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.
Figure 1 illustrates the MIRI readout scheme for the sensor chip assembly (SCA), where the “fast” direction of the readout is horizontal (across the rows) and the “slow” direction is 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. (Note that there is an additional "reference output" corresponding to blind pixels that are sampled continuously for various engineering and data quality purposes. Since this signal also appears as a 258 × 1024 pixel data stream that is interleaved with the 4 data channels, the SCA effectively presents a 1290 × 1024 pixel “image” to the outside world.)
MIRI offers two readout modes:
Figure 2 provides the general schematic for the MIRI timing pattern, which is defined by only 3 of the MULTIACCUM parameters:
nsample is the number of samples per pixel (for MIRI, this will either be 1 for FASTMode, or 10 for SLOWMode)
ngroups is the number of groups during an integration, where a group is the product of cycling through all the pixels
nint is the number of integrations during an exposure, where integration is defined as the time between resets.
By definition, for MIRI, there is exactly one frame per group. The value of nsample (the readout mode parameter given above) determines the time between frames, t1. The value of ngroup determines the integration time, tint, as follows: tint = ngroup × t1.
For example, 10 frames of FASTMode yield a tint = 10 × 2.775 = 27.75 s. An exposure consists of one or more identical integrations that are grouped together. The value for nint determines the exposure time as follows: texp = nint × tint. For instance, exposing for 5 integrations with a tint = 27.75 s, then texp = 138.75 s and during this exposure time there were 5 resets of the array.
In general, every exposure begins with a read-reset. The 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. The column and row shift registers must be clocked through the entire SCA before returning to a given odd-numbered row.
This approach enables a final read immediately before resetting the SCA, and thus captures the longest possible integration time. The disadvantage of this approach is that one cannot read the values immediately after reset, and thus the reset level (as in a traditional correlated double sample) cannot be obtained. All of the frames are passed through the ISIM Command and Data Handling (ICDH; ISIM is the Integrated Science Instrument Module), undergo lossless compression, and are stored in the solid-state recorder (SSR) for downlinking to the ground for processing.