JWST guide stars are selected for the Fine Guidance Sensor (FGS) 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.
The JWST proposal planning system is currently using the Guide Star Catalog (GSC) version 2.3 (that is 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 (SDSS), and the VISTA1 Hemisphere Survey (VHS) are also being incorporated. SDSS and VHS stellar data will improve the quality of the GSC data, particularly at the fainter magnitudes (J > 17), which is expected to significantly improve the JWST guide star availability at higher Galactic latitudes. This updated catalog will be referred to as GSC2.4, and will be ready for JWST observation planning no later than Nov. 2017.
The areas where the most gains are expected in guide star availability are, in order of decreasing importance:
- Improving the classification of objects in the guide star catalog as stellar or non-stellar. Analysis has shown that SDSS and VHS should be particularly helpful in this regard, especially for J > 17. Guiding on slightly extended sources can affect guiding accuracy.
- Adding 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 more than one bandpass 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.
- Filling in coverage gaps that exist in the GSC, 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 current Guide Star Catalog (GSC2.3), FGS sensitivity, and operational limitations, the probability of finding a guide star at low (0° < b < 30°), medium (30° ≤ b < 60°), and high (60° ≤ b ≤ 90°) Galactic latitude is: 99.9%, 98.9%, and 93.8%, respectively. The inclusion of SDSS and VHS catalog data into the GSC should improve this even further. SDSS provides complete coverage of northern high 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 requires only one guide star in one of the FGS fields for fine guiding during a given visit. (Roll control is provided separately by the star trackers.) The following criteria are used to select up to three guide candidates for each visit:
- Guide star candidates must be classified as point sources in the GSC; extended objects ("non-stars") are excluded.
- Compact objects classified as non-stars can be used as “reference stars” for the guide star identification algorithm.
- 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).
- There can be no bright spoiler stars within 8″ of a guide star candidate. A bright star in this context is defined to be a star that is less than two magnitudes fainter than the guide star candidate.
- Stars detected in only one of the catalog’s photometric passbands cannot be transformed into an FGS magnitude, and thus are not guide star candidates.
- Each guide star 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.
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 with an overlay of the Digitized Sky Survey (DSS) will be 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); Figure 1 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).