The Fine Guidance Sensor (FGS) is an optical sensor used on the Hubble Space Telescope to provide pointing information for the spacecraft and also, as a scientific instrument for astrometric science. An FGS contains a collection of mirrors, lenses, servos, prisms, beam-splitters and photomultiplier tubes.

There are three fine guidance sensors on Hubble located at 90-degree intervals around the circumference of the telescope. Two FGSs are used to point the telescope at an astronomical target and hold that target in a scientific instrument's field of view (FOV). The third FGS can then be used as a scientific instrument for astrometry.

Along with the gyroscopes, the FGSs are a key component of Hubble's highly complex but extraordinarily effective "pointing control system." Their level of stability and precision is comparable to being able to hold a laser beam focused on a dime that is 200 miles away (the distance from Washington D.C. to New York City).

The FGS that was returned on SM3A has been refurbished and upgraded for re-use on Hubble's Servicing Mission 4. This refurbished unit has an enhanced on-orbit alignment capability over the original FGS design.

SM4 Media Guide (PDF)
SM4 Images and Videos