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

Artifacts

Imager "scar"

The imager scar consists of a diagonal feature on the top-left part of the MIRI imager, centered at pixel (428,987) with a length of 16 pixels. This is related to a scratch on the surface of the detector. Although the pixels are responsive, this feature can scatter the light in both directions perpendicular to the scratch.

When the pixels associated with the scar are uniformly illuminated (no sources and just background), 2 diagonal features can run parallel to the scar (Figure 1). These stripes are brighter than the local background and their position and width change as a function of wavelength.

Figure 1. Location of the imager scar

 

If a bright source is imaged on these pixels, its light is scattered on the detector perpendicular to the scar creating a peculiar structure different from the classical point-like profile the same source would have in a different part of the imager (Figure 2).

Figure 2. Bright source imaged on the scar

 

Imager bad pixels

The MIRI imager detector has several pixels that are flagged based on ground testing and in-orbit observations. These pixels are flagged as "DO_NOT_USE" in the flight bad pixel mask (Figure 3), and the pipeline will not use those pixels. In addition, two of the columns (385 & 386) on the MIRI imager array are shorted. These are flagged in the flat field bad pixel mask as "UNRELIABLE_SLOPE".

Figure 3. MIRI imager bad/hot/dead pixel mask

 

Cosmic ray showers

See Shower and Snowball Artifacts

Imager knife edge

The MIRI imager has a protrusion from a knife edge structure that blocks a small portion of the field of view (FOV) on the left side, and is used for optical testing. The main effect will be on mosaics (see Knife Edge Gap in MIRI Imaging Mosaics). The default MIRI imaging mosaic overlap of 10% is used to address the gap.

Bonus Lyot FOV in imaging

In most cases, the Lyot FOV provides valid, calibrated data except where the Lyot occulting spot and support structure prevent light from reaching the detector.

However, there have been some datasets that show an offset between background levels in the Lyot compared to those in the rest of the imager. While the differences are not typically large, in those cases, they can be seen by eye in individual calibrated images, and can make background matching for larger mosaics that include both imager and Lyot regions more difficult. 

Edge brightening

The in-flight imaging FOV exhibits some stray light features on the left and bottom edges (Figure 5). These should not have a significant impact on combined dithered or mosaicked imaging. Additional analysis is underway.

Figure 5. Edge brightening in the MIRI imager field of view, also showing scattered light features in the MIRI coronagraphs

  

Time-dependent count rate loss

See MIRI Imaging Calibration Status.

Pipeline notes

None at present.

Summary of Common Issues and Workarounds

The sections above provide detail on each of the known issues affecting MIRI Imaging data; in this table we summarize some of the most likely issues for users to encounter along with any workarounds if available.  Note that greyed-out issues have been retired, and are fixed as of the indicated pipeline build.