If your observation encounters guiding problems, you may contact the JWST Help Desk. Additionally, the information below may help you understand what went wrong, or how to interpret error messages. The information is presented in order of the various steps of the guiding process, as described on the JWST Fine Guidance Sensor article. 

See also: JWST Pointing Performance, JWST Guiding Strategies and Challenging Fields

Guiding errors and problems

JWST typically succeeds on guiding with the correct guide star ~96% of the time. There are, however, many ways guiding may fail. Some of the more common causes of failure in guiding include the following: 

• Attitude Control System (ACS) pointing errors: Large pointing errors may result in failure to identify the guide star, or identifying the incorrect star as the guide star.
• Catalog errors: Errors in the expected count rates of the guide and/or reference stars may result in failure to identify the guide star, or identifying the incorrect star as the guide star. In addition, occasionally an object such as a galaxy or binary star is misclassified in the catalog as a single star. 
• Fine Guidance Sensor (FGS) flight software (FSW) errors: The FGS FSW is not 100% foolproof, and may occasionally fail to identify the guide star, or identify the incorrect star as the guide star.
• Operations Scripts Subsystem (OSS) checks: OSS checks the distance between where the guide star was expected and the location where it was found. If this distance exceeds a given threshold, OSS will declare an ID failure. OSS also performs a check of the guide star’s count rate in the ACQ2 acquisition step. If the star is outside a 40% allowed threshold, it will declare ACQ unsuccessful.
• Bad pixels (masked and unmasked): Both masked and unmasked bad pixels may cause the observed count rate of the candidate guide star to be outside the commanded count rate thresholds.
• Guider detector count rate variations, which may be due to issues such as photo response non-uniformity (PRNU), as well as other detector artifacts, may cause the observed count rate of the candidate guide star to be outside the commanded count rate thresholds.
  
  

Identification (ID)

If your visit had a guiding failure, and you have ID files but no acquisition files, then your visit failed in ID.

• Rarely, the FGS flight software (FSW) fails to identify the guide star. There may be up to 3 guide star candidates commanded per visit. Identification (ID) may be attempted 3 times for each guide star candidate. If all ID attempts are unsuccessful then that ID attempt will fail.
• Occasionally, the FGS FSW will identify the wrong guide star. This requires the incorrect guide star to be close to the expected position of the commanded guide star, and the observed count rate to be within the thresholds of the commanded guide star. For bright stars (those with a 3 × 3 pixel count rate of ~0.5 million counts/sec), the thresholds are much more open, so it is possible for stars far from the commanded count rates to pass the ID stage. 
• If the guide star's observed count rate is outside the expected range, due for example, to bad pixels, catalog error, detector count rate variations, or intra-pixel centering of the PSF, the guide star is eliminated from further consideration, and ID will either fail with the error "no guide star found" or will identify the wrong guide star.
• During the ID process, the brightest 100 stars in the field of view that satisfy the commanded count rate range are added to the "bright object list." If the field is crowded, it is possible for the guide star to not make it onto the "bright object list" because the list is full of brighter objects. In this case, the guide star is eliminated from further consideration in the ID process and ID will either fail with the error "no guide star found" or the wrong guide star will be identified.
• Large Attitude Control Subsystem (ACS) pointing errors (> 15") will cause the FGS FSW to report ID attempt unsuccessful.
• ACS pointing errors between 12" and 15" will cause OSS to report ID attempt unsuccessful.

Acquisition (ACQ)

If your visit had a guiding failure, and you have ACQ files but no TRACK files, then your visit failed in ACQ.

• ACQ1 consists of 12 integrations with 2 groups per integration. This results in one FITS file containing 6 correlated double sample (CDS) images. ACQ1 will fail if:
  • Too many centroids are marked as "bad"; this may happen when the point spread function (PSF) width or height is bigger than the threshold, the PSF brightness is too faint or too bright (i.e., outside the commanded thresholds based on the Guide Star Catalog), or the guide star is too close to the edge of the detector.
  • If the best candidate meets the location threshold but the "Bad Centroid Flag" is set.
  • If at least one candidate is detected, but no candidate meets both the distance and amplitude thresholds.
• ACQ2 consists of 10 integrations with 2 groups per integration. This results in one FITS file containing 5 CDS images. ACQ2 will fail if: 
  • The observed ACQ2 count rate is outside the 40% threshold of the expected value.

  • Too many centroids are marked as "bad"; this may happen when (1) the guide star location is too close to the edge of the detector, (2) the PSF width or height is bigger than the threshold, or (3) the guide star count rate is too bright or too faint (i.e., outside the commanded thresholds based on the Guide Star Catalog).

During the Acquisition phase, first ACQ1 followed by ACQ2 is attempted at the "ID attitude", the same spacecraft pointing and orientation at which the Identification images were taken, resulting in one ACQ1 FITS file and one ACQ2 FITS file. If ACQ at the ID attitude succeeds, the spacecraft performs a small slew to achieve the "science attitude", the pointing and orientation needed for the science exposure, and repeats the ACQ process, resulting in an additional ACQ1 FITS file and an additional ACQ2 FITS file, so successful guide star acquisitions have a total of one ACQ1 and one ACQ2 file from the "ID attitude" and one ACQ1 and one ACQ2 file from the "science attitude".

TRACK

If your visit had a guiding failure, and you have TRACK files but no fine guide files, then your visit failed in TRACK.

• If TRACK fails, STScI will attempt to re-try TRACK, for a total of 2 tries.
• TRACK will fail if too many continuous centroids are marked as "bad". This may happen when the guide star brightness is too bright or too faint (i.e., outside the commanded thresholds based on the guide star catalog)
• TRACK will also fail if the updated TRACK sub-window location is outside of the FGS field of view

Fine Guide (FG)

If your visit had a guiding failure, and you have FG files, then your visit failed in FG.

• FG will fail if too many continuous centroids are marked as "bad". This may happen when the guide star brightness is too bright or too faint (i.e., outside the commanded thresholds based on the guide star catalog).

Dithers 

• Re-acquisition after a dither maneuver may fail in any or all of the same ways as normal ACQ.
• A dither can cause a masked or unmasked hot pixel to be introduced that was not relevant at a previous pointing.