Last Updated Mar 24, 2017
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.
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
|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
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 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 Calculatorfor definitive estimates of performance in each observing mode.
Wide field slitless spectroscopy
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
|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.
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
Data calibration and analysis
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.
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)
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