The Fine Guidance Sensors are required to guide in a wide range of fields, from sparse fields at high galactic latitudes to very crowded fields near the center of the galaxy.

See also: JWST Guide Stars, Aperture Position Angle Special Requirements, APT Aladin Viewer (see Adjusting an observation's position angle)

Some fields are challenging for JWST guiding. Bright crowded fields (e.g., the Galactic center) with > 100 stars brighter than ~J = 15 in a guider's FOV, fields on or near large extended objects, and sparse fields all present unique challenges for guiding. The STScI guiding team has invested considerable work in improving guiding performance in these fields by addressing catalog errors and adjusting FGS flight software parameters. Furthermore, users may be able to mitigate the risks of guiding failure in some of these fields.

Guide star selection is an automated process, but 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. If the science target is in or near a bright crowded field (BCF), or a large extended object, use Aladin in APT to check for available guide stars and problematic sources. Visualize the location of the FGS apertures relative to the 2MASS survey, and adjust the aperture position angle range of your observation to optimize the FGS location. If necessary, use this information to constrain the aperture position angle or V3PA of your observation as a special requirement in APT. Examples of this are discussed below.

Guiding in challenging fields

The JWST Fine Guidance Sensors have proven to operate reliably over most of the sky. However, some fields challenge the FGSs and the Guide Star Catalog. STScI is currently working on guiding improvements for the types of challenging fields listed below. 

• Guide stars and/or reference stars in or near large extended objects
• Bright crowded fields (BCF)
• Galactic center
• Sparse fields

The following sections discuss challenges each of these types of fields present, and the measures that may be taken to lessen the risk for guiding in these fields. For some types of fields, there may be actions the proposer may take to improve the chances of successful guiding—such actions are covered below.

Guide stars in or near large extended objects

Targets with guide stars and/or reference stars in or near large extended objects have proven difficult for guiding. Compact star clusters, spiral arms, or extended nebulosity, dense or complex fields with moderate to high stellar density and/or significant spatial structure may affect whether a cataloged guiding candidate is an isolated single star.

Challenges

In some cases, the commanded guide star or reference stars turn out to not be a point source but instead a compact object (such as extragalactic star forming regions). In such cases, the object's surface brightness is not that of a PSF. Therefore, the resulting FGS-measured count rates are lower than expected, leading to a failed guide star acquisition. (FGS aperture photometry is measured over a 3 × 3 pixel subarray, i.e. 0.21" × 0.21".) These non-point sources are flagged as unusable for guiding.

In other cases, guide stars can be embedded in high background fields, which again can cause a star to fail an onboard count rate test. 

Mitigating guiding risks

The only mitigation to date for these observations is to visually inspect the fields to assess the risk, and perhaps select a different aperture position angle at which the observations are to execute, to place the FGS FOV on a more favorable patch of sky.

Guiding strategies

Proposers should check the availability of Guide Stars by inspecting the guiding scene in APT at various aperture position angles. In APT (1) select the observation (2) click the View in Aladin button, (3) click the JWST FOV button, and (4) click GS Check button . Changing the aperture position angle of your observation may facilitate the availability of better guide stars, for example, guide stars away from the spiral arms of a galaxy.

An example of the effects of aperture position angle on the availability of guide stars in and near NGC 104 (47 Tuc) is shown in Figure 1. The image in panel (a), generated using APT's View in Aladin tool, shows the Guider 1 and Guider 2 apertures falling within the core of the globular cluster as indicated by the overlay with DSS. Although the guide star selection system returned guide star candidates that meet selection criteria, the FGS failed to acquire any of these candidates due to a combination of catalog errors and high areal density of stars in the guide star identification images. The image in panel (b) shows the successful, second attempt to acquire the science with NIRSpec, but at an aperture position angle that placed the guider footprints on the globular cluster's outer perimeter.

Bright crowded fields (BCF) 

Fields with a high areal density (> 100 stars in an FGS FOV) of bright stars (brighter than ~J = 15) are considered bright crowded fields (BCF). 

Challenges

During the guide star and reference star identification process, only stars that make it onto the "bright object list" are considered by the FGS flight software for identification. The bright object list is limited to 100 objects. If there are more than 100 objects in the FOV, it is possible that the guide star and/or reference stars won't make it onto the bright object list, and identification will fail.

Mitigating guiding risks

Special commanding has been developed for these fields. Only the central 684 × 2048 pixels of the detector are read out during the guide star identification process, rather than the entire detector. This reduces the total number of stars vying for space on the bright object list. This special commanding is triggered automatically. The use of the Galactic Nucleus Survey (GNS, Nogueras-Lara, F. et al.) is also automatically applied, if the observation is covered by the catalog, with no special action required on the part of the observer.

Galactic center

The neighborhood of Sagittarius A* is an extreme example of a BCF. JWST visits to this part of the sky can encounter problems with acquiring the intended guide star, often attributable to incompleteness in the Guide Star Catalog (GSC). Some of this can be mitigated over time.

Challenges

As in other BCFs, the guide star and/or reference stars may not make it on to the bright object list. Only objects on the bright object list may be considered for identification by the FGS flight software.

Due to extreme reddening in the region of Sgr A*, the existing algorithm STScI employs to predict the stellar flux of a star under-predicts the count rate observed by FGS at 2–5 µm.

Mitigating guiding risks

To address the catalog errors and incompleteness in GSC 2.3 around Sgr A*, STScI has incorporated a specialized catalog, the Galactic Nucleus Survey (GNS) into GSC 3.0, which became operational in Spring of 2024. Comparison of simulated images based upon the GNS to images acquired by FGS show that the GNS is essentially complete down to magnitudes relevant to FGS operations with excellent astrometry. The GNS covers a non-contiguous 0.3 square degree area in regions towards Sgr A* in J,H,K.

Observed count rates were compared to the counts rates predicted from the GNS catalog. The observed and predicted count rates differed by an average factor (1.5x). This factor was then applied to the GNS count rates, to improve the predicted count rates. As more FGS data associated with Sgr A* visits become available, the FGS team may refine the algorithm for computing FGS count rates for reddened guide stars.

The BCF special commanding, with its reduced detector readout area, is triggered automatically. The use of the GNS is also automatically applied, if the observation is covered by the catalog. No special action is required on the part of the observer.

The recent improvements described above, put in place to support Sgr A* observations, resulted in a success rate of ~75%, as compared to the ~25% success rate near Sgr A* before the improvements were implemented.

See this JWST News item regarding the improved success rate for the Galactic nucleus. 

Sparse fields

Targets in sparse fields, with few and often faint guide star candidates, also pose challenges for guiding. 

Challenges

In sparse fields, there may be no guide star candidates available for the observation at some aperture position angles.

Mitigating guiding risks

The only mitigation to date for these observations is for proposers to to visually inspect the fields to assess the risk, and determine if a different aperture position angle yields more or better guide star candidates. 

Guiding strategies

Proposers should check the availability of guide stars by inspecting the guiding scene in APT at various aperture position angles. In APT (1) select the observation (2) click the View in Aladin button, (3) click the JWST FOV button, and (4) click GS Check button . Changing the aperture position angle of your observation may facilitate the availability of better, or more, guide stars.

References

Nogueras-Lara, F. et al. 2019, Astronomy & Astrophysics, 631, A20
Galactic Nucleus Survey (GNS)