JWST Near Infrared Imager and Slitless Spectrograph
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 over a 2.2' x 2.2' FOV.
Although NIRISS is packaged with the Fine Guidance Sensor (FGS), the two instruments are functionally independent of each other.
When used in specific combinations, optical elements in the NIRISS pupil and filter wheel enable four 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 that is enabled by a mask with seven sub-apertures.
- Imaging in seven wide- and five 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
|0.8–2.2||133 × 133||0.065||150 @ 1.4 μm||...|
|...||0.065||700 @ 1.4 μm||...||Subarrays are standard; |
|Aperture masking |
|5.2 × 5.2||0.065||...||...||Subarray is standard; |
|Imaging||0.8–5.0||133 × 133||0.065||4–10||2 pix @3.4 μm||Full-frame standard; |
only used in parallel mode
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
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
|Single object slitless spectroscopy (SOSS)||GR700XD||CLEAR|
Imaging (0.9 μm to 2.0 μm)
Imaging (2.7 μm to 4.8 μm)
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
|Subarray||Subarray size||Order||NGroups||J mag (Vega)†|
|SUBSTRIP256||256 × 2048||1||2|
|SUBSTRIP256||256 × 2048||2||2|
|SUBSTRIP96||96 × 2048||1||2|
† Reported magnitudes are simulated 2MASS J-band magnitudes. The bright limit varies as a function of wavelength and spectral type (see Figure 1).
Aperture masking interferometry (AMI)
Words in bold italics are buttons
or parameters in GUI tools. Bold
style represents GUI menus/
panels & data software packages.
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.
Magnitude in NIRISS filters assume an average A0V star has the same magnitude in all filters, and uses the Bohlin Sirius model (2020) as a template for this average star at magnitude -1.401. 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.
‡JWST ETC does not support Ngroups = 1, so the detected flux from the Ngroups = 2 calculation was scaled to estimate the bright limit for Ngroups = 1.
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
|Pupil Wheel Filters|
|Filter Wheel Filters|
† Magnitude in NIRISS filters assuming an average A0V star has the same magnitude in all filters and using the Bohlin Sirius model (2020) as a template for this average star at magnitude -1.401.
More about NIRISS
External NIRISS links and documents
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.
Bohlin, R. C., Hubeny, I, & Rauch, T. 2020, AJ, 160, 21
New Grids of Pure-hydrogen White Dwarf NLTE Model Atmospheres and the HST/STIS Flux Calibration
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)