The MIRI Team has implemented an improved correction to the long-wavelength count rate loss issue, available in the latest version of the JWST Calibration Pipeline (1.13.4). The previously provided correction, in September 2023, relied on a set of photometric calibration reference files, each applicable to a specific time range since the end of the commissioning period. This calibration was applied in discrete temporal steps based on the date of the observation.

In the latest pipeline release, the correction follows a continuous curve implemented via the analytical form of the loss profile for each Imager filter (see Figure 1), in combination with a single calibration file, mirroring the approach taken for the MIRI MRS photometric calibration. The change from discrete temporally-stepped correction factors to a smoothly varying function may result in changes in the imaging flux densities. This reflects the way in which the calibration is applied to the data, and for data taken since the end of commissioning this change should be no more than ~1%.

Reprocessing of archive data will take about 3-6 weeks; the MAST subscription service can be used to receive notifications of when the data has been reprocessed. Users can also reprocess their own data using the latest pipeline software and reference files. Full information about JWST pipeline builds can be found in JDox on the Operations Pipeline Build pages.

The MIRI Team continues to monitor the imager count rates through regular observations of flux calibrator stars. Figure 1 shows the latest measurement results in each filter; while some variances on the loss fractions have been noted since the correction was first implemented in September 2023, there is no evidence of any significant change in the observed trend (i.e. the behavior shows indications of stabilization). The most recent points in this figure represent data taken in early March 2024. Investigations into the root cause of the problem remain ongoing.

For further questions or support with this update, users are encouraged to reach out to the JWST Help Desk using the MIRI card. Users can also sign up for the reference file mailing lists to receive email notifications and additional information when new reference files are delivered.