Comparison of Old and New MIRI Detector Readout Patterns

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

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The FASTR1 and SLOWR1 MIRI detector readout patterns are now available in 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 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).

Figure 1. Up-the-ramp readout scheme for MIRI detectors

Left: MIRI FAST and SLOW readout patterns.
Right: MIRI FASTR1 and SLOWR1 readout patterns.

Table 1. Readout pattern comparison

Readout patternAvailable for science?Group time (FULL Array)Exposure time
FASTNo, available only for TA

2.775 s

Ngroups × Group_time × Nints

2.775 s

(Ngroups × Group_time × Nints) + (Group_time × (Nints-1) )

23.890 s

Ngroups × Group_time × Nints

23.890 s

(Ngroups × Group_time × Nints) + (Group_time × (Nints-1))

Assuming a single exposure with Ngroups and Nints. Consistent with the exposure time reported by APT and ETC.

Exposure time consistent with that reported in APT2021.2, ETC 1.6.1 and future versions.

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