New and improved calibrations for both the MIRI Low-Resolution Spectrometer (LRS) slit and the slitless subarray are now available for JWST pipeline context jwst_1295.pmap and later. Changes include updates to the specwcs, apcorr and photom reference files for the LRS in the slitless mode, and updated photom and apcorr reference files for the fixed-slit mode.

For slitless, the new wavelength calibration replaces the values from Commissioning. Figure 1 shows the measured offsets and the fitted polynomial used to modify the wavelength solution. The changes are less than 20 nanometers (nm) between about 7 and 12.5 µm, but they increase up to 80 and 175 nm at 6 and 5 µm, respectively. The estimated uncertainty is less than 10 nm over most of the LRS spectral range. The uncertainties are larger below 6 and beyond 13.2 µm, because the solution is extrapolated at both ends. The slitless calibration also includes a new aperture correction based on a super-sampled empirical point spread function (PSF). The new wavelength calibration and the update to the aperture correction required a new photometric correction.

For the LRS fixed slit, the new aperture correction also uses an empirical PSF, and the photometric correction removes the previously imprinted fixed-pattern noise that limited the signal-to-noise ratio (S/N) of calibrated spectra to roughly 50 across the wavelength range.

The new flux calibration for both the LRS slit and slitless modes have an overall uncertainty of about 1%. In addition, the repeatability of the shape (or tilt) of the spectra is accurate to about 1%, for both slit and slitless. Users with questions are encouraged to contact the MIRI team via the JWST Help Desk.