JWST's Fine Guidance Sensor (FGS) provides data for science attitude determination, fine pointing, and attitude stabilization using guide stars in the JWST focal plane. Absolute pointing and image motion performance is predicted on the JWST Pointing Performance page.
Parent page: Observatory Hardware
The FGS has an unfiltered passband from ~0.6 to 5.0 μm. Each focal plane array is a 2048 × 2048 HgCdTe sensor chip assembly that has a 2.3’ × 2.3’ FOV after correcting for internal field distortions. The central 2040 × 2040 pixels are light sensitive; the four outermost rows and columns are reference pixels for bias measurements. However, the usable FOV for guide star identification and guiding is 2.15' × 2.15' in order to provide sufficient light-sensitive pixels for flat field corrections for potential guide stars near the edge of the FOV.
The FGS has neither a shutter nor a filter wheel; therefore, its detectors are always exposed to the sky.
The JWST proposal planning system currently uses the Guide Star Catalog (GSC) version 2.4.1, which was updated in the fall of 2017. The Guide Star Selection System has been updated to use this new catalog, with improvements to astrometry, photometry, and number and distribution of stars that are available—for additional information, please refer to the JWST Guide Stars article.
FGS optical design
The optical assembly of the FGS is shown in Figure 2. Light from the telescope is focused onto the pick-off mirror (POM), collimated by the three-mirror assembly (TMA), and focused by an adjustable fold mirror (fine focus mechanism) onto the two focal plane arrays. The fine focus mechanism allows tuning of FGS focus.
FGS has three operating modes: "OFF", "STANDBY",1 and "OPERATE". In operational mode, it has five software functions: calibration, identification, acquisition, track, and fine guide. The calibration function allows the FGS to obtain necessary data for calibration by the ground system, while the remaining functions enable the identification, acquisition, and tracking of a guide star. These flight software functions are briefly described below.
In order to be able to calibrate the FGS, the ground system requires data collected with the "calibration" function. In this mode, the FGS acts like a camera, obtaining full-frame or subarray images with one guider while the other tracks a guide star. These data are then used to measure and correct for geometric distortion, intra-pixel non-uniformity, flat field response, bias, bad pixels, and other performance characteristics. The "calibration" function is only available for commissioning and calibration.
At the conclusion of a spacecraft slew, the telescope is pointing at the sky such that the selected guide star is near the center of one of the FGS detectors and the science target is in the desired science instrument, though not yet at the precise attitude for the scientific observation. To assure that the correct guide star is acquired, the FGS obtains an image of the sky and compares the observed positions of stars (and any other luminous objects) to a catalog of objects using a pattern-matching algorithm. To minimize smearing, the "identification" images are obtained in a sequence of "strips": 36 subarrays of 2048 × 64 pixels with an effective integration time of 0.3367 s each.
The approximate location of a guide star on the FGS detector is measured using the flight software "identification" function, or is determined at the end of a small angle maneuver that offsets the guide star from a previously known location in the FGS FOV. This is followed by executing the "acquisition" function. A 128 × 128 pixel (8.6” × 8.6”) subarray is centered at the expected position of the guide star. Images of the guide star within this subarray are obtained and autonomously analyzed by the FGS to locate the star. A second set of measurements using a 32 × 32 pixel (2.2" × 2.2") subarray, centered on the guide star position, is obtained. The FGS reports the position and intensity of the guide star to the ACS; this information is used by the ACS to update its knowledge of the spacecraft’s current attitude, and to bring the pointing of the telescope to within 0.45" (1-σ radial) of its commanded position.
Following the successful completion of the "acquisition" function, and ACS’s corrective maneuver of the observatory pointing, the FGS executes the "track" function. The FGS places a 32 × 32 pixel (2.2" × 2.2") subarray on the expected location of the guide star. High cadence subarray images are obtained from which the guide star’s position centroid is determined and reported to ACS every 64 ms. Once the guide star is within ~0.06" of its desired location, the FGS can transition to "fine guide" mode.
In "track" mode the FGS will adjust the position of the 32 × 32 pixel subarray on the detector to remain centered on the guide star if the guide star moves. Thus, "track" mode is used for moving target observations.
When the FGS transitions from "track" to "fine guide," a fixed 8 × 8 pixel (0.5" × 0.5") subarray is centered on the guide star position. The guide star centroid is computed from each subarray image and sent to the ACS every 64 ms, controlling the observatory pointing in a closed loop. In "fine guide" mode, the subarray location is fixed and cannot be changed without transitioning through the operating mode "STANDBY"1, which requires exiting fine guidance control and starting over in "track" mode.
Once in fine guide control, the absolute pointing accuracy of JWST with respect to the celestial coordinate system will be determined by the astrometric accuracy of the Guide Star Catalog and the calibration of the JWST focal plane model.
Each of the operational modes uses a different sized subarray and readout pattern. The frame readout time for each subarray can be calculated using the following equation:
where is the number of amplifiers used in the subarray, equal to 4 for CAL and ID or 1 for other functions; where is a constant that accounts for electronic overhead, equal to 12 for ACQ1 or 6 for all other functions; and where the number of rows and columns and are as specified in the table below.
FGS data utilizes correlated double sampling (CDS) to correct for detector effects within integrations, a method in which the 0th read is subtracted from the 1st read. The time between reads, or CDS time, is a function of the readout pattern and the frame readout time:
where is the number of dropped frames between reads and is the frame readout time.
Table 1. Subarray and readout definitions for each function
The Canadian Space Agency (CSA) has contributed the FGS to the JWST Observatory. Honeywell (formerly COM DEV Space Systems) of Ottawa, Canada, is CSA’s prime contractor for the FGS.