As reported in previous JWST Observer articles, both the MIRI Imager and Medium Resolution Spectrometer (MRS) are experiencing a wavelength-dependent decline in the observed count rate over time. The root cause of this loss is under investigation and, using observations of both internal and external calibration sources taken over the last 18 months, it has been possible to characterize the decline in detail.

As illustrated in Figure 1, for the MIRI MRS the count rate loss is most pronounced in Channel 4, reaching an effective count rate loss of approximately 50% at a wavelength of 26 microns over the first year of operations. Empirically, this loss can be well described by an exponentially decaying function of time and such a model is currently incorporated into both the JWST calibration pipeline (versions 1.11.0 and greater) and the Cycle 3 Exposure Time Calculator. Projecting this model forward into future cycles indicates that wavelengths longer than 25 microns in the MRS (channel 4C) may experience an additional loss of about 10% in the total count rate relative to commissioning over the next four years.

Observations indicate that the loss of count rate in the MIRI Imager is significantly smaller than in the MIRI MRS, but appears to be following a similar exponential decay trend (see Figure 2). At 25.5 microns, the count rate loss over the first year of operations has been about 21%; additional data is required to confidently estimate future performance. The Cycle 3 ETC does not incorporate the count rate loss observed in the MIRI imager. Observers planning to submit Cycle 3 proposals should add margins of 5% to the SNR derived in the F2100W and F2550W filters calculations to mitigate for the observed loss.

The MIRI team will continue to monitor instrument performance regularly, report on trending as needed, and update calibration reference files as necessary. Observers who are concerned about upcoming science programs or proposal preparations should contact the MIRI team via the JWST Help Desk.