Analysis shows that read noise in the MIRI SLOWR1 readout pattern is higher than expected. The new FASTGRPAVG8 readout pattern reduces the read noise by more than a factor of 2 relative to SLOWR1, which is significant for very faint sources, low-background observations, and high data volume and parallel programs. In addition, thermal transients when switching between FASTR1 and SLOWR1 cause artifacts in the data, which are not present when switching between FASTR1 and FASTGRPAVG8. Additional information about this change can be found in Kendrew et al. (2026) and the JWST User Documentation.

FASTGRPAVG8 should be used for all Cycle 6 proposals instead of SLOWR1. Existing programs using SLOWR1 will eventually be encouraged to switch to FASTGRPAVG8, unless they need to maintain consistency with existing exposures. The MIRI team can advise on readout pattern selection via the JWST Help Desk. This change will also be discussed in the upcoming JWebbinar 52 – Developments in Instrument Modes and Capabilities for JWST Cycle 6 taking place on August 20^th.