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|
1 The bright limit varies by +0.15 magnitudes for an A0V star to -0.10 magnitudes for an M5V star.
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.
|Filter||NGroups = 1||NGroups = 2|
The bright limits for Ngroups = 2 were calculated using the JWST ETC for an A0V star with no background and an aperture radius of 1". 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. Wideband filter point source imaging sensitivity for S/N = 10 in 10ks
† Vega magnitudes in NIRISS filters for which the colors of an average A0V star are 0.0.
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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
The JWST Fine Guidance Sensor (FGS) and Near-Infrared Imager and Slitless Spectrograph (NIRISS)
Doyon, R. JWST Community Webinar Series (2016 April 19)