The JWST Near Infrared Imager and Slitless Spectrograph (NIRISS) provides observing modes for slitless spectroscopy, high-contrast interferometric imaging, and imaging, at wavelengths between 0.6 and 5.0 μm over a 2.2' x 2.2' FOV.

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Although NIRISS is packaged with the Fine Guidance Sensor (FGS), the 2 instruments are functionally independent of each other.

NIRISS imaging 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).

NIRISS wide-field grism spectroscopy 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).

Observational capabilities

When used in specific combinations, optical elements in the NIRISS pupil and filter wheel enable 4 observing modes:

Wide field slitless spectroscopy (WFSS) over the entire field of view, using one or both of a pair of identical grisms and a selection of blocking filters to isolate specific wavelength intervals between 0.8 and 2.2 μm. The grisms are mounted to disperse light in orthogonal directions on the detector.
Single object slitless spectroscopy (SOSS) with a cross-dispersed grism designed to deliver broad wavelength coverage and spectro-photometric stability, optimized for time-series observations (TSOs).
Aperture masking interferometry (AMI) through specific filters, enabled by a mask with 7 sub-apertures.
Imaging in 7 wide- and 5 medium-band filters that are closely matched to the NIRCam filter set between 0.9 and 5.0 μm.

Table 1. Specific properties of NIRISS observing modes

Observing mode	Wavelength coverage (μm)	Field of view (arcsec)	Pixel scale (arcsec/ pixel)	Resolving power *R = λ⁄Δλ*	FWHM	Comment
Wide field slitless spectroscopy (WFSS)	0.8–2.2	133 × 133	0.066"/pixel	150 @ 1.4 μm	...	Orthogonal dispersion orientations available
Single object slitless spectroscopy (SOSS)	0.6–2.8	...	0.066"/pixel	700 @ 1.4 μm	...	Subarrays are standard; full frame allowed
Aperture masking interferometry (AMI)	2.8–4.8	5.2 × 5.2	0.066"/pixel	...	...	Subarray is standard; full frame allowed
Imaging	0.8–5.0	133 × 133	0.066"/pixel	4–10	F380M filter and longer wavelength filters are Nyquist sampled	Full frame standard

Optical elements

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 (3 reflections)
a user-selected element in the pupil wheel
a user-selected element in the filter wheel
a camera (3 reflections)
a detector in the focal plane assembly

Schematic of NIRISS optical path — © Honeywell

Optical elements in the Pupil Wheel and Filter Wheel — 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 9 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

Observing mode	Allowed pupil wheel elements	Allowed filter wheel elements
Wide field slitless spectroscopy (WFSS)	F090W F115W F150W F200W F140M F158M	GR150C GR150R
Single object slitless spectroscopy (SOSS)	GR700XD	CLEAR
Aperture masking interferometry (AMI)	NRM	F277W F380M F430M F480M
Imaging (0.9 μm to 2.0 μm) Imaging (2.7 μm to 4.8 μm)	F090W F115W F150W F200W F140M F158M CLEARP	CLEAR F277W F356W F444W F380M F430M F480M

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 4 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)

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. Colors correspond to the different blocking filters: blue - F090W, orange - F115W, red - F150W, green - F140M , purple - F158M, brown - F200W.

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" (N_groups) for the subarrays available for use with SOSS.

Table 3. SOSS saturation limits for a G2 V spectrum

Subarray	Subarray size	Order	N_Groups	J mag (Vega)^†
SUBSTRIP256	256 × 2048	1	2	8.4
SUBSTRIP256	256 × 2048	2	2	6.3
SUBSTRIP96	96 × 2048	1	2	7.4

^†Reported magnitudes are simulated 2MASS J-band magnitudes. The bright limit varies as a function of wavelength and spectral type.

Aperture masking interferometry (AMI)

Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.

Table 4 lists the bright limits for AMI for the SUB80 subarray.

Table 4. AMI saturation limits in Vega magnitudes for an A0V type star.

Filter^†	Magnitude (Sirius) N_Groups=1	Magnitude (Sirius) N_Groups=2
F227W	6.7	7.4
F380M	3.9	4.7
F430M	3.3	4.0
F480M	3.0	3.7

Table notes:

The magnitude in NIRISS filters are equivalent to the previous "Vegamag" system where the CALSPEC Sirius model from Bohlin 2022 is used as a template for an A0V star with a magnitude of -1.395 in all filters (Rieke et al. 2022). Note that quoted bright limits will vary for different spectral types.

†The magnitudes in NIRISS filters F277W and F380M roughly correspond to the WISE W1 magnitude.The magnitudes in NIRISS filters F430M and F480M roughly correspond to the WISE W2 magnitude. There is a ±0.05 magnitude uncertainty due to the conversion from NIRISS magnitude to WISE magnitudes, which is a function of the spectral shape of the source. The magnitudes of the WISE and NIRISS filters should match for an average A0V star and WISE magnitudes are predicted to be slightly smaller than the NIRISS magnitudes for later spectral types.

Imaging

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. Filter point source imaging sensitivity for S/N = 10 in 10ks

Filter	Flux density (nJy)	Magnitude^† (Sirius)
Pupil Wheel Filters
F090W	15.9	27.9
F115W	14.1	27.8
F140M	19.2	27.1
F150W	12.8	27.4
F158M	17.8	27.0
F200W	11.3	27.1
Filter Wheel Filters
F227W	11.7	26.4
F356W	12.6	25.8
F380M	33.5	24.6
F430M	46.1	24.0
F444W	20.5	24.9
F480M	59.5	23.5

† The magnitude in NIRISS filters are equivalent to the previous "Vegamag" system where the CALSPEC Sirius model from Bohlin 2022 is used as a template for an A0V star with a magnitude of -1.395 in all filters (Rieke et al. 2022).

More about NIRISS

External NIRISS links and documents

STScI NIRISS website

CSA NIRISS website

NASA NIRISS website

Lectures

JWST Community Lecture Series - NIRISS Overview (R. Doyon)

Acknowledgements

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.

References

Doyon, R., et al. 2012, SPIE, 8442, 2RD
The JWST Fine Guidance Sensor (FGS) and Near-Infrared Imager and Slitless Spectrograph (NIRISS)

Doyon, R. JWST Community Webinar Series (2016 April 19)
NIRISS Overview

Kammerer, J., Cooper, R. A., Vandal., T., et al. 2022, arXiv:2210.17528
The Near Infrared Imager and Slitless Spectrograph for JWST – V. Kernel Phase Imaging and Data Analysis

Sivaramakrishnan, A., Tuthill, P., Loyd, J. P. et al. 2022, arXiv:2210.17434
The Near Infrared Imager and Slitless Spectrograph for the James Webb Space Telescopes – IV. Aperture Masking Interferometry

Willott, C. J., Doyon, R., Albert, L., et al. 2022, PASP, 134, 1032
The Near-Infrared Imager and Slitless Spectrograph for the James Webb Space Telescope. II. Wide Field Slitless Spectroscopy

Notable updates	17 Jun 2017 Updated Table 3, SOSS saturation limits 26 Jan 2017 Updated several tables and figures
Originally published	30 Dec 2016