|MultiExcerptName||NIRISS Overview 1|
The JWST Near Infrared Imager and Slitless Spectrograph (NIRISS) provides observing modes for slitless spectroscopy, high-contrast interferometric imaging, and imaging, over wavelengths between 0.6 and 5.0 μm. Although NIRISS is packaged with the Fine Guidance Sensor (FGS), the two instruments are functionally independent of each other.
Figure 1a. NIRISS imaging mode field of view highlighted in the JWST focal plane
Figure 1b. NIRISS wide field grism spectroscopy (WFSS) mode field of view highlighted in the JWST focal plane
The 2.2' × 2.2' NIRISS field-of-view location in the JWST focal plane is highlighted in Figures 1a and 1b, which shows representative scenes for imaging (Figure 1a) and wide field grism spectroscopy (Figure 1b).
|MultiExcerptName||NIRISS Overview 2|
|When used in specific combinations, optical elements in the NIRISS pupil and filter wheel enable four observing modes:|
Table 1. Specific properties of NIRISS observing modes
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
- a pick-off mirror
- a collimator (three reflections)
- a user-selected element in the pupil wheel
- a user-selected element in the filter wheel
- a camera (three reflections)
- a detector in the focal plane assembly
Figure 2a. Schematic of NIRISS optical path
Figure 2b. Solid-body representation of NIRISS
Figure 3. NIRISS pupil and filter wheels
The pupil and filter wheels each contain 9 optical elements, which consist of 3 grisms, one aperture mask, 12 bandpass filters, and 2 "clear" holes.
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 Calculator for definitive estimates of performance in each observing mode.
Wide field slitless spectroscopy (WFSS)
Figure 4. Estimated sensitivity for WFSS
Estimated sensitivity for the WFSS mode of NIRISS, expressed as the limiting flux for an unresolved spectral line that is achieved with S/N = 10 in an integration of 10 ks.
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
|PageWithExcerpt||NIRISS Bright Limits|
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.
|PageWithExcerpt||NIRISS Bright Limits|
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
|PageWithExcerpt||MR:APT GUI footnote|
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.