The overall calibration status and estimated accuracy of the MIRI LRS are described in this article; please also see the article on known issues affecting MIRI LRS data.

Recent calibration updates

A complete list of the calibration pipeline and reference file updates can be found in the JWST pipeline change log and CRDS context history respectively. Selected recent major updates to LRS calibration are provided in Table 1.

Table 1. Recent major LRS calibration updates

Wavelength calibration

Date: 10 Dec 2024 

Wavelength calibration for the MIRI LRS was updated in December 2023 for the slit and August 2024 for the slitless, with reprocessing going until November 2024.

Slitless: It is generally better than 10 nm from 6-13 µm.

Slit: Generally good to ±20 nm, although the calibration is not sufficiently confirmed at ~5 µm.

See Beiler et al. 2023 for additional information on the status of the wavelength calibration in the slit prior to the update in December 2023.

Pointing distribution and consequences

Date: 19 Sep 2023 

The dispersion in the position of a star in the TA verification images is roughly ~0.10 pixels (radial, 1-σ). A shift of the target from the nominal pointing position will shift the dispersion solution for that spectrum an equal number of wavelength elements. The pipeline does not correct for that shift at this time.

Spectrophotometric calibration

Date: 23 May 2024 

Slit: The absolute flux calibration, as measured at 6–7 um for standard stars, is about 2% or better, but the overall shape of the extracted spectra for these stars show red excesses of 3–8%. The cause is under investigation. The limited quality of the current path loss correction will add errors to the photometry for sources not centered in the slit.

An update from June 2025 (jwst_1377.pmap) has corrected a ~2% fixed pattern noise issue that was introduced in a past flux recalibration. 

Slitless: The default pipeline concentrates on relative spectrophotometry for the study of exoplanetary transits and light curves and does not perform a background subtraction for slitless spectroscopy. Persistence in the slitless subarray (see next section) adds to the background. For data that have not been background subtracted, these issues affect fainter targets more than brighter ones. For standard stars at about 100–200 mJy at 10 um, the calibration can be better than 1% at 7 um. Standard stars at 50–70 mJy at 10 um can be several percent high at 7 um, and the problem grows worse for fainter targets, with excesses of 20%–30% seen in the fainter standard stars. Because stellar spectra are dropping rapidly with increasing wavelengths, the lack of a background subtraction can lead to strong red excesses in the extracted spectra. Observers interested in absolute photometry are strongly encouraged to remove the background manually from their slitless spectra integration by integration using apertures on either side of the region of interest in the spectral images.

Spectrophotometric precision (TSOs with LRS)

The spectrophotometric precision achieved during JWST commissioning was ~50 ppm at at a resolving power of ~50 in the core wavelength range for the LRS (7–8 um; Bouwman et al. 2023). This precision is achievable after an initial settling time; the time constant for this initial settling is of order 30 minutes, and its magnitude approximately 1%–3%. The shape of this initial settling ramp clearly depends on flux. Optimal spectrophotometric performance is also contingent on background subtraction, which is not yet automatically performed by the JWST calibration pipeline at this time. The background must be removed on a per-integration basis to mitigate for the 390 Hz noise signature in the SLITLESSPRISM subarray. 

Users are recommended to add 30 minutes of additional time at the start of their observation for transits and eclipses, and one hour for phase curves, to mitigate for the initial detector settling behavior. 

References

Beiler, S.A., Cushing, M.J., Kirkpatrick, J.D., et al. 2023, ApJ, 951, L48  
The First JWST Spectral Energy Distribution of a Y Dwarf

Bouwman, J., Kendrew, S., Greene, T. P., et al. 2023, PASP, 135, 038002
Spectroscopic Time Series Performance of the Mid-infrared Instrument on the JWST