MIRI LRS Calibration Status
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.
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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
| Date | JWST version | CRDS version | Description |
|---|---|---|---|
| 21 Feb 2024 | jwst_1201.pmap | Turning off shower code for the LRS while fixes are made. | |
| 13 Dec 2023 | jwst_1174.pmap | For the slit, the wavelength calibration, photometric correction and aperture correction have been updated to improve S/N and smooth at longer wavelengths to avoid introducing additional noise. For the slitless mode, the photometric and aperture corrections are improved, but the wavelength calibration is unchanged from October 2022. | |
| 18 Sept 2023 | 1.12.0 | Updates the MIRI LRS fixed slit path loss correction to default out-of-slit positions to the center of the slit, and adds the parameter user_slit_loc to allow specifying the source location to use. | |
| 15 Sep 2023 | jwst_1130.pmap | Updated MIRI gain files, including LRS. | |
| 24 Aug 2023 | jwst_1116.pmap | Update to the calwebb_spec2 parameter reference file "pars-spec2" to turn on the new "pixel_replace" algorithm to improve the quality of extracted products. | |
| 11 Aug 2023 | jwst_1107.pmap | LRS fixed slit path loss reference file changed to default all in-slit positions to center-of-slit, effectively defaulting the path loss correction to the center of the slit for all possible pointings. | |
| 11 May 2023 | jwst_1086.pmap | LRS "extract1d" file defines default apertures for spectral extraction for the MIRI LRS in calwebb_spec3 for both fixed slit and slitless modes to work around the issue of improper spectral extraction apertures based on WCS information. | |
| 3 April 2023 | 1.10.0 | Update to apply the correction array to all integrations when given a 3-D "rateints" input for MIRI LRS fixed slit data. | |
| 5 Oct 2022 | jwst_0995.pmap | General calibration update for LRS based on commissioning data, including first wavelength calibration update, for both slit and slitless modes. | |
| 1 Sept 2022 | 1.7.0 | Adjusts wavelength array to equivalent shape of data array. |
Wavelength calibration
Date:
The current calibration is from December 2023 and is based on data from commissioning and Cycle 1 calibration. The wavelength calibration for the slit was updated in December 2023; the slitless calibration is unchanged from October 2022.
Slitless: It is generally better than 20 nm, but possibly larger for wavelengths below ~7 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:
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:
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 (improvements are expected in Fall 2023).
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.