Hubble’s Fine Guidance Sensors are abbreviated FGS. These optical sensors measure the positions of guide stars and provide the pointing information needed to keep the telescope accurately aimed.
Hubble has three Fine Guidance Sensors arranged around the telescope. During an observation, two sensors can lock onto guide stars while the telescope’s control system uses their measurements, together with gyroscopes, to detect and correct tiny drifts. That stability lets Hubble collect sharp images and precise spectra.
The FGS units also serve as scientific instruments. Their exceptionally precise measurements have supported astrometric work, including studies of stellar positions, proper motions, parallaxes, binary stars, and angular diameters. Two sensors were replaced with refurbished units during servicing missions; the third remains an original sensor.
The distractors are other Hubble instruments: STIS is the Space Telescope Imaging Spectrograph, COS is the Cosmic Origins Spectrograph, and ACS is the Advanced Camera for Surveys. They do not expand the abbreviation FGS.