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James Webb Space Telescope
FGS - Fine Guidance Sensor

Find Guidance System

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 Chris Willott of the Herzberg Institute of Astrophysics, National Research Council of Canada.

Latest News

2015 August 27
Preparations for the third cryo-vacuum test (CV3) of the Integrated Science Instrument Module (ISIM) at NASA’s Goddard Space Flight Center continued throughout the summer. For the first time, the flight configuration of the ISIM was vigorously shaken – not stirred! – and bombarded by intense acoustical waves to simulate the harsh conditions of launch. Both NIRISS and FGS sailed through their “system functional tests” before and after these perturbations with no issues. Additional tests to confirm the electromagnetic compatibility of the subsystems of ISIM under conditions that simulate normal operations were also completed successfully. Now that the robustness of the ISIM has been demonstrated, it’s “full speed ahead” for the beginning of CV3 in late October!

2015 February 12
FGS/NIRISS became the first instrument to be reinstalled in the Integrated Science Instrument Module (ISIM) following the "Half-Time Show." All the planned hardware changes were successfully completed and both instruments passed their electronic check-outs at room temperature with flying colors. FGS/NIRISS is ready for the final series of tests at NASA's Goddard Space Flight Center!

NIRISS at GSFC cleanroom

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