MIRI Imaging Calibration Status

The overall calibration status and estimated accuracy of the MIRI imager are described in this article; please also see the article on known issues affecting MIRI Imaging data.

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Astrometric calibration

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The absolute astrometric accuracy can vary from 0.1 to 0.2 arcsec (pointing without target acquisition) to 2 arcsec (guide star issues). If the absolute astrometry is refined by means of the Gaia catalog and the TweakReg step of the stage 3 pipeline, the geometric distortion uncertainties (standard deviation of the positional residuals) can be 0.01–0.03 arcsec, depending on the filter. The astrometric accuracy and precision get worse from short to long wavelengths.

Photometric calibration

The MIRI imager experiences a time-dependent reduction in observed count rates at wavelengths longward of F1280W; this count rate loss is similar to the MRS, but of a significantly smaller magnitude. As indicated in Table 1, the observed reduction between the JWST commissioning period and July 2023 was about 18% at the longest wavelengths. Investigations are ongoing for the impact in the coronagraphic filters. The shorter wavelength filters (F560W, F770W, F1000W, F1130W) do not show a detectable effect.

Table 1. MIRI imager count rate reduction between the JWST commissioning period and July 2023 

FilterReduction in
count rate

The root cause of this issue is still under investigation, with a combined focus on the 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. As illustrated in Figure 1, the overall rate of loss is decreasing with time, and additional losses from July 2023 to January 2024 are minimal.

Figure 1. MIRI imager count rate loss

Click on the figure for a larger view.

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.

Users wishing to propose observations 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, adding margins of 5% on the SNR is recommended to mitigate for the observed loss.

To compensate for this count rate loss in the JWST calibration pipeline, as of Fall 2023, this loss is taken into account in the pipeline using a model of the evolving response function.  After this correction, uncertainties are of order 3% or better at all wavelengths, due to variations in measured values from one standard star to another. The corrected telescope/instrument stability are better than 0.5% over time based on repeated measurements of one star in F770W.

Notable updates
Originally published