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.

A FGS consists of a large structure housing 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 the scientific instrument's field of view. The third FGS can then be used as a scientific instrument for astrometry.