The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope I. Overview of the instrument and its capabilities (2202.03305v1)

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 $0.6-5.3!~\mu$m and higher resolution ($R!=500!-1340$ or $R!=1320!-3600$) grating spectroscopy over $0.7-5.2!~\mu$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.

• The paper presents a detailed assessment of NIRSpec’s design, emphasizing its MEMS-based multi-shutter system for enhanced multi-object spectroscopy.
• It explains the advanced optical and detector technologies, including an all-reflective design with silicon carbide components and Teledyne H2RG arrays.
• The paper outlines performance metrics that exceed benchmarks, underscoring NIRSpec's potential to advance studies of high-redshift galaxies and the early universe.

Overview of the Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope

This paper provides a comprehensive examination of the Near-Infrared Spectrograph (NIRSpec) mounted on the James Webb Space Telescope (JWST). NIRSpec is designed to advance our understanding of distant galaxies and the early universe by providing spectroscopic observations in the near-infrared range. The paper gives an in-depth description of NIRSpec's design, capabilities, performance metrics, and potential scientific impacts.

Instrument Design and Capabilities

NIRSpec is a sophisticated instrument capable of performing both single- and multi-object spectroscopy over a broad wavelength range from 0.6 to 5.3 micrometers. It is equipped to provide observations at three spectral resolutions—low resolution ($R=30-330$) with a prism and medium ($R=500-1340$) and high resolution ($R=1320-3600$) with diffraction gratings. Notably, NIRSpec includes three observing modes via the use of its Multi-Shutter Array (MSA), five Fixed Slits, and an Integral Field Unit (IFU).

The MSA is a central feature allowing for multi-object spectroscopy, comprised of micro-electro-mechanical system (MEMS) technology, offering the ability to open or close individual shutters selectively. This micro-shutter system allows NIRSpec to gather spectra from multiple targets simultaneously, with significant implications for the observation of countless galaxies in a single field of view.

Optical and Mechanical Design

NIRSpec enhances its functionality with an all-reflective optical train and employs advanced materials like Silicon Carbide (SiC) for its optical bench and powered mirrors. These materials ensure low thermal expansion and high thermal conductivity, crucial for maintaining stability at the operational temperature of approximately 37 K. The optical design focuses on optimizing the light path from the telescope to the NIRSpec detectors, balancing the field of view and ensuring high spectral resolution across its range.

Detector Technology

NIRSpec employs a detector subsystem consisting of two Teledyne H2RG arrays, each with a 2048x2048 pixel grid and sensitive to wavelengths up to 5.3 micrometers. These arrays are pivotal in achieving the faint-end sensitivity required for detecting distant galaxies. The detector system was enhanced through advanced readout technologies like the IRS$^2$, allowing for improved noise performance critical to observing faint astronomical targets.

Sensitivity and Performance

NIRSpec's sensitivity allows it to detect continuum and emission lines from faint galaxies, a capacity that exceeds its formal requirements due to its high photon conversion efficiency and advanced noise reduction techniques. The instrument's capabilities are further augmented by the JWST's large collecting area and NIRSpec's varied observing modes, ensuring both versatility and depth in astrophysical observations.

The paper outlines comprehensive signal-to-noise calculations, demonstrating the instrument's ability to exceed expected performance benchmarks. These calculations consider various operational scenarios and environmental conditions the instrument will face in space, including the impact of cosmic ray events, which are crucial for understanding and optimizing observing strategies.

Implications and Future Prospects

NIRSpec's advanced capabilities hold significant implications for astrophysical research. By allowing detailed spectroscopic analysis of high-redshift galaxies, it opens avenues for understanding galaxy formation, cosmic reionization, and the evolution of the early universe. Furthermore, its multi-object observation capability is expected to yield insights into faint and distant astronomical phenomena that have so far been unattainable.

Looking forward, NIRSpec's contributions to spectroscopy will undoubtedly push the boundaries of our cosmological understanding. The data obtained could potentially refine existing theories and spark new lines of inquiry in astrophysics, particularly concerning the epochs of first light and reionization. As the field progresses, the flexibility and sensitivity of NIRSpec will likely adapt to new scientific challenges, continuing to provide critical observations that enhance our comprehension of the cosmos.