Former MIRI detector patterns FAST and SLOW were replaced with FASTR1 and SLOWR1 for use in MIRI observations on June 26, 2021. This was done to enable an improved response to detector characteristics and operations. 

The FASTR1 and SLOWR1 MIRI detector readout patterns have been available since APT 2021.2 and ETC 1.6.1. 

The FAST detector readout is now only available for target acquisition. SLOW is no longer available to general observers.

These updated patterns have been designed to mitigate detector response anomalies in multiple integration data; the introduction of an additional reset between integrations improves the calibration and makes it possible to achieve flux accuracy requirements (see Figure 1). The additional reset results in slightly increased overheads and charged times for multiple integration exposures only. Since these are overheads and do not contribute to photon collection time, the ETC SNR results will remain unchanged. 

The FAST and SLOW readout patterns have been deprecated for science observations in favor of FASTR1 and SLOWR1, respectively (see Table 1). 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 Target Acquisition exposures, which always use single integrations, the readout patterns will remain unchanged (i.e., will use FAST and the FASTGRPAVG patterns).

Table 1. Readout pattern comparison

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