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One of the three Fine Guidance Sensors photographed during Second Servicing Mission in 1997

The Fine Guidance Sensor (FGS) is an interferometric instrument on board the Hubble Space Telescope (HST) that provides high-precision pointing information as input to the observatory's attitude control systems.

There will also be a fine guidance sensor system for the James Webb Space Telescope, but it takes a different technical approach.

In some specialized cases, such as astrometry, the FGS's can also be used as scientific instruments. [1]

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.

The camera can image two adjacent fields of view, each approximately 2.4 arc-minute × 2.4 arc-minute in size, and can also be configured to read out small 8 × 8 pixel subarrays at a rate of 16 times per second. Even with these short integration times, the FGS is sensitive enough to reach 58 µJy at 1.25 µm (~Jab = 19.5). This combination of sky coverage and sensitivity ensures that an appropriate guide star can be found with 95% probability at any point in the sky, including high galactic latitudes.

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 planned observatory's attitude control system (ACS).

The Canadian Space Agency will provide the Fine Guidance Sensor to the James Webb Space Telescope (JWST) Project. 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-Guider has three main functions: The first is to obtain images for target acquisition. Full-frame images are used to identify star fields by correlating the observed brightness and position of sources with the properties of catalogued objects selected by the observation planning software. The second is to acquire pre-selected guide stars. During acquisition, a guide star is first centred in an 8 × 8 pixel window. Small angle manoeuvres are then executed to translate this window to a pre-specified location within the field of view, so that an observation with one of the science instruments will be oriented correctly. And the third is to provide the ACS with centroid measurements of the guide stars at a rate of 16 times per second. These measurements will be used to enable stable pointing at the milli-arc-second level.

• FGS page at STScI
• HST Fine Guidance Sensor (FGS1R)
• The HST Fine Guidance Sensor
• The HST Fine Guidance Sensor at ESA/Hubble

v • d • e Hubble Space Telescope
Current instruments	Advanced Camera for Surveys  · Corrective Optics Space Telescope Axial Replacement  · Fine Guidance Sensor  · Near Infrared Camera and Multi-Object Spectrometer  · Space Telescope Imaging Spectrograph  · Wide Field and Planetary Camera 2
Future instruments	Cosmic Origins Spectrograph  · Wide Field Camera 3
Previous instruments	Faint Object Camera  · Faint Object Spectrograph  · Goddard High Resolution Spectrograph  · High Speed Photometer  · Wide Field and Planetary Camera
Shuttle Missions	Launched Hubble: STS-31  · Servicing missions: STS-61  · STS-82  · STS-103  · STS-109  · Upcoming: STS-125