JWST Guide Stars

JWST uses a single guide star in one of the FGS fields for fine guiding during a given visit.

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

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.

Guide Star Catalog

See also: JWST Pointing Performance

The JWST proposal planning system currently uses a Guide Star Catalog (GSC) that is the result of a merger of the previous HST catalog with data from 2MASS, Sloan Digital Sky Survey (SDSS) DR13, Gaia, PanSTARRS, Skymapper, and VISTA 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:

  1. 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.

  2. 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 2 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.

  3. The Gaia data release catalogs provide 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.

  4. Coverage gaps in the previous catalog have been filled, for example, in areas around very bright stars and in regions of very high extinction.

  5. Improvements continue to be made, based on operational experience.

Whenever possible, schedulers select 3 potential guide stars.  Operations onboard then has two additional chances in addition to the first guide star in case onboard autonomous operations fails on a given star for any reason.

If you have need for more detailed information about the current GSC is use for JWST, contact the JWST Help Desk.

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 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. 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 ≤ 17.8 (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 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.

  • Compact objects classified as non-stars in the GSC can still be used as “reference stars” for the guide star identification algorithm.

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.  Also, improvements to the catalog in extremely crowded fields are also being implemented over time.

On-orbit Performance

To date, trending of the guide star acquisition success rates shows that about 3.8% of all visits fail to acquire science due to issues related to the JWST attitude control system (ACS). The causes of guide star acquisition failures range from large (>10”) ACS initial coarse pointing errors, unmasked bad pixels on the FGS detector in the vicinity of faint (FGS_Mag > 17) guide stars, GSC catalog errors, such as surprise binaries, compact galaxies, unresolved spoilers that FGS resolves. Note that most catalog errors generally will not cause loss of science if more than one guide star candidate is available for the visit (presented with a guide star acquisition failure, OSS commands an attempt on the next candidate).

Latest updates
      Updated maximum number of references stars allowed.

  • Removed outdated text section and Figure on visualizing guide stars.

    Further clarifications to text and additional links added.

    New Figure 1; Major updates to text
Originally published