Fine guidance sensor

To achieve the very high pointing accuracy Hubble needs, the FGSs have been constructed as interferometers to exploit the wavelike features of the in-coming starlight.

During on-orbit commissioning of the JWST, the FGS also provided pointing error signals during activities to achieve alignment and phasing of the segments of the deployable primary mirror.

[3] The JWST FGS, designed and built by COM DEV International, was supplied by the Canadian Space Agency.

To save on mass and volume it was assembled into a single unit together with the Near Infrared Imager and Slitless Spectrograph, but they are separate optical instruments.

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.

One of the three fine guidance sensors photographed during Servicing Mission 2 in 1997
After locking on to a bad guide star , Hubble's tracking system captured this image: the prominent red streaks are actually stars in globular cluster NGC 288 . [ 2 ]
Image produced by the JWST FGS.