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 0.6–5.0 μm.


Introduction


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



Figure 1a. NIRISS imaging mode field of view highlighted in the JWST focal plane

NIRISS imaging mode field of view highlighted in the JWST focal plane

 

Figure 1b. NIRISS wide field grism spectroscopy (WFSS) mode field of view highlighted in the JWST focal plane

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 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

Observing
mode

Wavelength
coverage (μm)

Field of
view
(arcsec)

Pixel scale
(arcsec/
pixel)

Resolving power
R = λ⁄Δλ

FWHMComment

Wide field
slitless spectroscopy 
(WFSS)
 

0.8–2.2133 × 1330.065150 @ 1.4 μm
...

Orthogonal dispersion
orientations available

Single object
slitless
spectroscopy
(SOSS)

0.6–2.8

...0.065700 @ 1.4 μm...Subarrays are standard;
full-frame allowed
Aperture masking
interferometry 
(AMI)

2.8–4.8

5.2 × 5.20.065......Subarray is standard;
full-frame allowed
Imaging0.8–5.0133 × 1330.0654–102 pix @3.4  μmFull-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



Figure 2a. Schematic of NIRISS optical path

Schematic of NIRISS optical path

 

Figure 2b. Solid-body representation of NIRISS

 



Figure 3. NIRISS pupil and filter wheels

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 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

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)GR700XDCLEAR

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 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 Calculator for definitive estimates of performance in each observing mode.

Wide field slitless spectroscopy (WFSS)



Figure 4.  Estimated sensitivity for WFSS

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 (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

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.

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

 


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

JWST technical documents





Last updated

June 17, 2017

  • Updated Table 3, SOSS saturation limits


January 26, 2017

  • Updated several tables and figures 


Published December 30, 2016


 

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Published March 02, 2017