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. 

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.

Artifacts

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

 

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

 

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.