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Fine Guidance Sensors (FGS)

The Fine Guidance Sensors are three instruments that help keep the Hubble Space Telescope pointed precisely in the right direction. The fine guidance sensors work with the gyroscopes to point Hubble at a target. These sensors are more sensitive to Hubble’s motion than the gyroscopes and provide the “fine tuning” for pointing the telescope. Astronauts replaced one of Hubble’s fine guidance sensors during SM3A in December 1999 and returned it to Earth for refurbishment Astronauts will install the refurbished sensor during SM4.


A gyroscope is a spinning wheel mounted on a movable frame that assists in stabilizing and pointing a space-based observatory. Gyroscopes are important because they measure the rate of motion as the observatory moves and help ensure the telescope retains correct pointing during observations. The gyroscopes provide the general pointing of the telescope while the fine guidance sensors provide the “fine tuning.” Gyroscopes are used in navigational instruments for aircraft, satellites, and ships. The Hubble Space Telescope has six gyroscopes for navigation and sighting purposes.

Hubble Space Telescope (HST)

An automated telescope that orbits above Earth’s atmosphere. Launched in April 1990, the telescope is operated jointly by the National Aeronautics and Space Administration and the European Space Agency. Hubble contains an array of instruments capable of carrying out a variety of astronomical observations in ultraviolet, optical, and infrared wavelengths.