MIRI Detector Readout SLOWR1

JWST MIRI's SLOWR1 readout pattern mode offers slightly lower detector noise than the FASTR1, making it a good choice for faint source medium-resolution spectroscopy where the sky backgrounds are very low.

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For all MIRI science observations, FASTR1 is the default readout pattern; SLOWR1 is highly recommended for reducing data volume and rate (e.g., in parallel observations and MRS mosaics).

Long MULTIACCUM exposures can maximize signal to noise in detector noise-dominated regimes. At least 5 groups in SLOWR1 are recommended to optimize the calibration procedure, particularly for cosmic ray removal; in fact, ngroups > 10 is recommended.

Following MIRI detector readout properties, SLOWR1 mode samples the pixel 8 times within a 9 sample wide window (Nsample = 9; t1 = 23.890 s). The first sample is ignored and the remaining 8 samples are averaged to output a single result before being returned to the ICDH (ISIM Command and Data Handling). Nsample = 9 is a fixed parameter in SLOWR1 mode and cannot be altered by the observer. The user can alter the number of groups (Ngroups) and integrations (Nint). All groups will be stored and downloaded.

Pixel perspective 

While not integrating, the MIRI detectors continue to be clocked but the pixels in the MIRI detectors are reset every frame. Once the exposure starts, the SLOWR1 readout pattern executes Ngroups − 1 read-only non-destructive frames. The focal plane electronics (FPE) dwells at each pixel for Nsample = 9 (90 µs in total). However, the FPE will only record the final 8 measurements; this allows ample pixel settling time during the skipped sample. Those 8 samples will be averaged in the FPE signal chain and reduced to 16 bits before being sent to the ICDH. The time between sampling an individual pixel is t= 23.890 s because the entire sensor chip assembly (SCA) is read before returning to a particular pixel. The final group of the integration is a read-reset, yielding the total of Ngroups reads. In multiple integration data the read-reset is followed by an additional reset frame that takes 23.890 s. This integration timing pattern may be repeated Nint times during an exposure, with the last integration ending in a read-reset. The duration of a single exposure in SLOWR1 will therefore be:

Exposure_duration = (Ngroups × 23.890 s × Nints) + (23.890 s × (Nints - 1))

Figure 1. SLOWR1 mode sampling pattern from the output amplifier point of view

The horizontal axis is time and the vertical axis is pixel signal strength. Each signal level corresponds to a different pixel. The pixel signal levels chosen here are just for illustrative purposes, but the horizontal line highlights that the values are assumed to be relatively constant over the 80 µs of the sampling. Pixel sampling is represented by small blue lines. Note that the FPE dwells at each pixel for 9 samples, and will only record the final 8 to allow for ample settling time. The time between samples is 10 µs.

Notable updates
    Updated to replace SLOWR with SLOWR1 readout pattern. 
    Figure updated.
Originally published