For a given visit, JWST maintains fine guiding using a guide star in one of the FGS fields. Guide stars are selected from the GSC2.4 catalog by the Guide Star Selection System (GSSS), but users may assess guide star availability for a target using tools in APT.
JWST uses a single guide star in one of the FGS fields for fine guiding during a given visit. Roll control is provided separately by the spacecraft star trackers. It is not the user's responsibility to pick specific guide stars to be used for their observations. JWST guide stars are selected from the GSC2.4 catalog by the Guide Star Selection System (GSSS) based on several factors related to telescope pointing and suitability of the star. Tools and reports are available to visualize the availability of guide stars for a target.
Guide Star Catalog
See also: JWST Pointing Performance
The JWST proposal planning system currently uses Guide Star Catalog (GSC) version 2.4 for the selection of guide stars and reference stars. GSC 2.4, released for JWST use in November 2017, is a major update to the GSC used for many years for HST operations. GSC 2.4 is a merger of GSC 2.3 with data from 2MASS, Sloan Digital Sky Survey (SDSS) DR13, Gaia DR1, and VISTA1 Hemisphere Survey (VHS) DR4. SDSS and VHS stellar data improve the quality of the GSC, particularly at the fainter magnitudes (J > 17), which significantly improves the JWST guide star availability at higher Galactic latitudes.
Improvements in the catalog that have resulted in increased guide star availability include:
- Guide star catalog object classifications as stellar or non-stellar have been improved. Analysis has shown that SDSS and VHS are particularly helpful in this regard, especially for J > 17. Guiding on slightly extended sources can reduce guiding accuracy.
- Additional photometric information, especially in the near-infrared, has been made available. In order to be considered a valid JWST guide star candidate, a GSC object must have photometry available in two or more bandpasses so interpolations or extrapolations can be made as needed. If a given star has more photometry bands, its total observed FGS count rate can be more accurately predicted, making it possible to determine if it’s a guide star that will provide successful acquisition, tracking, and fine guidance functions.
- The Gaia DR1 catalog provides significant astrometric improvement (0.001” vs >0.1”) for essentially all stars near the galactic plane, and for about half the stars at higher galactic latitudes.
- Coverage gaps in GSC 2.3 have been filled, for example, in areas around very bright stars and in regions of very high extinction.
Guide star availability
Guide Star Catalog objects need to meet a number of requirements, discussed below, in order to be considered a JWST guide star candidate for a given observation. The area density of guide star candidates is strongly correlated with Galactic latitude, with the density falling sharply for regions about 35° above or below the plane of the galaxy. The FGS field of view and sensitivity, along with the depth of known stars in the Guide Star Catalog, determine the availability of guide stars for any particular pointing and orientation of the telescope. Mission requirements call for a 95% probability of acquiring a guide star and maintaining pointing stability for any permitted pointing of the telescope. The statistical availability of guide stars as a function of galactic latitude is used by APT to determine the visit splitting distance it assumes for each target/observation.
Given the GSC 2.4 content, FGS sensitivity, and operational limitations, the probability of finding a guide star is 98% or higher at all galactic latitudes. VHS southern sky coverage is nearly complete but there are small areas of the southern sky that are not yet in the catalog (see www.vista-vhs.org).
Photometric measurements of the guide star candidates contained in the GSC are used to predict the count rate of the star at the FGS detector (which is needed by the FGS to successfully acquire the guide star). This involves transforming the catalog’s optical photometric measurements into the near-infrared (if 2MASS data are unavailable), and then applying wavelength dependent telescope and FGS throughput factors over the 0.6–5.0 μm passband of the FGS.
Guide star selection criteria
JWST uses a single guide star in one of the FGS fields for fine guiding during a given visit. Roll control is provided separately by the spacecraft star trackers (see JWST Attitude Control Subsystem.) The following criteria are used to select up to 3 guide star candidates for each visit:
Guide star candidates must be classified as point sources in the GSC; extended objects ("non-stars") are excluded.
Guide star candidates must be in the magnitude range of 12.5 ≤ J ≤ 18.3 (the limits vary slightly with spectral type of the star).
No bright spoiler stars exist within 6″ of a guide star candidate. A spoiler star in this context is defined to be a star that is less than two magnitudes fainter than the guide star candidate.
A guide star candidate must be detected in two or more of the catalog’s photometric passbands so its brightness in FGS count rate can be derived.
Each guide star candidate may be (but is not required to be) augmented by up to 10 “reference stars,” which will be used in the guide star identification pattern matching algorithm to identify the correct guide star.
Compact objects classified as non-stars in the GSC can be used as “reference stars” for the guide star identification algorithm.
Retrieving and visualizing JWST guide stars
See also: JWST APT Aladin Viewer
It is not the proposer's responsibility to pick specific guide stars to be used for observations. However, there are cases where you may want to understand the availability of potential guide stars for a particular target, and the Astronomer Proposal Tool (APT) provides a way to help you evaluate this.
At the visit level in APT, select View in Aladin in the main menu. Then, in the same window, click on the FoV and JWST GS buttons in the APT Aladin Controls.
In the separate Aladin pop-up window, the JWST focal plane and guide star candidates are displayed. These candidates are stars that have met the constraints applied by the guide star selection system (basically the guide star selection criteria described above). An overlay of the DSS can be included by either clicking the DSS icon in the Aladin display or opening the folder icon and choosing the DSS option. (Note: this displays the DSS view with a dark background.) For planning observations, the former method is recommended and is shown in Figure 1, which illustrates this for a NIRCam observation. Green squares denote the guide star candidates that are available for the visit, with only those within the FGS apertures being applicable for that particular roll angle of the observatory.
One potential use case for this functionality would be a situation where the user has fixed the desired position angle to some value (or a range), and run the Visit Planner only to find the observation is declared unschedulable due to lack of available guide stars. The user could view the observation in Aladin and assess the guide star situation, and determine an alternate position angle or range where a guide star would be available.
Fine Guidance Sensor, FGS
Astronomer's Proposal Tool