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JWST NIRISS detector subarrays are designed to support imaging and spectroscopy observations of targets with a wide range of brightnesses. The smaller subarrays are configured to avoid the risk of saturation and offer high time resolution for bright target observations.

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Introduction

NIRISS offers four observing modes including imaging and spectroscopy that enables a variety of science observations (e.g, exoplanet transits, high-redshift galaxies, high resolution imaging of stellar sources, AGNs, etc). The NIRISS detector is configured for full-frame and subarray readout readout to enable observations of science targets with a wide range in brightnesses. 


Full-frame readout

In full-frame readout, the entire NIRISS detector array is read out through 4 parallel output channels, each reading a 512 × 2048 pixel region with its own readout amplifier. Full-frame readout is the only supported readout for the NIRISS WFSS and NIRSS imaging observing modes.


Subarray readout

NIRISS detector subarrays are read out faster and permit shorter integration times, thereby extending the dynamic range of NIRISS to bright sources. Subarrays also enable observations at high cadence which is particularly useful for science targets that require high time resolution (e.g; exoplanet transits, eclipsing binaries, etc). All subarrays use single channel readout. The subarrays are essential for the NIRISS SOSS and NIRISS AMI modes.

The frame time for a subarray depends on the size of the subarray. The frame times for the SOSS and AMI observing and target acquisition modes are summarized in Tables 1 and 2, respectively. The subarray sizes are given in (x,y) detector coordinates, where x is the slow read direction and y is the fast read direction. Both observing modes can also be observed with full-frame readout.


Single object slitless spectroscopy (SOSS)

Table 1. Available NIRISS SOSS subarray modes

Subarray ID

Subarray Size

(Pixels)

Frame Time

(ms)

Comments
SUBSTRIP9696 × 20482213

SOSS for bright targets

Samples only 1st order

SUBSTRIP256256 × 20485491

SOSS mode default

Samples orders 1-3

SUBTASOSS64 × 6446SOSS target acquisition

 


Aperture masking spectroscopy (AMI)

Table 2. Available NIRISS AMI subarray modes

Subarray ID

Subarray Size

(Pixels)

Frame Time

(ms)

Comments
SUB8080 × 8075AMI mode for bright targets
SUBTAAMI64 × 6446AMI target acquisition
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Related links

NIRISS Overview

NIRISS Observing modes

NIRISS Detector

NIRISS Detector Readout

NIRISS Detector Readout Patterns

NIRISS Operations

JWST User Documentation Home

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References

JWST technical documents

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Last updated

Published July 11, 2017


 

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Published March 02, 2017


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UpdatedJuly 11, 2017
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TeamNIRISS 
AuthorRavindranath, La Massa 
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UpdatedJuly 11, 2017
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