Spectroscopic Modes

Banner image showing various science themes within JWST mirror segments

Spectroscopic Mode Capabilities with JWST

MIRI, NIRSpec

Illustration of IFU image slicing.

Both MIRI and NIRSpec contain integral field units (IFUs) for obtaining three-dimensional “Integral Field Spectroscopy (IFS)” with wavelength along one axis and sky position along the other two. The IFUs all use an “image slicer” design, in which the field-of-view is sliced into multiple long slits, each of which is re-imaged on a different location on the detector.

MIRI uses four IFUs to produce dispersed images over its full wavelength range (4.9 to 28.8 μm) with resolving powers between λ/Δλ~1500 and 3500, depending on wavelength. The MIRI IFS mode is called “Medium-Resolution Spectroscopy” (MRS). Each MIRI IFU covers a different wavelength region (channel), with a pixel scale that optimally samples the PSF at the central wavelengths of each channel. The fields of view of the MRS channels range from 3.9″x3.9″ to ~7.7″x7.7″ with increasing wavelength, while the number of slices decreases from 30 to 12.

NIRSpec is equipped with an IFU that divides a 3x3” field-of-view into 30 slices, each 0.1 arcsec wide, covering 0.6 to 5.3 μm. Three NIRSpec spectral resolving power settings are available (λ/Δλ~100,1000, and 2700).
NIRSpec

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

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

JWST offers capabilities for Single-object spectroscopy from 0.6 to 28.3 μm.

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.