JWST uses a single guide star in one of the Fine Guidance Sensors (FGS) fields for fine guiding during a given visit.

Fine guiding is provided via the selection of a single guide star. Roll control is provided separately by the spacecraft star trackers.

Telescope guiding and guide star selection are not directly controlled by the observer and cannot be specified in APT. Target acquisition, when necessary, is a separate process whereby the telescope pointing is refined relative to a reference target.

JWST guide stars are currently selected from a catalog within the guide star selection system (GSSS) based on several factors related to telescope pointing and suitability of the star. It is not the user's responsibility to pick specific guide stars to be used for their observations. This is done as part of the scheduling process. However, users may be able to increase the chances of finding good guide stars for some challenging fields by changing the aperture position angle of their observations.

Whenever possible, schedulers select 3 potential guide stars. This allows 3 separate chances for the onboard autonomous guide star acquisition process, in case one or two chances fail for any reason.

If you have need for more detailed information about the current Guide Star Catalog (GSC) used for JWST, contact the JWST Help Desk.

Guide star availability

Guide Star Catalog (GSC) 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 beyond 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 GSC, 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 contents, FGS sensitivity, and operational limitations, the probability of finding a guide star is 97% or higher at all Galactic latitudes. VISTA Hemisphere Survey (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 2MASS J-band (Vega scale) magnitude range of 12.5 ≤ J ≤ 18.0 (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 2 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 accompanied by up to 5 “reference stars,” which are used in the guide star identification pattern matching algorithm to identify the correct guide star. Reference stars must also be classified as “stars.” Early operational experience showed that extended objects are not suitable as “reference stars” for the GS Identification pattern match.

Operational experience post-launch has shown that there are still some guide star candidates in the catalog that are unsuitable for use, so the catalog will continue to be improved over time. Improvements to the catalog in extremely crowded fields are also being implemented over time.

Occasionally, guide stars are found to be fainter than predicted, leading to an increased noise equivalent angle (NEA, an equivalent jitter angle based on centroid position and SNR). This could be due to catalog error, photo response non-uniformity  (PRNU) in the detector, unflagged bright pixels, flagged bad pixels coincident with a guide star, mistakenly guiding on a slightly extended object, or a guide star at the edge of the centroiding box in the FGS. Work is ongoing to reduce these variations.  

Moving target guide star faint limits are J = 16.5, and 17.0 in FGS 1 and 2, respectively.  

Guide star acquisition failures

Occasionally, a visit is skipped or unsuccessful because the guide star could not be acquired. This can happen for several reasons, including:

• Guide Star Catalog error; e.g., the commanded guide star turned out to be a galaxy misclassified as a star, or a binary, or the star was too faint or too bright. When identified, these objects will no longer be considered for use as Guide Stars.
• Attitude control system pointing error; guide star acquisition will fail if the observed position of the guide star is farther away than expected
• Bad FGS pixels; bad pixels can affect the observed (3 × 3 pixel) count rate of the guide star. If the observed count rate is outside the expected range, the guide star will not be acquired.

Users may wish to request a repeat of the failed visit. These requests typically must be submitted within 90 days. See Policies for the Telescope Time Review Board for details.