JWST NIRSpec detectors support subarrays in the fixed slit (FS) and bright object time-series (BOTS) observing modes.

The NIRSpec detectors (sensor chip assemblies; SCAs) have a subarray readout mode that reads a contiguous rectangular subset of pixels from each SCA. In this mode, a single output channel is used instead of the usual 4 output channels in full frame readout mode.

Subarrays are only offered in the fixed slit (FS) and bright object time-series (BOTS) templates with NIRSpec, and can only be read out using the NIRSpec traditional readout, not the IRS² noise reduction mode. Only one rectangular subarray can be used per SCA, and its size must be identical for both SCAs. Each dimension of the subarray must be a power of 2 that is larger than 8 (e.g., 32 × 1024 pixels) and the subarrays must contain at least 1024 pixels.

During NIRSpec subarray exposures, the detector area outside the subarray pixel range may still be exposed to light, either through the other fixed slit apertures or from failed-open micro-shutters. 

The wavelength coverage for science using NIRSpec subarrays is described in the relevant articles on wavelength ranges and gaps for FS and BOTS observing modes.

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NIRSpec subarrays in FS and BOTS modes

NIRSpec subarrays offer shorter frame times than the full frame readout modes, and can therefore be used to observe brighter science sources before pixel saturation occurs. Subarray sizes optimized for the supported FSs in the FS and BOTS science modes are listed in Tables 1 and 2. The subarrays are automatically adjusted in pixel position for the grating-filter optical configurations used for a given exposure, to ensure that science spectra lie within the subarray detector region. The conversion gain for subarrays is about a factor of 1.43 higher compared to full frame data for both SCAs, resulting in a saturation limit of ~77,000 e^–. This provides a greater dynamic range for observing bright sources. The ETC imposes a slightly more constrained saturation limit of ~65,000 e^– for the subarrays.

The NIRSpec FS mode uses the matched detector subarrays, which should be used for targets that would saturate within the minimum full frame readout time of 10.737 s. BOTS mode observations are defined for bright sources, so full frame readout is not supported; observations should use the subarray that best matches the desired spectral coverage and avoids target saturation. Most subarrays used in FS mode include reference pixels; these are un-illuminated pixels that are used in pipeline processing to provide an improved estimate of the noise characteristics of the detector. The reference pixel correction for subarray data has been implemented for pipeline calibration. 

However, the BOTS mode subarrays SUB512 nor SUB512S are just 512 pixels in spectral extent and do not sample any reference pixels, thus the reference pixel correction is not possible in these cases. The only exception is the BOTS prism multistripe subarrays, in which 4 reference pixels are interleaved at the start of each integration (stripe). The reference pixel correction step can therefore be applied to reductions of prism multistripe observations.  

In FS mode, the maximum exposure duration is 10,000 s to allow for re-adjustment of the high gain antenna (HGA) to maintain communications. In BOTS mode, the limit on the maximum exposure duration has been extended to ride out any HGA moves. Table 2, column 6, shows the longest possible BOTS mode exposure time using the shortest available integration time with each subarray. This corresponds to an exposure containing the maximum number (65,535) of integrations with only one group of one frame (NRSRAPID readout) per integration.

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Multistripe subarrays

Beginning in Cycle 6, NIRSpec BOTS will offer 4 new "multistripe" PRISM subarrays (SUB256M2_PRISM, SUB128M4_PRISM, SUB64M8_PRISM, SUB32M16_PRISM). These subarrays are distinct as they only read out a portion (or "stripe") of the full subarray footprint in any given frame, but are still able to cover nearly the same detector footprint as the standard SUB512 subarray by sequentially stepping the stripe across the detector after each integration is complete. The sizes of the individual stripes for each subarray are indicated by the numerical value in their parameter names, and the number of stripes for each subarray is defined such that almost all of the pixels in the standard SUB512 subarray are read out, sacrificing some columns for the interleaved reference pixels. This slight loss of coverage in the red is described in Table 2 of NIRSpec BOTS Wavelength Ranges and Gaps. The key trade-off of these multistripe readouts is that the frame time can be dramatically reduced, improving the achievable bright limit, at the expense of an overall reduction in exposure efficiency due to only a portion of the full subarray footprint being read out at a given time. 

For more information on the functionality of multistripe, see JWST Multistripe, Superstripe, and Substripe Detector Modes. 

Table 1. NIRSpec subarrays for the FS observing mode

In FS mode, the subarrays are matched to the FS aperture used for science, and will sample all spectra measured on the detectors. Some smaller subarrays are available for the S1600A1 aperture only (see Table 2) to observe brighter targets, particularly in the BOTS observing mode.

Table 2. Subarrays and exposure parameters for BOTS mode

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