FGS - Fine Guidance Sensor

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 arcmin × 2.4 armin 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.

The FGS-Guider has three main functions:

  1. 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 cataloged objects selected by the observation planning software.
  2. To acquire pre-selected guide stars. During acquisition, a guide star is first centered in an 8 × 8 pixel window. Small angle maneuvers 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.
  3. 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-arcsecond level.
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.

Tunable Filter Imager (TFI)

A science instrument known as the FGS Tunable Filter Imager (FGS-TFI) is packaged with the guide camera, but is functionally independent. The FGS-TFI will be used solely for science observations and will have a selectable band of resolution R~100 across its 2.2 x 2.2 arcmin field between 1.5-5 microns.

The Canadian Space Agency will provide the Fine Guidance Sensor/TFI to the the JWST Project. The prime contractor is ComDev. The Principal Investigator for the FGS is John Hutchings of the Herzberg Institute of Astrophysics, National Research Council of Canada. Rene Doyon (Univ. Montreal) is the PI of the TFI.

FGS Optical Assembly.
Figure 1: Optical-mechanical packaging of the FGS. (Click on the figure to enlarge.)