September 19, 2023

About This Article

The MIRI Team has delivered significant updates to the Imaging and Coronagraphic Imaging flux calibration reference files. The updates are the result of continued analysis of calibration observations and improvements in code and algorithms.

The flux calibration is now based on a larger sample of stars with well-characterized total fluxes. By using observations of different types of stars, the impact of uncertainties in the model spectra for any one type of star is reduced. In addition, the analysis of the calibration observations benefited from improved aperture corrections. Aperture corrections are used to correct finite measurement aperture to infinite apertures fluxes, as the model predictions are for the total stellar flux densities.

The magnitude of the changes to the calibration factors compared with those derived from the commissioning data varies per filter, and can be significant. For MIRI Imaging the calibration factors are 1% to 8% lower than the existing values, except for the F560W and F770W filters. For the F560W and F770W filters, the calibration factors are 15% and 21% lower, respectively. These larger changes are mostly due to an improved accounting for the detector scattering effect in the aperture corrections. This effect, also known as the “cruciform artifact” (see Gáspár et al 2021), predominantly affects MIRI’s shortest-wavelength filters F560W and F770W.

In the Coronagraphic Imaging filters, the calibration factors are 66%, 57%, 73%, and 56% higher than the existing factors for the F1065C, F1140C, F1550C, and F2300C filters, respectively. These significant changes are mainly due to a better understanding of how to use the WebbPSF tool to determine the finite to infinite aperture corrections, and applying it to all data taken since commissioning.

Finally, the new reference files contain a correction to the calibration factors for the time-dependent count rate loss reported on August 24, 2023. For each filter, a set of reference files is now available, each representing the appropriate flux calibration for a particular epoch. To apply the correction, the JWST calibration pipeline will match the date of execution of an exposure to select the correct reference file. In the longer term, this time-dependent correction will be performed by the pipeline, as already implemented for the MIRI MRS. Automatic reprocessing of all MIRI imaging and coronagraphic data will begin promptly and will take about 3 to 6 weeks. Please use the MAST subscription service to receive notifications of when the data has been reprocessed. Users can also reprocess their own data using the updated PHOTOM and APCORR reference files in CRDS.

With this update, the accuracy is 3% or better in all filters based on the 1-sigma scatter of the measured flux calibration stars. For details of the stars observed for flux calibration, see Gordon et al. (2022, AJ, 163, 267). This updated MIRI Imager and Coronagraphic flux calibration will be reported in Gordon et al. (in prep).

For further questions or support with these new flux calibration reference files, users are encouraged to contact the MIRI team via the JWST Help Desk. Please note that STScI welcomes help desk tickets as they provide us with better insight into the difficulties users encounter and how they impact researchers. To keep up to date with new reference file deliveries, users can also sign up for the reference file mailing lists to receive email notifications and additional information when new reference files are delivered.

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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.