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.
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.
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
|SUBSTRIP96||96 × 2048||2213|
SOSS for bright targets
Samples only 1st order
|SUBSTRIP256||256 × 2048||5491|
SOSS mode default
Samples orders 1-3
|SUBTASOSS||64 × 64||46||SOSS target acquisition|
Aperture masking spectroscopy (AMI)
Table 2. Available NIRISS AMI subarray modes
|SUB80||80 × 80||75||AMI mode for bright targets|
|SUBTAAMI||64 × 64||46||AMI target acquisition|