NASA's James Webb Space Telescope

Spectroscopic Modes

0.4 0.5 0.6 0.7 0.8 0.9 1 1.5 2 3 4 Wavelength (µm) 5 6 7 8 9 10 15 20 30 B V R I J H K L NIRISS NIRCam NIRSpec MIRI MIRI ~100 (@7.5μm) 0.51” x 4.7” 1550 6.7” x 7.7” 2000 5.2” x 6.2” 2650 3.5” x 4.4” 3250 3.0” x 3.9” 250k microshutters 100, 1000, 2700 3.4’ x 3.6’ / 0.2” x 0.46” 100, 1000, 2700 3.0” x 3.0” 100, 1000, 2700 0.2” x 3.3”, 0.4” x 3.65”, 1.6” x 1.6” IFU 1450 2.2’ x 2.2’ 7.8” x 46” 150, 700 2.2’ x 2.2’ FOV λ/Δλ Slit λ/Δλ FOV λ/Δλ FOV/Slit λ/Δλ Slitless Single Slit Multi-Object ~100 (@7.5μm)


Multi-Object Spectroscopy 

Available for NIRSpec

Illustration of JWST NIRSpec MSA mode
Illustration of the NIRSpec MSA spectroscopy mode.  Selectable micro-mechanical shutters can be opened to obtain spectra for multiple objects simultaneously.

The wide-field mode of NIRSpec includes a Micro-Shutter Assembly (MSA) of about 250,000 selectable shutters, which can be opened or closed to permit simultaneous observations of more than 100 sources over a field of view of 3.2 x 3.3 arcminutes.  Targets in the Field of View are normally selected by opening groups of shutters in a MSA to form multiple apertures. The micro-shutters are arranged in a waffle-like grid with each cell projecting to 0.2” (dispersion) by 0.46” (spatial) on the sky. Sweeping a magnet across the surface of the MSA opens all operable shutters. Individual shutters may then be addressed and closed electronically. NIRSpec provides spectroscopy from 0.6 to 5.3 μm at a spectral resolution λ/Δλ of R~100 (3000 km/s) and from 1 to 5 μm at R~1000 and 2700 (110 km/s). The four quadrants of the MSA are imaged onto two detectors, resulting in some gaps that need to be considered when planning observations.

Integral Field Unit Spectroscopy 

Available for NIRSpec and MIRI

IFU image slicing illustration
Illustration of IFU image slicing

Both NIRSpec (0.6-5.3 μm) and MIRI (5-28.3 μm) contain integral field units (IFUs), which re-image a few-arcsecond, rectangular region of the focal plane onto a long slit and disperse it into a spectrum. This results in a spectrum for each pixel in the aperture (typically a few arcseconds on a side), resulting in a three dimensional data cube with wavelength along one axis and sky position along the other two. In the case of NIRSpec, the IFU divides a field-of-view of 3x3” into 30 slices, each 0.1 arcsec wide. All three NIRSpec spectral resolutions (R=100,1000 and 2700) are available.

MIRI uses four image slicers to produce dispersed images of the sky on two 1024x1024 pixel detectors to provide R~3000 spectroscopy over its full wavelength range. The spectral window of each IFU channel is covered using three separate gratings. The IFUs are designed with different pixel scales, providing roughly optimal sampling of the PSF at the central wavelengths of each spectral window. The fields of view of the channels are thus different, ranging from 3.6″x3.6″ to ~7.6″x7.6″ with increasing wavelength. The number of slices decreases from 30 to 12 with increasing wavelength.

Single Object Spectroscopy 

Available for NIRSpec, MIRI, NIRISS and NIRCam

Illustration of single-object spectra
JWST offers capabilities for Single-object spectroscopy from 0.6 to 28.3 μm.

JWST offers several options for observing single targets. Some of these modes are intended for bright targets (e.g. exoplanet spectroscopy), but all can be used for observing any astronomical target. To avoid saturation in the time it takes to read out the full frame, fast-readout modes are available that use only a sub-array of the detector.

NIRSpec provides observations through fixed slits of 0.2 x 3.3, 0.4 x 3.8, and 1.6 x 1.6 arcsec with all three spectral resolutions (R~100,1000 and 2700). These permanently-open slit apertures are intended primarily for high-dynamic-range observations of a single target (e.g., exo-planet transits).

The NIRISS Single-Object Slitless Spectroscopy (SOSS) mode is specifically optimized for bright-target spectroscopy. The optical design defocuses the light from the bright targets and disperses it into three orders, allowing observations without saturating the detector.  This mode provides a spectral resolution of R=750 from 0.6 to 2.5 μm.

The MIRI low-resolution spectrometer (LRS) provides R ~ 100 long-slit and slitless spectroscopy from 5 to 12 μm and is optimized for observations of compact sources. The slit mode provides a 5.5" long and 0.6" wide entrance aperture, while the slitless mode provides very accurate spectrophotometry for Time Series Observations, such as transiting exoplanets. The prism disperses the 5 to 10 micron spectral band of light from the target over approximately 140 detector pixels.

NIRCam has a grism with a spectral resolution R~2000 in the 2.4-5 micron channel. Its primary purpose is to assist with the coarse phasing of JWST. But the grism can also be used for scientific observations, including slitless spectroscopy of bright targets.

JWST Grism mode illustration
NIRCam and NIRISS offer simultaneous slitless spectroscopy of multiple objects.  Each instrument has optics that disperse the spectra in two orthogonal directions fot mitigate the problem of overlapping sources.

Slitless Spectroscopy 

Available for NIRISS and NIRCam

To enable blind searches for faint emission-line galaxies, NIRISS provides low-resolution slitless spectroscopy from 1-2.5 μm over a 2.2x2.2 arcminute field with a spectral resolution R~150.

NIRCam has a grism with a spectral resolution R~2000 in the 2.4-5 micron channel. Its primary purpose is to assist with the coarse phasing of JWST. But the grism can also be used for scientific observations, including faint galaxies.