The readout patterns for all MIRI science observations have been revised to mitigate certain detector response anomalies that significantly impacted the expected calibration accuracy for multi-integration exposures. These new readout patterns introduce an additional reset between integrations, which allows MIRI to meet its flux accuracy requirements.

The additional resets result in slightly increased overheads and charged time for observations using multi-integration exposures. Since the overheads do not contribute to photon collection time, sensitivity predictions remain unchanged in the ETC.

The change is implemented in APT 2021.2 and ETC 1.6.1, as the FAST and SLOW readout patterns are replaced by FASTR1 and SLOWR1, respectively. Target acquisition exposures will still use FAST, and the FASTGRPAVG patterns remain unchanged.

All science exposures from approved JWST Cycle 1 programs will automatically be updated to use FASTR1 and SLOWR1 when loaded in APT 2021.2 and programs will be adjusted in an appropriate manner.

For more detailed information, see the JDox article on MIRI readout patterns. The newly released ETC version 1.6.1 also includes an update to how MIRI integrations with less 5 groups are handled for consistency with the JWST pipeline. For a complete list of ETC changes, see the release notes.