MIRI Detector Readout Overview
See also: Understanding Exposure Times
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where Nsamples 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.
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, Nint, the exposure time is determined as follows:
texp = (Nint x tint) + (Nint - 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 Nsamples determines the time, t1, between each group (i.e., frame) up the ramp. The value of Ngroups determines the integration time, tint, as follows:
tint = Ngroup × t1
For example, 10 frames of FASTR1 mode yield a tint = 10 × 2.775 = 27.75 s. An exposure with 10 frames and 5 integrations in FASTR1 mode will yield texp= (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.
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.
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