Known issues specific to MIRI 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, along with some guidance on using the pipeline data products in the Pipeline Notes section, and a summary of some common issues and workarounds in the summary section.

For mode-specific issues, see the coronagraphy, imaging, LRS, and MRS issues pages. For a detailed discussion of many aspects of the MIRI detectors and their impact on science calibration, see Morrison et al. 2023. Also see MIRI Calibration Status for an overview of the current calibration of MIRI data products.

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Artifacts

Brighter-fatter correction

Saturated pixels affect the flux of neighboring pixels and "fatten" the PSF. A correction for this effect is still under development. For more details see Argyriou et al. 2023B.

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Column/row pull up/pull down

The column/row pull up/pull down effect is a form of electronic cross talk. It manifests itself as an overall signal increase (pull-up) or decrease (pull-down) of the flux in columns and rows with strong sources. There is a flux dependency in the effect, and exhibits differences between the impact in rows and columns. See Dicken et al. 2022 for further discussion.

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Cosmic ray showers

See Shower and Snowball Artifacts

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Cruciform artifact

There is a feature caused by internal scattering within the MIRI detectors that manifests as a cruciform shape in imaging and spectroscopy at wavelengths ≤ 10 μm.

The Si:As detectors used by MIRI are tasked with observing a large wavelength range, from 5.6 to 30 μm. Inherently, these detectors are lower in quantum efficiency at the shorter wavelength range of this window, absorbing less than 27% of the photons on their first pass through the absorptive layer at 5.6 μm. The remaining photons are diffracted by the square pixel lattice, resulting in a classic cruciform-shaped diffraction pattern that follows the row and column directions of the detector.

In the MIRI imager, the field of view (FOV) is rotated by 4.835° from the V3 axis, and the cruciform artifact is thus also rotated by this amount from the OTE/hexagon diffraction spikes that constitute the JWST PSF. The cruciform artifact is strongest at the shorter wavelengths and disappears mostly by 12 μm (see Figure 1 below. The inner region of the PSF core does not show the cruciform artifact due to photons being able to escape the detector wafer within the total internal reflection limit. For more information, please see Gaspar et al. (2021).

As discussed by Argyriou et al. 2023, in the MIRI MRS this scattering occurs between the detector pixel metalization and the anti-reflection coating on the back surface of the detector (the MRS detectors are backside illuminated) and is most pronounced in channel 1.

This photon scattering in the MRS detectors manifests in the data in 2 ways: 

1. A broader PSF FWHM at short wavelengths compared to diffraction-limited PSF predictions.

2. A secondary diffraction at narrow gaps between the pixels, which act as narrow slits in the mid-infrared. This superimposes a traditional Airy diffraction pattern on the detector that is centered on the PSF. 

Because neighboring slices on the detector do not correlate to neighboring pixels on the sky (and are similarly offset in wavelength), the wings of the Airy diffraction pattern will appear in the MRS 3-D reconstructed spectral cubes as faint sources far away from the observed point source both spatially and spectrally. This stray light signal is automatically removed in the pipeline (see the straylight step) for all MRS observations by convolving the observed detector scene with a model of the known cruciform profile (Figure 2), and does a reasonable job of removing both the horizontal "band" type features and the small "spike" features. However, the features may be either over- or under-subtracted at the percent level, and users should thus be cautious about the existence of such spikes when performing PSF subtraction and/or looking for faint companions within the field of view.

A faint scattered light component has also been observed in the spectral direction (detector vertical direction) as well. This component is difficult to disentangle from the spectral continuum of a source, and can manifest as either a slight broadening or faint secondary peaks in the spectral line spread function (LSF). At present, this component is still under investigation and no correction is currently available. Likewise, the performance of the spatial correction degrades in the vicinity of extremely bright emission lines, which can produce artifacts in adjacent slices on the detector.  In channel 1 particularly, this can lead to spurious positive or negative spectral features (depending on the degree of under- or over-subtraction of the cross artifact) anywhere in the composite data cube for wavelength regions adjacent to extremely bright spectral lines.

Electromagnetic interference (EMI) pattern noise

All MIRI data show coherent pattern noise from electromagnetic interference. While the 10 Hz heater noise affecting all MIRI data is subtle, 390 Hz noise is prominent in most MIRI subarray data whose readouts are out of phase with this signal (SUB128, SUB64, SLITLESSPRISM and all coronagraphic subarrays are affected by 390 Hz noise; BRIGHTSKY and SUB256 subarrays are unaffected). In jwst pipeline version 1.13.0 a new step was added to the calwebb_detector1 pipeline to correct for this correlated noise by calculating the phase of each detector pixel and searching for and removing periodic amplitude variations at the known EMI frequencies. In the future, the MIRI subarray locations may be moved to be better in phase with any EMI.

In the meantime however, the emicorr step in the pipeline typically does a good job of correcting for the feature (see Figures 3 and 4).

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Pipeline notes

N/A; see known issues pages for coronagraphy, imaging, LRS, and MRS modes.

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Summary of common issues and workarounds

 N/A; see known issues pages for coronagraphy, imaging, LRS, and MRS modes.

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References

Argyriou, I., et al. 2023, A&A, 675, A111 (MRS Overview)
JWST MIRI flight performance: The Medium-Resolution Spectrometer

Argyriou, I., et al. 2023B, A&A submitted (MIRI brighter-fatter effect)
The Brighter-Fatter Effect in the JWST MIRI Si:As IBC detectors I. Observations, impact on science, and modelling

Dicken, D., et al. 2022, SPIE 12180, 2(MIRI pull up/down effect)
Row and column artifacts in JWST MIRI's Si:As blocked impurity band detectors

Morrison, J., et al. 2023, PASP, 135, 5004 (MIRI Detector Effects)
JWST MIRI Flight Performance: Detector Effects and Data Reduction Algorithms

Wright, G. S., et al. 2023, PASP, 135, 048003 (MIRI Overview)
The Mid-infrared Instrument for JWST and Its In-flight Performance

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