September 29, 2023

About This Article

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.

MIRI MRS count rates
Figure 1: MRS relative count rate produced by internal calibration lamps as a function of time for each of the 12 MRS spectral bands. Filled symbols show measurements based on in-flight observations, while solid/dotted/dashed lines represent best-fit empirical models. Repeat observations of standard stars show the same trend.

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.

MIRI imager count rates
Figure 2: Imager relative count rates obtained using data from standard stars as a function of time for each of the nine imager filters. Most points correspond to observations of BD+60 1753. For the 15 microns and longer wavelength filters, observations from δUMi and HD 37962 have been used to fill in a time gap between the BD+60 1753 observations. The purple line represents an exponential fit to the data with a time constant τ . The amplitude (A) and standard deviation (σ) around the best fit are provided at the top left of each line.

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.


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The NASA James Webb Space Telescope, developed in partnership with ESA and CSA, is operated by AURA’s Space Telescope Science Institute.