The JWST Near Infrared Spectrograph (NIRSpec) provides near-IR spectroscopy from 0.6–5.3 μm within a 3.4 × 3.6 arcmin field of view using micro-shutter arrays (MSAs), an integral field unit (IFU) and fixed slits (FSs).
The JWST Near Infrared Spectrograph (NIRSpec) enables 0.6–5.3 μm spectroscopy at resolving powers of ~100, ~1000, and ~2700 in four observing modes. NIRSpec is designed to be particularly powerful for multiplexing spectroscopy and high contrast, high throughput single object spectroscopy.
Key science uses of NIRSpec include, but are not limited to: statistical survey spectroscopy for galaxy formation and evolution studies, characterization of stellar populations, spatially resolved spectroscopy of extended targets and characterization of exoplanet atmospheres using transit observations.
The 4 observing modes of NIRSpec are:
- Multi-object spectroscopy (MOS) with the micro-shutter assembly (MSA)
- 3-D imaging spectroscopy with the integral field unit (IFU)
- High contrast single object spectroscopy with the fixed slits (FSs)
- High throughput bright object time-series (BOTS) spectroscopy with the NIRSpec wide aperture
NIRSpec offers 4 different observing modes: (1) multi-object spectroscopy with the MSA, (2) integral field spectroscopy with the IFU, (3) fixed slit spectroscopy with one of 5 available slits, and (4) bright object time-series spectroscopy with the optimized wide aperture. Table 1 summarizes the NIRSpec modes including wavelength coverages, aperture sizes and average (central wavelength) resolving powers.
Table 1. Characteristics of NIRSpec observing modes
|Aperture or slit size (arcsec)||Pixel scale|
0.6–5.3 μm (prism)
0.20 × 0.46
|IFU spectroscopy||3.0 × 3.0|
|Fixed slit spectroscopy||0.2 × 3.2|
0.4 × 3.65
1.6 × 1.6
|Bright object time series||1.6 × 1.6|
** These resolving powers correspond to the values at the central wavelength in the measured spectral range.
Figure 2 shows the NIRSpec optical design. The key instrument elements that are important for science observation specifications are the filters, dispersers, science apertures and detectors.
- Filter wheel assembly (FWA): NIRSpec has a filter wheel equipped with: (1) four long-pass filters and a clear filter for spectroscopy, (2) two short wavelength filters that can be used for target acquisition and (3) an opaque blocking filter used to block the light entering NIRSpec when the instrument is not in use.
- Grating wheel assembly (GWA): The NIRSpec grating wheel assembly has a low-resolution (R ~ 100) prism, three medium-resolution (R ~ 1000) gratings, three high-resolution (R ~ 2700) gratings and a mirror for target acquisition imaging.
- Apertures: NIRSpec has 3 types of apertures; they are MSA shutters, integral field unit (IFU) and fixed slits (FSs).
- Multi-object spectroscopy (MOS) with the MSA: NIRSpec MOS capabilities are enabled by the micro-shutter assembly, which is a 4-quadrant grid of individually configurable shutters, each 0.2" × 0.46" in extent on the sky.
- IFU spectroscopy: NIRSpec's integral field spectroscopy mode is enabled by the IFU to acquire three-dimensional (3D) imaging spectroscopy over a small 3" × 3" field of view with 0.1" sampling.
- Fixed slits spectroscopy: Five fixed slits are available for high-contrast spectroscopy on single objects. One of these slits is a 1.6" × 1.6" aperture that has been optimized for exoplanet transit observations in the bright object time-series spectroscopy mode.
- Detectors: NIRSpec's focal plane is equipped with two 5.3 μm cutoff Teledyne-Hawaii-2RG HgCdTe arrays, each having 2048 × 2048 pixels. The projected detector pixel size on the sky is 0.1". There is a physical gap between the detectors which can result in wavelength loss in a single NIRSpec exposure in the MOS mode, and in the R = 2700 resolutions in all science modes. In order to meet instrument sensitivity requirements, NIRSpec has a specialized low noise readout mode called "increased reference sampling and subtraction (IRS2)," which intersperses more reference pixel reads to better remove noise effects.
Sensitivity and performance
Figure 3 shows NIRSpec predicted sensitivity in MOS mode observations for a point source observed in ten 966 s exposures, for all filter+grating combinations available for science. Observers testing NIRSpec performance and preparing proposals should always use the JWST Exposure Time Calculator (ETC) to obtain the most recent sensitivity estimates.
Each mode in NIRSpec has its own planning interface template in the Astronomer's Proposal Tool (APT) software. Follow the links below to access the documentation for each of the observing modes.
- NIRSpec IFU Planning in APT
- NIRSpec Fixed Slit Planning in APT
- NIRSpec Bright Object Time Series Planing in APT
- NIRSpec MOS Planning in APT
Data calibration and analysis
NIRSpec was built for the European Space Agency by Airbus Industries; the micro-shutter assembly and detector sub-systems were provided by NASA. Dr. Peter Jakobsen guided NIRSpec's development until his retirement in 2011. Dr. Pierre Ferruit is the current NIRSpec PI and ESA JWST project scientist.
JWST User Documentation Home
NIRSpec Observing Modes
NIRSpec Multi-Object Spectroscopy
NIRSpec IFU Spectroscopy
NIRSpec Fixed Slits Spectroscopy
NIRSpec Bright Object Time-Series Spectroscopy
JWST Astronomers Proposal Tool, APT
JWST APT website
JWST Exposure Time Calculator
JWST ETC website
Dorner, B., Giardino, G., Ferruit, P. et al. 2016, A&A, 592, A113
A model-based approach to the spatial and spectra calibration of NIRSpec onboard JWST