On April 21, 2023, the JWST team announced that the MIRI Medium Resolution Spectrometer (MRS) exhibited a reduced count rate. The initial analysis of the MIRI imager did not show a similar effect. At that point, however, the available calibration data were not sufficient for a robust measurement. As part of the MRS investigation, additional monitoring observations were taken with the MIRI imager. In combination with earlier calibration data, these new calibrations have revealed a reduced count rate in the long-wavelength filters of the MIRI Imager. The effect in the Imager is at a lower level than the effect in the MRS.

The data show a measurable reduction in count rates from the F1280W filter onwards, with the effect increasing with wavelength. The observed reduction per filter between the JWST commissioning period and July 2023 is shown in the table below. Investigations are ongoing for the impact in the coronagraphic filters. The shorter wavelength filters (F560W, F770W, F1000W, F1130W) do not show a detectable effect.

Users wishing to propose observations for Cycle 3 in filters F560W through F1800W can assume the SNR calculations in the ETC are sufficiently accurate (nominally, the SNR goes as the square root of the count rate). For the F2100W and F2550W filters, we recommend adding margins of 5% on the SNR to mitigate for the observed loss.

Filter	Reduction in count rate
F1280W	3%
F1500W	3%
F1800W	8%
F2100W	12%
F2550W	18%

 

The root cause of this issue is still under investigation, with a combined focus on MRS and Imager. Regular monitoring observations are being taken with the MIRI imager to continue measuring the photometric response, and to characterize the temporal trend alongside what has been observed in the MRS. The JWST team is working to improve the temporal trending to evaluate if the imager loss rate is stabilizing, as is observed in the MRS data.

To compensate for this count rate loss in the JWST calibration pipeline, the MIRI team will update the calibration reference files and the pipeline to include a time-dependent correction to the flux calibration. These changes are expected to be operational in mid-autumn 2023. A similar correction has already been implemented for the MIRI MRS. Users are encouraged to sign up to the reference files mailing list to keep informed of reference file changes.

MIRI science operations will remain unchanged at this time. Observers who are concerned about their recent or planned science programs using the Imager long wavelength filters should contact the MIRI team via the JWST Help Desk.