FGS Optical Assembly
- FGS provides the data for fine pointing and attitude stabilization
- Two channels imaging separate regions of sky onto independent 2048 × 2048 HgCdTe sensor chip arrays (SCAs). The central 2040 × 2040 pixels are sensitive to light while the outer four rows and columns serve as reference pixels for bias measurements.The field of view is 2.4' × 2.4' with a plate scale of 69 milliarcsec⁄pixel.
- SCAs are responsive from 0.6 - 5 µm.
- Sensitive to reach 58 µJy at 1.25 µm (j=18.6).
- 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.
- FGS will not have a shutter or a filter wheel, and the detector will always observe the sky.
The FGS has three main functions:
- 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.
- 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.
- 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.