The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope I. Overview of the instrument and its capabilities

TL;DR

The paper presents a comprehensive overview of JWST’s Near‑Infrared Spectrograph (NIRSpec), detailing its all‑reflective optical chain, three slit configurations (MSA, fixed slits, IFU), and the complementary calibration and target‑acquisition systems. It documents the instrument’s modular design, high throughput across 0.6–5.3 μm, and multiple spectral resolutions ($R\approx 30-330$ with PRISM and $R\approx 500-1340$ or $R\approx 1320-3600$ with gratings), powered by a Micro‑Shutter Array and two Teledyne H2RG detectors with IRS^2 readout. The work quantifies anticipated performance, including photometric throughput, slit losses, and limiting sensitivity, and discusses the impact of detector noise, cosmic rays, and field distortions on data quality and multiplexing. It further outlines expected scientific capabilities for studying the distant universe, along with practical considerations for instrument modeling, calibration, and on‑orbit operations, and points to companion papers for deeper dives into MSAs, IFU, and exoplanet observations.

Abstract

We provide an overview of the design and capabilities of the near-infrared spectrograph (NIRSpec) onboard the James Webb Space Telescope. NIRSpec is designed to be capable of carrying out low-resolution ($R\!=30\!-330$) prism spectroscopy over the wavelength range $0.6-5.3\!~μ$m and higher resolution ($R\!=500\!-1340$ or $R\!=1320\!-3600$) grating spectroscopy over $0.7-5.2\!~μ$m, both in single-object mode employing any one of five fixed slits, or a 3.1$\times$3.2 arcsec$^2$ integral field unit, or in multiobject mode employing a novel programmable micro-shutter device covering a 3.6$\times$3.4~arcmin$^2$ field of view. The all-reflective optical chain of NIRSpec and the performance of its different components are described, and some of the trade-offs made in designing the instrument are touched upon. The faint-end spectrophotometric sensitivity expected of NIRSpec, as well as its dependency on the energetic particle environment that its two detector arrays are likely to be subjected to in orbit are also discussed.

Figures (19)

• Figure 1: Optical path through the NIRSpec instrument. The dispersion direction is out of the image.
• Figure 2: CAD rendering of NIRSpec with its three major mechanisms and the calibration source identified.
• Figure 3: Location and orientation of the NIRSpec field of view on the shared JWST focal sphere with respect to NIRCam and the telescope optical axis. The entrance apertures of the other two JWST instruments, MIRI and NIRISS, are situated to the right of and below NIRCam.
• Figure 4: Layout of the NIRSpec Slit Plane. (a) The four quadrants of the Microshutter Array, the five Fixed Slits, the entrance aperture of the Integral Field Unit, and the locations of its 30 virtual slit images. The dispersion direction is horizontal. Black areas in the MSA depict inoperable vignetted or permanently closed shutters. (b) Close-up of the left-most group of four Fixed Slits and the IFU entrance aperture. (c) Close-up of the right-most redundant Fixed Slit.
• Figure 5: Wavelength ranges and spectral resolutions achievable with the seven dispersers of NIRSpec. The nominal wavelength range covered by each disperser is shown as a full line, and the anticipated actual usable range as a dotted lines, including the blue extensions of the G140M and G140H gratings when used with the F070LP filter.