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.
Figure 1a. 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
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).
|When used in specific combinations, optical elements in the NIRISS pupil and filter wheel enable 4 observing modes:|
Table 1. Specific properties of NIRISS observing modes
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
Figure 2b. Solid-body representation of NIRISS
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.
Figure 3. NIRISS pupil and filter wheels
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.
Table 2. Allowed combinations of optical elements for 4 observing modes
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
Figure 4. Estimated sensitivity for WFSS
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
|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