JWST Fine Guidance Sensor Commissioning and Performance During Operations

The Fine Guidance Sensor (FGS) Instrument on the James Webb Space Telescope (JWST) provides accurate Guide Star information every 64 ms to the Attitude Control System (ACS) to maintain the pointing stability needed for science operations. This involves identifying and tracking the correct Guide Star in different types of astrometrical fields and it is run as an event driven operation working through the different guider modes (Identification, Acquisition, Track and Fine Guide) with interleaved slews and operations with the Attitude Control System. This paper summarizes activities undertaken during the Commissioning period, including guiding with different Point Spread Functions (PSFs) as the JWST mirrors were being aligned and focused, and instances in which the Guide Stars selected by the Guide Star Catalogue had to be updated to match what was expected to be imaged on the FGS detector based on the mirror’s state. A significant amount of ground testing with simulated images was required in order to prepare for on orbit operations. This paper also covers several calibrations and optimizations completed during this period.

The current normal operations performance is also presented. For normal operations there are sparse fields, bright crowded fields and Guide Stars of different brightnesses which require different accommodations. JWST also observes nearby objects in the solar system that require a different ‘moving target’ mode for guiding. Bad pixel maps need to be updated at a regular cadence. Performance to date exceeds requirements with typical pointing stability of ~ 1 mas. The information from FGS is also used to identify updates needed in the Guide Star Catalogue by identifying photometric inaccuracies, duplicate Guide Star entries, misclassified objects (e.g. classified as stars but are actually galaxies), etc.

A continuous improvement operational concept has evolved which identifies unsuccessful Visits to understand the reason behind any performance issues (whether they are attributable to FGS parameters/algorithms, ACS pointing errors, Guide Star Catalogue errors, or something else). Visits which needed retries and Visits with high Noise Equivalent Angle are also studied and mitigation approaches have been implemented. The current success rate for finding and guiding on the correct commanded Guide Star is ~ 96%.