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.) 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 availability of guide stars for a target.
Guide Star Catalog
The JWST proposal planning system is currently using the 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.
The areas where the most gains are expected in guide star availability are, in order of decreasing importance:
- Improved the classification of objects in the guide star catalog as stellar or non-stellar. 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. 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 used as needed. The more bands in which a given star has photometry available, the more accurately its total observed FGS count rate can be predicted, and the more likely guide star identification, acquisition, track, and fine guide functions will succeed.
Improved astrometry. 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.
Filled in coverage gaps that existed in GSC 2.3, for example, around very bright stars or 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 areal 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 FOV 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 three 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
It is not the user's responsibility to pick specific guide stars to be used for their observations. However, there are cases where a user may want to understand what the availability of potential guide stars is for a particular target, and the Astronomer Proposal Tool (APT) provides a way for an observer to evaluate this.
See also: JWST APT Aladin Viewer
At the visit level in APT, select View in Aladin in the main menu, then click on the FoV and JWST GS buttons under 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 opening the folder icon and choosing DSS, or by clicking the DSS icon in the Aladin display. For planning observations, the former method is recommended, and is shown in the Figure 1 below, 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 (the actual permitted roll angles depend upon the date of the observation as well as the ecliptic latitude of the target field).