All 17 observing modes of JWST undergo regular performance monitoring. Analysis of MIRI Medium Resolution Spectrometer (MRS) flux calibration data has revealed that the throughput at the longest wavelengths (primarily channel 4) has decreased since commissioning. No effect has been seen for MIRI imaging. There is no risk to the instrument and its science operations will remain unchanged. 

A team of experts is investigating to find the root cause of the effect, and to identify possible mitigating actions. STScI will examine whether the science objectives of any past or planned observations, including Cycle 2 proposals (after selection), can not be met, and whether appropriate modifications are possible. Affected observers will be informed if modifications to their programs are needed.