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James Webb Space Telescope
FGS Instrument Design

FGS Optical Assembly

FGS optical assembly FGS and OTE

The main components of the FGS assembly consist of the pick-off mirror (POM), three-mirror assembly (TMA), fine focus mechanism with fold mirror, and the detector assembly with two focal point arrays (FPA). All these components are built on a stiff bench that is mounted on the ISIM by kinematic mounts. Light coming from the optical telescope element (OTE) is focused onto the Guider POM, which feeds it to the TMA. The light is collimated by the TMA and then is focused by a fold mirror onto two focal point arrays that are part of the detector assembly. The NIRISS is mounted on the opposite side of the FGS.


The focal plane arrays (FPAs) used by the FGS will be a single 2048 x 2048 HgCdTe sensor chip arrays (SCAs), with the central 2040 x 2040 pixels being light sensitive, while the pixels on the outer four rows and columns serve as reference pixels for bias measurements.

Operating Modes

The FGS will have three operating modes, Off, Standby, and Operate. There will be five functions performed under Operate namely Identification, Acquisition, Track, Fine Guiding, and Calibration. The FGS enters these modes and executes these functions upon receipt of commands from the ICDH. In Off, the FGS (hardware) is powered down and does not send telemetry. In Standby, flight application software is running and the FGS is ready to transition to Operate to execute any commandable function. The FGS flight software (FSW) actively controls all physical, electrical, and detector conditions to which the FGS and NIRISS performance is sensitive. The FGS FSW shall be capable of sending telemetry and receiving commands, calibration data, reference data, and software updates. Standby can be entered from only from Operate.