JWST MIRI detectors can be read out using several different modes, each with their own advantages. They're associated with specific parameters used to define an observation.

See also: Understanding Exposure Times

The MIRI readout patterns fall within the framework of the general MULTIACCUM readout patterns adopted by the JWST mission so that all instruments will have similar exposure interfaces. MIRI offers 2 readout patterns for science observations:

1. SLOWR1 (N_samples = 9, t₁ = 23.890 s)
2. FASTR1 (N_samples = 1,  t₁ = 2.775 s)

where N_samples is the number of samples per pixel per frame and t₁ is the resulting frame time.

Target Acquisition exposures make use of a different set of readout patterns.

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Timing

In MULTIACCUM mode, an exposure consists of one or more identical integrations that are grouped together. For MIRI FASTR1 and SLOWR1 multiple integrations exposures also include an additional frame reset in between integrations. Based on the number of integrations, N_int, the exposure time is determined as follows: 

t_exp = (N_int x t_int) + (N_int - 1) × t₁ 

Each integration is a ramp composed of a number of groups. Unlike the other instruments, each MIRI group is limited to only one frame. The value of N_samples determines the time, t_1, between each group (i.e., frame) up the ramp. The value of N_groups determines the integration time, t_int, as follows: 

t_int = N_group × t₁

For example, 10 frames of FASTR1 mode yield a t_int = 10 × 2.775 = 27.75 s. An exposure with 10 frames and 5 integrations in FASTR1 mode will yield t_exp= (5 x 27.75) + (4 x 2.775) = 149.85 s.

The optimal combination of groups and integrations depends on the specific science case. The MIRI Best Practices article explains how to optimize the number of samples, groups, integrations, and exposures. 

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Readout scheme

See also: MIRI Detector Readout FASTR1, MIRI Detector Readout SLOWR1

The MIRI readout scheme for the sensor chip assembly (SCA) includes a “fast” direction (horizontal across the rows) and a “slow” direction (vertical along the columns). The detector has a total of 1024 × 1024 active pixels. There are 4 additional reference pixels at both the beginning and end of each row. All pixels are read out through 4 interleaved data outputs (i.e., 258 × 1024 pixels per output). The outputs are read simultaneously, resulting in a full frame readout in just under 3 s given the sampling rate of 10 μs per pixel. 

In a MIRI exposure, pixels are reset by row pairs (i.e., 2 rows, 2064 pixels, at a time). For example, row 1 will be read, then row 2 will be read, then they will be reset together, then row 3 will be read, etc. This approach enables a final read immediately before resetting the SCA.

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References

Ressler, M. E. et al. 2015, PASP, 127, 675
The Mid-Infrared Instrument for the James Webb Space Telescope, VIII: The MIRI Focal Plane System

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