as a replacement during the Second Servicing Mission and another re-certified Fine Guidance Sensor was installed as a replacement during Servicing Mission 3A. The Fine Guidance Sensors were built by Hughes Danbury Optical Systems in Danbury, Conn.

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