Last Updated Mar 24, 2017

JWST's Near Infrared Imager and Slitless Spectrograph (NIRISS) provides slitless spectroscopy, high-contrast imaging and direct imaging over a 2.2' × 2.2' FOV from 0.6–5.2 μm.

Introduction

The JWST Near Infrared Imager and Slitless Spectrograph (NIRISS) provides observing modes for grism 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.

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 spectrophotometric stability.
Aperture masking interferometry (AMI) through specific filters that is 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 $// R = \lambda / \Delta\lambda //$	FWHM	Comment
Wide field slitless spectroscopy (WFSS)	0.8–2.2	133 × 133	0.065	150 @ 1.4 μm	...	Orthogonal dispersion orientations available
Single object slitless spectroscopy (SOSS)	0.6–2.8	...	0.065	700 @ 1.4 μm	...	Subarrays are standard; full-frame allowed
Aperture masking interferometry (AMI)	2.8–4.8	5.2 × 5.2	0.065	...	...	Subarray is standard; full-frame allowed
Imaging	0.8–5.0	133 × 133	0.065	4–10	2 pix @3.4 μm	Full-frame standard; only used in parallel mode

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 (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

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) can be read out in 45.5 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 Sensitivty (2016 Dec. Estimates) — 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

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

Subarray	Order	NGROUPS	J magnitude
256 × 2048	1	2	8.05
256 × 2048	2	2	6.75
96 × 2048	1	2	7.05
96 × 2048	1	1	6.35

Aperture masking interferometry (AMI)

The saturation limit for AMI corresponds to a magnitude of M ~ 4 with NGROUPS = 2.

Imaging

Table 4 lists the estimated point-source sensitivity for Imaging through broad-band filters. The limits are expressed as the limiting flux achieved with S/N = 10 in an integration of 10 ks.

Table 4. Estimated point-source sensitivities in broad-band filters

Filter	nJy
F090W	14.3
F115W	12.6
F150W	11.1
F200W	9.7
F277W	10.7
F356W	10.0
F444W	15.0

Data calibration and analysis

coming soon...

Acknowledgements

NIRISS is a contribution of 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

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