The overall calibration status and estimated accuracy of the MIRI MRS are described in this article; please also see the article on known issues affecting MIRI MRS 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 the MRS calibration are provided in Table 1.

Table 1. Recent major MRS calibration updates

Astrometric calibration

The astrometric calibration of the MIRI MRS is described in detail by Patapis et al. 2023. In brief, each of the 12 bands was calibrated using a combination of ground testing and in-flight observations of the bright star 10 Lac using a 57-point raster scan that moved the star throughout the field of view.

When using target acquisition (TA), the relative astrometric alignment of the 12 MRS bands is generally good to about 0.03" (1-σ radial) based on analysis of multiple point sources observed throughout Cycle 1. As illustrated in Figure 1, this is driven by the repeatability of the MRS dichroic & grating wheel assembly (DGA) in the along-slice (alpha) direction. This alignment is generally sufficient for most scientific analyses at 1/10 to 1/30 of the PSF FWHM, but can nonetheless be noticeable in spectra of individual data cube spaxels on the shoulder of bright point sources (especially at short wavelengths), for which the astrometric jump between bands can produce discontinuous jumps in combined data cubes. Likewise, this non-repeatability in the alignment means that programs that rely upon detailed modeling of the PSF should use data cubes produced for individual bands. 

The absolute astrometric accuracy, however, is typically about 0.3" (1-σ radial) for MRS data products, with some known cases in which the WCS was incorrect by well over an arcsecond. This is driven by an observatory-wide WCS accuracy limit and is true even in cases where target acquisition was performed, such that the source can be properly centered in the field of view but the coordinates embedded into the data products are incorrect. This is due to a combination of uncertainties in the guide star catalog, misidentified guide stars during observations, and uncertainty in the spacecraft roll angle at the large separation of the MRS from the Fine Guidance Sensors (FGS). This in turn can produce errors in the locations used by the pipeline for extracting 1-D spectra from the data cubes. See MIRI MRS Known Issues for further details.

Without target acquisition, the pointing accuracy of the MRS is 0.45" (1-σ radial), again based upon analysis of multiple point sources observed throughout Cycle 1.  This is driven by a combination of genuine uncertainty in the target coordinates for some programs and the 0.3" observatory-wide WCS accuracy limit.

Wavelength calibration

As discussed by Argyriou et al. 2023, the wavelength calibration of the MIRI MRS has been revised multiple times since commissioning to take advantage of ever more detailed calibration information. The original commissioning wavelength solution (distortion version FLT-2) was based upon small adjustments from the pre-flight wavelength solution and had errors of up to 0.04 μm (~1000 km/s) in some bands as well as an offset in some slices around 17.4 μm resulting in splitting of emission lines crossing these slices. This solution was improved early in Cycle 1 using observations of bright nebular emission lines from planetary nebula NGC 6543 to rederive an entirely flight-based solution. The corresponding FLT-4 solution (implemented in CRDS context jwst_0970.pmap in September 2022) had a typical accuracy of 10–20 km/s throughout the MRS wavelength range but could be in error by as much as 70–80 km/s near the ends of individual bands where no nebular lines were available to constrain the solution.

FLT-5 (implemented in CRDS context jwst_1082.pmap in May 2023) radically revised the approach to MRS wavelength calibration, using observations of molecular features in the giant planets Jupiter and Saturn paired with NEMESIS atmosphere models to constrain the wavelength solution throughout the FOV. This calibration dramatically improved the wavelength calibration throughout bands 1A–3B, as illustrated in Figure 2 (see details provided by Argyriou et al. 2023 and Harkett et al. 2023). Channels 3C–4C could not be constrained by these giant planet observations, but were updated at the same time to fold in additional information provided by planetary nebula NGC 7027 and the Be stars HD 76534 and HR 2787 (Figure 3). The typical calibration accuracy of FLT-5 is a few km/s in bands 1A–3B, and roughly 30 km/s in bands 3C–4C.

Additional minor updates provided in FLT-6 (implemented in CRDS context jwst_1094.pmap in June 2023) further adjusted the Channels 1A and 1B wavelength solutions to adjust the across-slice wavelength tilt at the few km/s level, while FLT-7 kept the wavelength solution fixed and adjusted only the spatial distortion solution of Ch4B slightly based on observations of Uranus.  FLT-8 (implemented in CRDS Context jwst_1179.pmap in December 2023) revised the approach to wavelength calibration in channels 3C–4C, making wavelength corrections of up to a few tens of km/s based on a template cross-correlation analysis of observations of protoplanetary disk FZ Tau.

There are two ways that the wavelength solution can be given for MIRI MRS data cubes and extracted spectra. Per-band or per-channel cubes are constructed using a linear wavelength solution, the parameters of which are given using the usual FITS CRVAL, CDELT, etc., header keywords. FITS headers use a 1-based counting convention while Python uses a 0-based convention, so the wavelength solution using NumPy for a given data cube would be:

wave = (np.arange(hdr['NAXIS3']) + hdr['CRPIX3'] - 1) * hdr['CDELT3'] + hdr['CRVAL3']

Multichannel cubes use a nonlinear wavelength solution that cannot be described by FITS header keywords alone. These cubes use the WAVE-TAB convention in which the wavelength coordinates are stored in a FITS table extension.

Note that the wavelength solutions reported by the pipeline have already been corrected for spacecraft motion and are given in the barycentric vacuum rest frame.

Table 2. Estimated MRS wavelength calibration accuracy in km/s based on observations of Jupiter/Saturn (Channels 1A–3B) and FZ Tau (3C–4C)

Photometric calibration

The initial first-year photometric calibration of the MRS (as described by Argyriou et al. 2023) was based on commissioning observations of the A7Vm star HD 163466. These commissioning observations, however, used only a single star and had limited SNR (particularly in the longer wavelength bands). Continued observations of additional stars throughout the Cycle 1 observing program permitted an updated calibration (implemented in CRDS context jwst_1094.pmap in June 2023) using multiple stars. The primary flux calibrator for the reference files delivered to CRDS in June 2023 is the O9V star 10 Lac, which has the highest SNR of all standard stars observed to date. Flux calibration is achieved by comparing the pipeline-extracted 1-D spectra of 10 Lac against BOSZ stellar atmosphere models (Bohlin et al. 2017). Note that there is a nearly 10% global systematic uncertainty in the overall flux calibration due to differences in the calibration factors for various standard stars (Figure 4), driven largely by uncertainties in the CALSPEC models at mid-infrared wavelengths.

Since traditional flux calibration standard stars are faint, and long wavelengths can be biased by background subtraction systematics, the Channel 4A–4C flux calibration was updated in November 2023 (jwst_1146.pmap) to be based on bright red targets instead. These include the asteroid 515 Athalea and circumstellar disk SAO 206462, details of which will be provided in a forthcoming publication (Law et al, in prep).

The MRS long-wavelength photometric calibration is additionally complicated by a time-dependent evolution in the effective count rate registered by the instrument that was observed during the first year of operations (as announced by the JWST Observer). As illustrated by Figure 5, this loss is most pronounced in channel 4, reaching approximately 50% over the first year of operations in band 4C. The origin of this evolution is under investigation, but the effective loss at all wavelengths can be well described by an asymptotically decaying function of time. This time-dependent evolution is taken into account in JWST pipeline versions 1.11.0 and greater, and in the JWST Exposure Time Calculator released for Cycle 3.

Based on repeated observations of standard star HD 163466, the overall stochastic flux calibration uncertainty is below 0.5% in channels 1–3 and increases to 5% in Channel 4C (where the star becomes faint relative to the thermal background). This is likely a lower limit however, as observations using different numbers of groups/integrations in all channels can show systematic differences of 1%–2%. The MRS photometric calibration is thus most reliable below about 20 µm.  While the MRS long wavelength cutoff extends to 28.7 µm, the wavelength range longward of 27.9 µm is particularly uncertain as the effective system throughput at these wavelengths is extremely low. Further study and observations will permit improvements of the MRS long wavelength photometric calibration throughout future observing cycles.

Please also see the known Issues section on the 12 µm MRS spectral leak.

Point vs extended sources

As with ordinary imaging observations, the surface brightness in the MRS data cubes is a convolution of the intrinsic surface brightness profile of an object with the beam of the instrument. The surface brightness measured for extended sources will thus depend on the PSF, with larger PSF widths (e.g., at longer wavelengths) resulting in lower peak surface brightnesses.

In the limit of an infinitely-extended uniform slab, PSF losses outside of a given region will be exactly offset by gains into that region. In the limit of spatially unresolved point sources, spectral extraction must take into account the fraction of light lost outside a given aperture as a function of wavelength. The MRS photometric calibration is tied to such point source observations, and will give properly calibrated results in either of these two regimes so long as the pipeline aperture correction factors are applied to extracted point source spectra.

In the intermediate regime of finite-size extended objects such correction factors will be complex and scene-dependent. Users are therefore encouraged to forward model their data if precise spectrophotometric results are required, constructing a 3-D RA/Dec/wavelength model of their scene, convolving it with the wavelength-dependent MRS PSF, and dialing the brightness of the model until the PSF-convolved model is a good match to the observations.

References

Argyriou, I., et al. 2020, A&A, 641, A150 (MRS Fringing)
The nature of point source fringes in mid-infrared spectra acquired with the James Webb Space Telescope

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

Bohlin, R., et al. 2017, AJ, 153, 234 (BOSZ atmosphere models)
A New Stellar Atmosphere Grid and Comparisons with HST/STIS CALSPEC Flux Distributions

Harkett, J., et al. 2023, in prep

Kavanagh, P., et al. 2024, in prep (MRS Residual Fringing)

Law, D. R., et al. 2023, AJ, 166, 45 (MRS Cube Building)
A 3D Drizzle Algorithm for JWST and Practical Application to the MIRI Medium Resolution Spectrometer

Law, D., et al. 2024, in prep (MRS Flux Calibration)

Patapis, P., et al. 2023, A&A submitted (MRS Distortion)
Geometric distortion and astrometric calibration of the JWST MIRI Medium Resolution Spectrometer

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