Spectroscopic Mode Capabilities with JWST
Spectroscopic Modes
MIRI, NIRSpec
NIRSpec
NIRSpec offers multi-object spectroscopy (MOS) using 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 3.6'x3.4'. The shutters are arranged in a waffle-like grid with each cell projecting to 0.2” (dispersion) by 0.46” (spatial) on the sky. Targets in the Field of View are selected by opening groups of shutters in the MSA to form multiple apertures. NIRSpec provides spectroscopy between 0.6 and 5.3 μm, with spectral resolving powers of λ/Δλ ~100, 1000, or 2700.
MIRI, NIRCam, NIRISS, NIRSpec
All instruments have spectroscopic modes optimized for observations of single targets. Some of these modes are intended for Time-Series Observations (TSO) of bright targets (such as exoplanet transit spectroscopy), while others optimize sensitivity for observations of faint sources.
The MIRI low-resolution spectrometer (LRS) provides single-object long-slit and slitless spectroscopy with resolving powers λ/Δλ~100 from 5 to 12 μm. The slit mode uses a 4.7"x0.51" fixed slit intended for a broad range of science, whereas the slitless mode is used for time-series observations.
The NIRCam grism can be used with time-series spectroscopy, offering λ/Δλ~1600 at 2.4 to 5.0 μm for very bright targets. In this mode, the NIRCam short-wavelength channel simultaneously obtains time-series imaging of the target using a weak lens to defocus the image to avoid saturation.
NIRISS is equipped with a Single-Object Slitless Spectroscopy (SOSS) mode optimized for time-series spectroscopy. A cylindrical lens defocuses the source image over many pixels and a grism disperses the light in least two usable orders, allowing observations of very bright targets without saturating the detector. This mode provides a spectral resolving power of λ/Δλ~700 from 0.6 to 2.8 μm.
NIRSpec is equipped with permanently open fixed slits (FS), including a large, square 1.6”x1.6“ aperture, in combination with all dispersers (0.6 to 5.3 μm; λ/Δλ~100, 1000, and 2700). The large aperture is used for bright object time-series spectroscopy (BOTS).
NIRCam, NIRISS
NIRISS and NIRCam offer complementary Wide-Field Slitless Spectroscopy (WFSS) modes.
NIRCam is equipped with a WFSS mode with a resolving power of λ/Δλ~1600 at 2.4 to 5.0 μm in the long-wavelength channel. In the WFSS mode, the short-wavelength channel is used to obtain imaging of the same field-of-view. Similar to NIRISS WFSS, two grisms with perpendicular dispersion directions are available to help separating overlapping spectra.
NIRISS has a WFSS mode with resolving powers of λ/Δλ~150) between 0.8 and 2.2 μm, over a 2.2’x2.2’ field-of-view. This mode can be used efficiently in parallel with other instruments. The NIRISS WFSS mode uses a pair of identical grisms with perpendicular dispersion directions. Data acquired with both dispersion directions helps to disentangle blended spectra in crowded fields.
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The NASA James Webb Space Telescope, developed in partnership with ESA and CSA, is operated by AURA’s Space Telescope Science Institute.