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
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.