NIRSpec Detector Subarrays

The 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 four 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 two 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 not included within 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 are described in the two relevant articles on wavelength ranges and gaps for FS and BOTS observing modes.



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, but some of the BOTS mode subarrays are just 512 pixels in spectral extent and do not sample reference pixels. Neither the SUB512 nor SUB512S subarrays include reference pixels.  Reference pixels are un-illuminated pixels that may be 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. It will become available in November 2018

In FS mode, the maximum exposure duration is 10,000 seconds 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 exposure time on the brightest objects (using the shortest integration time) that can be observed without saturating this subarray. This corresponds to an exposure containing the maximum number (65,535) of integrations with only one group of one frame (NRSRAPID 1 readout) per integration.

1  Bold italics font style is used to indicate parameters, parameter values, and/or special requirements that are set in the APT GUI.


Table 1. NIRSpec subarrays for the FS observing mode

Subarray nameApertures availableHeight (pixels)Width (pixels)Frame time (s)
FULL All FSs20482048

10.73677

14.58889 (IRS2)

ALLSLITS

All FSs,

option: S200A1 and S200A2 **

25620485.49400
SUBS200A1S200A16420481.55800
SUBS200A2S200A26420481.55800
SUBS200B1S200B16420481.55800
SUBS400A1S400A16420481.55800
SUB2048S1600A13220480.90200
SUB1024A*S1600A13210240.45144
SUB1024BS1600A13210240.45144
SUB512S1600A1325120.22616

FULL is not a subarray, but listed here for completion. Detectors are read with all four outputs, and IRS2 readout mode is available for full frame readouts.

SUB1024A captures the shorter wavelength portion of the spectrum on the NRS1 detector, and the longer wavelength portion of the spectrum on NRS2. Alternatively, SUB1024B captures the longer wavelength portion on NRS1 and the shorter wavelength portion on NRS2. The medium resolution (M) grating and PRISM spectra from the fixed slits map entirely to NRS1, but for the high resolution (H) gratings, the use of SUB1024B will enable capturing a (semi-)contiguous mid-wavelength portion of the spectrum across the detector gap.

* SUB1024A (available for the S1600A1 aperture only) should not be used with the PRISM since the spectra do not project to the region of the detector covered but this subarray.

** Automatically observing with both the S200A1 and S200A2 slits is an available option; this selection must use the ALLSLITS subarray or FULL.

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

Subarray name

Height
(pixels)

Width
(pixels)

Recomended
Disperser

Frame time
(s)

Maximum exposure duration, in seconds (or hours)

Comments
SUB2048322048any0.90200119,567.3 (33.21)Full spectrum range
SUB1024A‡*321024any, except PRISM0.4514460,512.4 (16.81)Short wavelength half of the spectrum
SUB1024B321024any0.4514460,512.4 (16.81)Long wavelength half of the spectrum
SUB51232512PRISM0.2261630,984.9 (8.6)Both detectors are read out but no illumination of detector NRS2
SUB512S16512PRISM0.1439220,205.7 (5.61)Both detectors are read out but no illumination of detector NRS2

All BOTS mode observations use the S1600A1 fixed slit. Maximum exposure durations are achieved with NRSRAPID readout and the maximum number of integrations/exposure (65535). These are only allowed after adding the time series observation (TSO) special requirement.

SUB1024A captures the shorter wavelength portion of the spectrum on the NRS1 detector, and the longer wavelength portion of the spectrum on NRS2. Alternatively, SUB1024B captures the longer wavelength portion on NRS1 and the shorter wavelength portion on NRS2. The medium resolution (M) grating and PRISM spectra for BOTS map entirely to NRS1. However, for the high resolution (H) gratings, the use of SUB1024B will enable capturing a (semi-)contiguous mid-wavelength portion of the spectrum across the detector gap.

SUB1024A should not be used with the PRISM since the spectra do not project to the region of the detector covered but this subarray.




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