Fine guidance sensor

A fine guidance sensor (FGS) is an instrument on board a space telescope that provides high-precision pointing information as input to the telescope's attitude control systems. Interferometric FGSs have been deployed on the Hubble Space Telescope; a different technical approach is used for the James Webb Space Telescope's FGSs. In some specialized cases, such as astrometry, FGSs can also be used as scientific instruments. Fine Guidance Sensor (HST) 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. Fine Guidance Sensor and Near Infrared Imager and Slitless Spectrograph A guiding system, also called FGS, but using different technology is used 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 also provided 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.