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

See also: NIRISS Detector Readout

NIRISS offers 4 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 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. While the JWST Exposure Time Calculator offers subarray readout options for both the WFSS and imaging modes, these are only supported for engineering or calibration observations and not for science observations.

Subarray readout

See also: NIRISS Bright Limits

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). 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) science coordinates (i.e., the coordinate frame of the images retrieved from the MAST archive), where x is the slow read direction and y is the fast read direction. The subarray corner locations refer to the "bottom left" pixel of the subarray on the detector, and they are 1-indexed. The locations of the science subarrays on the detector are also depicted in the NIRISS Detector Readout article (see its Figure 1). The SOSS and AMI observing modes can also be observed with full frame readout.

Single object slitless spectroscopy (SOSS)

Table 1. NIRISS SOSS subarray modes

Note that for the SOSS observing mode, the location of the subarrays on the detector does not reflect the location of objects on the sky as they would in imaging mode, due to the optical power of the GR700XD grism used in the SOSS mode. This is explained in more detail in the NIRISS Apertures article. 

Aperture masking interferometry (AMI)

See also: NIRISS Aperture Masking Interferometry, NIRISS Target Acquisition

Table 2. NIRISS AMI subarray modes