MIRI Coronagraphy Known Issues

Known issues specific to MIRI coronagraphy 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 MIRI Coronagraphy Calibration Status for an overview of the current astrometric and flux calibration of MIRI coronagraphic data products.


Why are the coronagraph masks at an angle in the focal plane?

The support structure for the Lyot coronagraph, and the quadrant boundaries for the 4QPM coronagraphs, are tilted at 4.835° relative to the rows and columns of the imaging detector (see MIRI Coronagraphic Imaging). Since MIRI is tilted by this same amount in the telescope focal plane, the tilt in the coronagraphs is necessary to align the coronagraph masks with the pupil structures in the OTE.

TA PSF asymmetry in 4QPM coronagraphs

The MIRI coronagraphic imaging target acquisition (TA) process uses one of 4 broadband filters: F560W, F1000W, F1500W, and the neutral density filter (FND). In-flight data shows that the point spread function (PSF) exhibits an asymmetry caused by internal reflection in the 4QPM germanium optics. This asymmetry is a function of the difference between the central wavelengths of the TA filter and the operating wavelength of the specific 4QPM. The most extreme asymmetry is for the F560W TA filter and the F1550C coronagraph. Tests of the TA process have shown that this asymmetry is uniform across the coronagraph FOVs—it only imparts a small offset to the TA which has been compensated for in the TA process. Because there are no optics at the Lyot coronagraph occulting spot and support structure, the TA PSFs for the Lyot coronagraph are unaffected.

Glow sticks in the MIRI 4QPM coronagraphs

Fight data has shown that there is light being scattered into the coronagraphs. This is particularly apparent for the 3 4QPMs, where it manifests as a glow along the horizontal boundaries of the phase masks. These are referred to as glow sticks; they can be removed by (1) a dedicated background observation, (2) angular differential imaging (ADI) processing where a telescope roll has been used for 2 or more observations of the science target, or (3) by reference star PSF subtraction if the reference star has the same brightness and exposure time as the science target. At present, these glow sticks appear to mostly increase the shot noise.

Figure 1. Scattered light in the 4QPM coronagraphs


Click on the figure for a larger view.

Comparison between the ground and flight images of the F1550C coronagraph when uniformly illuminated. The glow sticks feature is clearly visible, and "hides" the attenuation of the 4QPM phase mask along the horizontal axis. This feature is seen in all 3 4QPMs.

Edge brightening around the Lyot spot and lower edge of the Lyot FOV

In-flight data show that, as for the imager, there is light scattered into the Lyot FOV that slightly illuminates the rich-hand edge of the occulting spot and the support structure. This scattered light can be removed by either (1) a dedicated background observation, (2) ADI processing where a telescope roll has been used for 2 or more observations of the science target, or (3) by reference star PSF subtraction if the reference star has the same brightness and exposure time as the science target.

Figure 2. Scattered light in the Lyot coronagraph


Click on the figure for a larger view. 

Comparison between the ground and flight images for the Lyot coronagraph when uniformly illuminated. The scattered light is evident along the upper edge of the Lyot spot, and the lower edge of the coronagraph FOV.

Pipeline notes

None at present.

Summary of common issues and workarounds

The sections above provide detail on each of the known issues affecting MIRI coronagraphic data; the table below summarizes some of the most likely issues 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.

SymptomsCauseWorkaroundFix buildMitigation Plan

MR-CI02: Users will want to perform their own PSF subtraction with the data products.

PSF subtraction has too many free parameters to capture them in a one-size-fits-all automated processing pipeline.

The public "Coronagraphy_ExampleNB" repository contains a suite of 6 notebooks that will guide the user through reprocessing coronagraphic data through all steps of the pipeline, as well as for customizing parameters at each step. Users will find notebooks suitable for processing "uncal" files retrieved from MAST through stage 2 to yield background-subtracted photometrically-calibrated data products suitable for use in PSF subtraction. Older notebooks are still available in the "old_pipeline_demos" subfolder.


Updated issue

The MIRI team is always considering the scope of what data products the calwebb_coron3 pipeline stage should provide.

MR-CI03: Tweak stage 1 and 2 parameters to optimize data calibration.

Default parameters may not be optimal for all datasets.

spaceKLIP, provided by the ERS team, can run pipeline steps as well as perform highly tunable PSF subtraction on the data products. There are examples of different pipeline parameter values they have found helpful, which can be found in the MIRI config file under the "tests" folder.

Pipeline stages 1 and 2 are shared with the MIRI imager, so users may look there for more examples.


Created issue

None at this time.

MR-CI04: Current Lyot flat-fielding causes a sharp discontinuity underneath the coronagraphic mask.A sharp edge in the Lyot coronagraph's flat-fielding reference file creates a sharp discontinuity underneath the coronagraphic mask, in stage 2 pipeline products. This can result in artifacts during certain image processing steps that are sensitive to abrupt changes.

For now, users should be aware that this sharp discontinuity is an artifact of the calibration pipeline and does not represent astrophysical information. They should take this into account during post-processing.


Created issue

Mitigation strategies are under investigation.

MR-CI01: Absolute flux calibration is incorrect.An error was made when computing the aperture correction for coronagraphic PSFs.Multiply fluxes by a factor that will be available by the end of August, 2023.10.0

Updated Operations Pipeline

Flux calibration "photom" reference files were updated in October. STScI will reprocess affected data products with updated calibration reference data. Reprocessing of affected data typically takes 2–4 weeks after the update.

Notable updates

Originally published