New And Improved James Webb Space Telescope MIRI MRS Photometric Calibration Now Available

The MIRI team has updated the photometric calibration reference files for the Medium Resolution Spectrometer (MRS), effective from CRDS context version 1263. This update reflects a more comprehensive analysis of photometric calibrator stars observed over the course of JWST Cycles 1 and 2.

The previous calibration relied on O9V star 10 Lac as the photometric calibrator at wavelengths below 18 µm. The updated approach now uses the average normalization of 10 different standard stars of O, A, and G spectral types.

The primary effect of this update is to make all calibrations brighter by 2% at short wavelengths and 5% at long wavelengths; this update brings the agreement between the MRS and MIRI Imaging modes to within 1% in the 4.9 to 18 µm wavelength range.

At longer wavelengths (MRS Channel 4), these calibrator stars do not provide sufficient signal-to-noise to derive a reliable photometric calibration; beyond 18 µm the calibration remains tied to an infrared-bright asteroid and a young stellar object, with absolute normalization joining the stellar calibrators at the Ch3/Ch4 boundary.

A detailed description of these changes, and the overall approach to photometric calibration for MIRI MRS, is provided by Law et al. 2024.

Users are encouraged to contact the MIRI team via the JWST Help Desk with questions or concerns about these changes.

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