A guide star is crucial for the James Webb Space Telescope (JWST) to achieve precise pointing and maintain stability during observations. JWST uses a single guide star within one of the Fine Guidance Sensors (FGS) fields for fine guiding throughout a specific observation period. This article will delve into what a guide star is, its selection process, and why it’s essential for the success of JWST missions.
How JWST Uses Guide Stars for Precise Pointing
Unlike many telescopes where users can specify guide stars, JWST autonomously selects them. While observers don’t directly control the guide star selection, they can influence the availability of suitable guide stars in challenging fields by adjusting the aperture position angle during observation planning. The spacecraft’s star trackers provide separate roll control, complementing the fine guiding offered by the selected guide star.
Alt text: JWST Fine Guidance Sensor (FGS) field of view illustration, highlighting the area used for guide star acquisition and telescope pointing.
The telescope’s guiding and guide star selection process are automated and not directly controlled by the observer through the Astronomer’s Proposal Tool (APT). Target acquisition is a separate, subsequent process used to refine the telescope’s pointing relative to a reference target. The scheduling process aims to select three potential guide stars whenever possible, providing redundancy in case the initial acquisition attempts fail.
Guide Star Catalog and Selection Criteria
JWST’s guide stars are sourced from a catalog within the Guide Star Selection System (GSSS), considering factors related to telescope pointing and the star’s suitability. This catalog, maintained and updated, ensures a high probability of finding appropriate guide stars for observations. For detailed information about the current Guide Star Catalog (GSC), users can contact the JWST Help Desk.
Several criteria must be met for an object in the Guide Star Catalog (GSC) to qualify as a JWST guide star candidate:
- Point Source Classification: The candidate must be classified as a point source (“star”) in the GSC, excluding extended objects like galaxies.
- Magnitude Range: The candidate’s 2MASS J-band (Vega scale) magnitude must fall within the range of 12.5 ≤ J ≤ 18.0 (with slight variations based on the star’s spectral type).
- Absence of Spoiler Stars: No bright “spoiler” stars can be present within 6 arcseconds of the candidate. A spoiler star is defined as being less than 2 magnitudes fainter than the guide star candidate.
- Multiple Detections: The candidate must be detected in at least two of the catalog’s photometric passbands to allow for accurate brightness calculation in FGS count rates.
- Reference Stars: Each guide star candidate can be accompanied by up to 5 “reference stars,” used in the guide star identification pattern matching algorithm. These reference stars must also be classified as point sources.
Alt text: Diagram depicting a spoiler star near a guide star candidate, illustrating the potential impact on guide star selection for JWST observations.
Guide Star Availability and Galactic Latitude
The areal density of guide star candidates is closely linked to Galactic latitude. Candidate density decreases significantly beyond 35° above or below the plane of the galaxy. The FGS’s field of view and sensitivity, combined with the depth of known stars in the GSC, influence the availability of guide stars for any given telescope pointing and orientation.
Mission requirements necessitate a 95% probability of acquiring a guide star and maintaining pointing stability for any permitted telescope pointing. APT (Astronomer’s Proposal Tool) uses the statistical availability of guide stars as a function of Galactic latitude to determine the visit splitting distance for each target and observation.
Even with limitations, the probability of finding a suitable guide star is generally high. Considering the GSC contents, FGS sensitivity, and operational constraints, the probability of finding a guide star is 97% or higher across all Galactic latitudes.
Photometric measurements of guide star candidates in the GSC are used to predict the star’s count rate at the FGS detector. This involves converting optical photometric measurements into the near-infrared (if 2MASS data is unavailable) and applying wavelength-dependent telescope and FGS throughput factors over the FGS’s 0.6–5.0 μm passband.
Addressing Guide Star Acquisition Failures
On occasion, a visit might be skipped or become unsuccessful due to the inability to acquire a guide star. This can occur due to:
- Guide Star Catalog Errors: The guide star candidate might be misclassified (e.g., a galaxy mistaken for a star) or have incorrect brightness information.
- Attitude Control System Pointing Errors: Inaccuracies in the telescope’s initial pointing can prevent the FGS from locating the guide star within its expected field of view.
- Bad FGS Pixels: Defective pixels on the FGS detector can interfere with the accurate measurement of the guide star’s count rate.
If a guide star acquisition failure occurs, the principal investigator is typically notified. Users can request a repeat of the failed visit, usually within 90 days.
The Ongoing Importance of Guide Stars
In conclusion, a guide star is a fundamental component for JWST’s successful operation, allowing for the precise pointing and stability necessary for groundbreaking astronomical observations. Although users don’t directly select guide stars, understanding the selection process and factors that influence guide star availability is crucial for optimizing observation plans. Ongoing improvements to the Guide Star Catalog and operational procedures will continue to enhance the reliability and efficiency of JWST’s guide star acquisition process, ensuring the continued success of its scientific mission.
