The FGS is a sensitive camera that provides dedicated, mission-critical support for the observatory's Attitude Control System (ACS). The camera can image two adjacent fields of view, each approximately 2.4' X 2.4' in size, and can also be configured to read out small 8 X 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.
The main functions of FGS include obtaining images for target acquisition, acquiring pre-selected guide stars and to provide the ACS with centroid measurements of the guide stars at a rate of 16 times per second. 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.
A science instrument known as the Near-InfraRed Imager and Slitless Spectrograph (NIRISS) is packaged with the guide camera, but is functionally independent.
The Canadian Space Agency will provide the Fine Guidance Sensor/NIRISS to the the JWST Project. The prime contractor is Com Dev. The Principal Investigator for the FGS is John Hutchings of the Herzberg Institute of Astrophysics, National Research Council of Canada.