Figure 1. NIRSpec, highlighted on the left, in the JWST focal plane
The JWST focal plane with NIRSpec on the left. The 4 magenta rectangles represent the 4-quadrants of the micro-shutter assembly (MSA). Fixed slits (in red) and the IFU (in yellow) are located between the MSA. The NIRSpec aperture position angle is rotated by approximately 137.5° in comparison to NIRCam, NIRISS and the FGS fields of view.
For more information, see the NIRSpec optics page.
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
** 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.
Figure 2. 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
Figure 3 shows NIRSpec predicted sensitivity in MOS mode observations for a point source observed in ten 966 s exposures, with the medium resolution R = 1000 paired filter and grating settings used 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.
Figure 3. An example of NIRSpec point-source sensitivity in R = 1000 spectral observations
Four curves show the NIRSpec point-source sensitivity in the main spectral settings of the R = 1000 observations, based on instrument sensitivity information derived from the ground test calibration campaign in winter 2016. 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 966s exposures with a 0.2" MSA slit width.
Each mode in NIRSpec has its own planning interface template in the Astronomer's Proposal Tool (APT) software. The NIRSpec MOS mode has a specialized MSA planning tool (MPT) to help optimize NIRSpec MSA science.
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.