The optical path of NIRISS is illustrated schematically in Figure 2a. A solid-body representation of the instrument is shown in Figure 2b.
Light from the Optical Telescope Element of JWST is processed sequentially by
Four observing modes are enabled by specific combinations of the nine optical elements in the pupil and filter wheels. Allowed combinations are indicated in Table 2.
Table 2. Allowed combinations of optical elements for 4 observing modes
NIRISS has a single Teledyne H2RG detector with 2040 × 2040 pixels sensitive to light. The pixels, measuring 18 μm on a side, are made of HgCdTe with a composition tuned to provide a long wavelength cutoff near 5.2 μm. In its full frame format, the detector is read out non-destructively every 10.74 s through four readout channels. Subarray formats are available for most modes to decrease the readout time. The smallest subarray (64 × 64 pixels, used for target acquisition) can be read out in 50.16 ms.
Sensitivity and performance
Please consult the JWST Exposure Time Calculatorfor definitive estimates of performance in each observing mode.
Wide field slitless spectroscopy (WFSS)
Single object slitless spectroscopy (SOSS)
Table 3 lists the J-band magnitude for which saturation first occurs in the specified order, with the specified number of samples "up the ramp" (Ngroups) for the subarrays available for use with SOSS.
Table 3. SOSS saturation limits for a G2 V spectrum
Aperture masking interferometry (AMI)
Table 4 lists the bright limits for AMI for the SUB80 1 subarray.
Table 4. AMI saturation limits in Vega magnitudes for an A0V type star.
Table 5 lists the estimated point source sensitivity for imaging through broadband filters. The limits are expressed as the limiting flux achieved with S/N = 10 in an integration of 10 ks.
Table 5. Wideband filter point source imaging sensitivity for S/N = 10 in 10ks
NIRISS is a contribution by the Canadian Space Agency to the JWST Project. The Principal Investigator of NIRISS is Professor René Doyon of the Université de Montréal. Honeywell International designed and built the instrument, with additional technical support from the National Research Council of Canada.
Doyon, R., et al. 2012, SPIE, 8442, 2RD
Doyon, R. JWST Community Webinar Series (2016 April 19)