NIRSpec Overview

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).

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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:

Figure 1. NIRSpec, highlighted on the left, in the JWST focal plane

NIRSpec, highlighted on the left, in the JWST focal plane

The JWST focal plane with NIRSpec on the left. The four magenta rectangles represent the 4-quadrants of the Micro-Shutter Assembly (MSA). Fixed Slits (in red) and the IFU (in orange) are located between the quadrants of the MSA. The NIRSpec aperture position angle is rotated by approximately 138.5° in comparison to NIRCam, NIRISS and the FGS fields of view. For more information, see the NIRSpec optics article.


Observational capabilities

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 modeAperture or slit size (arcsec)Wavelength
coverage
(μm)
Pixel scale
(arcsec/pixel) 
Resolving
power
MSA spectroscopy

0.20 × 0.46
(individual shutter size in the dispersion direction × spatial direction)


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)






0.1


~100 (Prism),

~1,000 (medium-resolution gratings),

~2,700 (high-resolution gratings)

IFU spectroscopy3.0 × 3.0
Fixed slit spectroscopy0.2 × 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

See also: NIRSpec Optics, NIRSpec 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.

Figure 2. NIRSpec optical elements

NIRSpec Optical Elements

A schematic layout of the NIRSpec instrument, including the key filter wheel assembly, grating wheel, detector housing and apertures used for science. Also shown are the locations of pickoff mirrors, fore optics, refocus mechanism and calibration assembly.


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. 

Figure 3. An example of NIRSpec point source sensitivity

The curves show the NIRSpec point source sensitivity using the NIRSpec NRSIRS2 readout mode. The plots show curves of limiting sensitivity (defined as S/N of 10 on a source of the presented brightness) that can be achieved in ten 966 s exposures with a 0.2" MSA slit width. In many cases, the curves have gaps. The gaps are present when observing in any NIRSpec mode, but the location of the gap in wavelength depends on the aperture or shutter the target is in, so those shown in the figure are representative only. Details about the wavelength gaps and cutoffs for each mode can be found in the NIRSpec FS Wavelength Ranges and Gaps, NIRSpec IFU Wavelength Ranges and Gaps, and NIRSpec FS Wavelength Ranges and Gaps articles.


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 astrometricflux, 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.



Proposal planning 

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:

There are many helpful videos that can found below.



Training videos and proposing tools

General Videos

NIRSpec Lectures

Several JWST Community Lecture Series presentations that highlight NIRSpec functionality are available for viewing at the JWST Events web page.

The JWST Mission Office at the Space Telescope Science Institute organized a series of webinars aimed at training the astronomical community on how to use JWST.  Recordings of the webinars are posted on the STScI webcast archive, and the presentation slides are also available for download. Several of these webinars have focused on NIRSpec features and capabilities.

NIRSpec Presentations in the JWST community lecture series:

Relevant NIRSpec information has also been archived in these Webinars:

JWST Proposing Tools of Interest to NIRSpec Users



Acknowledgements

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.



References

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




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