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 a Micro-Shutter Assembly (MSA), 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, ~1,000, and ~2,700 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 four observing modes of NIRSpec are:
- Multi-object spectroscopy (MOS) with the Micro-Shutter Assembly (MSA)
- 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
As noted above, NIRSpec offers four different modes. Table 1 summarizes these modes including wavelength coverages, aperture sizes and average (central wavelength) resolving powers.
Table 1. Characteristics of NIRSpec observing modes
|Observing mode||Aperture or slit size (arcsec)||Wavelength|
0.20 × 0.46
0.6–5.3 μm (prism)
0.7–1.27 μm (f070lp)
0.97–1.89 μm (f100lp)
1.66–3.17 μm (f170lp)
2.87–5.27 μm (f295lp)
~1,000 (medium-resolution gratings),
~2,700 (high-resolution gratings)
|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.
†† Multiple shutters can be combined to form a slit.
Optical elements and detectors
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 and target acquisition, (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 ~ 1,000) gratings, three high-resolution (R ~ 2,700) gratings and a mirror for target acquisition imaging.
- Apertures: NIRSpec has three 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 (2D spatial plus 1D spectral) 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 = 2,700 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
See also: NIRSpec Predicted 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. Sensitivity estimates in the other science modes are similar. Observers testing NIRSpec performance and preparing proposals should always use the JWST Exposure Time Calculator (ETC) to obtain the most recent sensitivity estimates.
See also: NIRSpec APT Templates
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 Planning in APT
- NIRSpec MOS Planning in APT
There are many helpful videos that can found on the NIRSpec Training Webinars and Webcasts page.
Data calibration and analysis
Information about calibration is available at Data Processing and Calibration Files. This article and its links point to content about absolute astrometric, flux, and wavelength calibration, as well as information on calibration reference files.
Details about the data can be found in the JWST File Names, Format, and Data Structures article. The JWST pipeline is described in JWST Data Reduction Pipeline and some information about post-pipeline processing can be found at JWST Post-Pipeline Data Analysis.
NIRSpec was built for the European Space Agency by Airbus Industries; the Micro-Shutter Assembly and detector subsystems 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.
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