Fine Guidance Sensor

A fine guidance sensor (FGS) is an interferometric instrument on board a space telescope that provides high-precision pointing information as input to the observatory's attitude control systems. FGSs have been deployed on the Hubble Space Telescope, and will be deployed with a different technical approach on the James Webb Space Telescope.

In some specialized cases, such as astrometry, FGSs can also be used as scientific instruments.

The Hubble Space Telescope has three fine guidance sensors (FGSs). Two are used to point and lock the telescope onto the target, and the third can be used for position measurements - also known as astrometry. Because the FGSs are so accurate, they can be used to measure stellar distances and also to investigate binary star systems.

The three FGSs are located at 90-degree intervals around the circumference of the telescope's field of view. To achieve the very high pointing accuracy Hubble needs, the FGSs have been constructed as interferometers to exploit the wavelike features of the in-coming starlight. With this kind of accuracy and precision, the sensors can search for a wobble in the motion of nearby stars that could indicate the presence of a planetary companion, determine if certain stars really are double stars, measure the angular diameter of stars, galaxies, etc.

Due to the sensitivity of the FGS they can not be used whilst the HST is pointed within 50 degrees of the Sun.

A guiding system, also called FGS, but using different technology, is also planned for the James Webb Space Telescope (JWST). It provides input for the observatory's attitude control system (ACS). During on-orbit commissioning of the JWST, the FGS will also provide pointing error signals during activities to achieve alignment and phasing of the segments of the deployable primary mirror.

The JWST FGS, designed and built by COM DEV International, was supplied by the Canadian Space Agency. To save on mass and volume it was assembled in a single unit also containing the Near Infrared Imager and Slitless Spectrograph, but they are separate optical instruments.

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