NIRCam Coronagraphy Known Issues
Known issues specific to NIRCam coronagraphic data processing in the JWST Science Calibration Pipeline are described in this article. This is not intended as a how-to guide or as full documentation of individual pipeline steps, but rather to give a scientist-level overview of issues that users should be aware of for their science.
On this page
Specific artifacts are described in the Artifacts section below. Guidance on using the pipeline data products is provided in the Pipeline Notes section along with a summary of some common issues and workarounds in the summary section.
Please also refer to NIRCam Coronagraphy Calibration Status for an overview of the current astrometric, photometric, and target acquisition accuracy of NIRCam coronagraphic data products.
Artifacts
Information on NIRCam instrument artifacts are found on the main NIRCam Known Issues article.
Pipeline notes
For issues that affect all observing modes, see NIRCam Known Issues. Currently, there are no pipeline notes for coronagraphy.
Summary of common issues and workarounds
The sections above provide detail on each of the known issues affecting NIRCam coronagraphic data; this table summarize some of the most likely issues that users may encounter along with any workarounds, if available. Note that greyed-out issues have been retired, and are fixed as of the indicated pipeline build.
| Symptoms | Cause | Workaround | Fix build | Mitigation Plan |
|---|---|---|---|---|
| NC-CI01: Dark correction leads to worse outcome than not applying any. | On-sky darks are not high enough in SNR and lead to residuals and erroneous jumps in the "rateints" images. | Skip dark correction during calwebb_detector1. | N/A | Created issue Mitigation is not yet scheduled because the data are satisfactory without dark subtraction. |
| NC-CI02: An excessive number of pixels are flagged as outliers in some subarray data. | The coronagraph subarrays do not have reference pixels on all sides. Without a reference pixel correction, the data become noisier and the jump step in calwebb_detector1 sometimes identifies too many pixels as outliers. | The spaceKLIP open-source package (provided by the community) implements a pseudo-reference pixel correction using a few not-illuminated pixels around (at the edge of) the subarray and forces them to be reference pixels. This way, global DC offsets frame to frame are taken care of. | N/A | Updated issue Implement something similar to spaceKLIP for calwebb_coron3. Timeline TBD. |
| NC-CI03: Absolute flux calibration is incorrect. | The wavelength dependent throughput of the bar occulters are currently a copy of throughputs of the round occulters, despite a slight difference of about 2% due to the different Lyot stops. In addition, flux calibration for all masks is still from pre-flight expectations. Note: previous text here mistakenly said that the COM (coronagraph optical mount) substrate throughput was not taken into account. That is not the case. | spaceKLIP uses the spectral energy distribution (SED) from published photometry (VO table from VizieR) to assess the expected flux from the star and calibrate the contrast curves, then determines the flux and position of any point source using MCMC and a forward model of the off-axis PSF. | 10.0 | Updated issue Data was reprocessed with an enhanced calibration reference file (photom) in CRDS. An update made on October 31, 2023 takes the full coronagraphic throughput into account using in-flight data. Reprocessing of old data typically takes 2–4 weeks after the update. |