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

See also: JWST Guiding Errors and Problems

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, sensitivity, and the allowed magnitude range (from J = 12.5 to 18.0 mag), 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.

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

• 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.
  
  

• To be considered as a guide star candidate, a star must be detected in two or more of the GSC catalog’s photometric passbands so that 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 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, guider detector count rate variations (e.g., photo response non-uniformity [PRNU], cross hatching), 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.  

Guide star and reference star information

A user may determine what guide star was used for a particular science observation from the GDSTARID keyword in the primary header of the science fits file.

Up to 3 guide stars (GS) may be commanded for each visit. Each GS may have up to 5 reference stars. Identification (ID) will try each commanded guide star up to 3 times. If all 3 attempts for a given GS fails, ID moves on to the next GS. If all attempts on all guide stars fail, then ID fails.

The Identification data files (jwppppvvvooo_gs-id_*_cal.fits) will provide lists of reference stars in data product table extensions. There are two sets of reference lists: the planned ground list in the Proposal Planning System (PPS) database and the flight list in the onboard engineering data. Planned reference stars contains the information for all of the commanded guide star and reference star candidates for a given visit. Flight reference stars contains the information for the guide star and reference stars used in flight for a given visit.

The "PLANNED REFERENCE STAR" table extension holds the following information for each reference star.

Table 1 gives the contents of the PLANNED Reference STAR table extension for calibrated ID data files (i.e., jwpppppvvvooo_gs-id_*_cal.fits).

Table 1. Content of PLANNED REFERENCE STAR table extension for ID data files

The "FLIGHT REFERENCE STAR" table extension of calibrated ID data files (i.e., jwpppppvvvooo_gs-id_*_cal.fits) will hold the following information for each reference star.

Table 2 gives the contents of the FLIGHT REFERENCE STAR table extension for calibrated ID data files (i.e., jwpppppvvvooo_gs-id_*_cal.fits).

Table 2. Content of FLIGHT REFERENCE STAR table extension for ID data files

Guide star acquisition failures

If a visit is skipped or unsuccessful because the guide star could not be acquired, then the principal investigator is generally notified via email. 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 should also be aware that it is possible for the FGS to "successfully" guide on the wrong guide star. While rare, this leads to an offset in the expected target position in the science aperture, and the user is not notified because the visit apparently succeeded. Users are strongly encouraged to review their data promptly. 

More details are provided on the Guiding Errors and Problems article.

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.