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. Performance for absolute pointing and image motion is predicted on the JWST Pointing Performance page.
JWST's Fine Guidance Sensor (FGS) is a near-infrared (NIR) camera residing in the Integrated Science Instrument Module (ISIM). It has a passband from ~0.6 to 5.0 μm and operates at a temperature of ~37 K, similar to near-infrared science instruments. The FGS has two channels, each with 2.3′ × 2.3′ field of view (FOV).
The FGS functions are:
- to identify and acquire a guide star, measure its position in one of the two guider channels, and provide this data to the JWST attitude control subsystem (ACS) for attitude determination.
to provide fine pointing data to the ACS for attitude stabilization. The FGS can provide this data for both fixed target pointings and for moving target observations.
Guide star position data is used by the ACS for absolute (right ascension and declination) pointing knowledge and pointing control in the plane of the sky (pitch and yaw). ACS uses the data from off-axis star trackers to control the spacecraft’s roll orientation.
In addition to its critical role in executing observations, the FGS also serves as an integral part in the commissioning of the JWST Observatory, and in observation planning. FGS pointing data are archived for every science observation and may be valuable for post-observation data analysis.
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 4 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 2.3 (that's spatially correlated and augmented with the 2MASS Point Source Catalog) for the selection of guide stars and reference stars. This is the same catalog currently in use for HST. The STScI Archive Sciences Branch is in the process of incorporating Gaia astrometric catalog data into the GSC as they become available to improve catalog positional accuracy. Stellar data from the Sloan Digital Sky Survey, and the VISTA Hemisphere Survey are also being incorporated. This updated catalog will be referred to as GSC 2.4, and will be ready for JWST proposers no later than November 2017.
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.
The flight software functions and corresponding operational modes of the FGS associated with the identification, acquisition, and tracking of a guide star are briefly described below.
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 36 subarrays of 2048 × 64 pixels with an effective integration time of 0.320 s each.
With the approximate location of the guide star on the FGS detector known, either following a measurement from the "Identification" function, or at the end of a small angle maneuver that offsets the guide star from a previously known location in the FGS FOV, the FGS executes 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 0.68 arc seconds (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. The ACS then enters its “Fine Guidance Mode” to control the observatory pointing in a closed loop using the FGS position centroids. 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 x 32 pixel subarray 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. In "Fine Guide" mode, the subarray is fixed and cannot be changed without transitioning through "STANDBY"1, which exits fine guide control, and starting again with "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.
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.