Fine Guidance Sensor
Encyclopedia
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
(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
James Webb Space Telescope
The James Webb Space Telescope , previously known as Next Generation Space Telescope , is a planned next-generation space telescope, optimized for observations in the infrared. The main technical features are a large and very cold 6.5 meter diameter mirror, an observing position far from Earth,...
, but it takes a different technical approach.
In some specialized cases, such as astrometry
Astrometry
Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of our Solar System and our Galaxy, the Milky...
, the FGS's can also be used as scientific instruments.
The FGS on the Hubble Space Telescope (HST)
The Hubble Space TelescopeHubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
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
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
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.
FGS on the James Webb Space Telescope
A guiding system, also called FGS, but using different technology, is also plannedfor the James Webb Space Telescope
James Webb Space Telescope
The James Webb Space Telescope , previously known as Next Generation Space Telescope , is a planned next-generation space telescope, optimized for observations in the infrared. The main technical features are a large and very cold 6.5 meter diameter mirror, an observing position far from Earth,...
(JWST). It provides input for the planned
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 will be supplied by the Canadian Space Agency. To save on mass and volume it will be assembled in a single unit also containing the Tunable Filter Imager, but they are separate optical instruments.
JWST FGS Functions
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. Finally, 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.The FGS will be sensitive enough to reach 58 µJy at 1.25 µm (~Jab = 19.5), and has a 2.4x2.4 arcminute square field of view. 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.