NIRSpec (Near-Infrared Spectrograph)

Near-Infrared Spectrograph
General information
Organization ESA with contributions from NASA
Major contractors Astrium GmbH (Germany)
Launch date October 2018 (planned)[1]
Launch site Guiana Space Centre ELA-3
Kourou, French Guiana
Launch vehicle As part of JWST onboard Ariane 5
Mission length 5 years (design)
10 years (goal)
Mass 196 kg (432 lb)[2]
(Instrument and Electronic boxes)
Orbit period 1-year
Location 1.5 million km from Earth
(Earth-Sun Lagrangian point L2 halo orbit)
Telescope style Spectrograph
Wavelength 0.6 (orange) to 5.0 µm (near-infrared)
Website ESA Europe
Astrium Germany
NASA United States

NIRSpec, the Near-Infrared Spectrograph, is one of the four scientific instruments which will be flown on the James Webb Space Telescope (JWST).[3] The JWST is the follow on mission to the Hubble Space Telescope (HST) and is developed to receive more information about the origins of the universe by observing infrared light from the first stars and galaxies.[4] In comparison to HST its instruments will allow looking further back in time and will study the so-called Dark Ages during which the universe was opaque. The Dark Ages are in the range from 150 to 800 million years after the big bang.[5] The JWST main science themes are:[6]

The NIRSpec instrument is a multi-object spectrograph and is capable of simultaneously measuring the near-infrared spectrum of up to 100 objects like stars or galaxies with low, medium and high spectral resolutions. The observations are performed in a 3 arcmin × 3 arcmin field of view over the wavelength range from 0.6 micrometer to 5.0 micrometer. It also features a set of slits and an aperture for high contrast spectroscopy of individual sources, as well as an integral-field unit (IFU) for 3D spectroscopy.[7] The instrument is a contribution of the European Space Agency (ESA) and is built by Astrium together with a group of European subcontractors.[8]

Key science objectives and requirements

Operational modes

In order to achieve the scientific objectives NIRSpec has four operational modes:[7]

Multi-Object Spectroscopy (MOS)

NIRSpec in Multi-Object Mode; The image shows the spectra of a Spectral Line Calibration Lamp (Fabry-Perot type) imaged onto the 2 detector Sensor Chip Assemblies (SCA). For this the instrument was in MOS mode and several vertical shutter lines in the MSA have been openend and illuminated.
Basic principle of Multi-Object Spectroscopy
Microshutter Closeup
NIRSpec Microshutter Arrays

In MOS the total instrument field of view of 3 × 3 arcminutes is covered using 4 arrays of programmable slit masks. These programmable slit masks consist of 250 000 of micro shutters where each can individually be programmed to 'open' or 'closed'. The contrast between an 'open' or 'closed' shutter is better than 1:2000.[10] If an object like e.g. a galaxy is placed into an 'open' shutter than the spectra of the light emitted by the object can be dispersed and imaged onto the detector plane. In this mode up to 100 objects can simultaneously be observed and the spectra be measured.

NIRSpec in IFU Mode; The image shows the spectra of a Spectral Line Calibration Lamp (Fabry-Perot type) imaged onto the 2 detector SCA's. For this the instrument was in IFU/PRISM mode and the illuminated square aperture has been sliced and re-imaged into a long vertical slit which generates a stack of spectra on the detector.
Basic principle of Integral Field Spectroscopy

Integral Field Unit Mode (IFU)

The integral field spectrometry will primarily be used for large, extended objects like galaxies. In this mode an 3 × 3 arcsecond field of view is sliced into 0.1 arcsecond bands which are thereafter re-arranged into a long slit. This allows to obtain spatially resolved spectra of large scenes and can be used to measure the motion speed and direction within an extended object. Since measured spectra in the IFU mode would overlap with spectra of the MOS mode it can not be used in parallel.

High-Contrast Slit Spectroscopy (SLIT)

A set of 5 fixed slits are available in order to perform high contrast spectroscopic observations which is e.g. required for spectroscopic observations of transiting extra-solar plants. Of the five fixed slits, three are 0.2 arcseconds wide, one is 0.4 arcsecond wide and one is a square aperture of 1.6 arcseconds. The SLIT mode can be used simultaneously with the MOS or IFU modes.

Imaging Mode (IMA)

The imaging mode is used for target acquisition only. In this mode no dispersive element is placed in the optical path and any objects are directly imaged on the detector. Since the microshutter array which is sitting in an instrument intermediate focal plan is imaged in parallel, it is possible to arrange the JWST observatory such that any to be observed objects fall diretly into the center of open shutters (MOS-mode), the IFU aperture (IFU-mode) or the slits (SLIT mode).

Overview

The NIRSpec instrument operates at -235 °C and utilizes a highly a-thermal concept with all mirrors, the mirror mounts and the optical bench base plate all manufactured out of Silicon Carbide Ceramic SiC100. The instrument size is approximately 1900 mm × 1400 mm × 700 mm and weighs 196 kg with about 100 kg of SiC. The operation of the instrument is performed with three electronic boxes.

NIRSpec includes 4 cryo-mechanisms which are:

Further NIRSpec includes two electro-optical assemblies which are:

The Calibration Assembly, one component of the NIRSpec instrument which will form part of the James Webb Space Telescope, at University College London prior to integration.

And finally the Integral Field Unit (IFU) image slicer, used in the instrument IFU mode.

The optical path is represented by the following SiC mirror assemblies:

Industrial partners

NIRSpec has been built by Astrium Germany with subcontractors and partners spread over Europe and with the contribution of NASA from the US which provided the Detector Subsystem and the Micro-shutter Assembly.

NIRSpec Industrial Partners

The individual subcontracters and their corresponding contribution were:[11]

- Filter Wheel Assembly
- Grating Wheel Assembly
- Calibration Assembly
- Optical Ground Support Equipment (Shack-Hartman-Sensor, Calibration Light Source)
- Detector Subsytem
- Microshutter Subsystem

Performance parameter

The NIRSpec key performance parameters are:[7][8][12]

PARAMETER VALUE
Wavelength Range 0.6 µm - 5.0 µm
When operating in R = 1000 and R = 27000 mode, split in three spectral bands:
1.0 µm - 1.8 µm Band I
1.7 µm - 3.0 µm Band II
2.9 µm - 5.0 µm Band III
Field of View 3 × 3 arcmin
Spectral Resolutions R = 100 (MOS)
R = 1000 (MOS + fixed Slits)
R = 2700 (fixed Slits + IFU)
Number of commendable open/ closed spectrometer slits MEMS technology based on micro-shutter arrays with 4 times 365 × 171 = 250 000 individual shutters, each of thme with a size of 80 µm × 180 µm
Detector 2 MCT Sensor Chip Assemblies (SCA's) of 2048 × 2048 pixels each. Pixel pitch = 18 µm × 18 µm
Wavefront Error, including Telescope Diffraction limited at 2.45 µm at MSA: WFE = 185 nm RMS (Strehl = 0.80)
Diffraction limited at 3.17 µm at FPA: WFE = 238 nm RMS (Strehl = 0.80)
Limiting Sensitivity * In R = 1000 mode, using one single 200 mas wide shutter or fixed slit, NIRSpec will be capable of measuring the flux in an unresolved emission line of 5.2 from a point source at an observed wavelength of 2 µm at SNR = 10 per resolution element in a total exposure of 105 s or less
* In R = 100 mode, using one single 200 mas wide shutter or fixed slit, NIRSpec will be capable of measuring the continuum flux of 1.2×10−33 Wm−2Hz−1 from a point source at an observed wavelength of 3 µm at SNR = 10 per resolution element in a total exposure of 104 s or less
NIRSpec Optics Envelope Approximately 1900 mm × 1400 mm × 700 mm
Instrument Mass 195 kg with about 100 kg SiC parts, Electronic Boxes: 30.5 kg
Operating Temperature 38 K (-235 °C)

'

References

  1. ↑ "JWST factsheet". ESA. 2013-09-04. Retrieved 2013-09-07.
  2. ↑ "Extracting Information From Starlight". NASA. 2010-03-30. Retrieved 2014-04-09.
  3. ↑ Greenhouse, M. (2013). MacEwen, Howard A; Breckinridge, James B, eds. "The JWST science instrument payload: mission context and status". Proceedings of SPIE. UV/Optical/IR Space Telescopes and Instruments: Innovative Technologies and Concepts VI 8860: 886004. doi:10.1117/12.2023366.
  4. ↑ JWST
  5. ↑ Chronology of the universe#Dark Ages
  6. ↑ "The James Webb Space Telescope". Jwst.nasa.gov. Retrieved 2015-01-20.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 7.6 Ferruit, P. et al. (2012). "The JWST near-infrared spectrograph NIRSpec: status". Proceedings of SPIE. Space Telescopes and Instrumentation 2012: Optical, Infrared, and Millimeter Wave 8442: 84422O. Bibcode:2012SPIE.8442E..2OF. doi:10.1117/12.925810.
  8. ↑ 8.0 8.1 "ESA Science & Technology: NIRSpec – the Near-Infrared Spectrograph on JWST". Sci.esa.int. 2013-09-06. Retrieved 2013-12-13.
  9. ↑ Zaroubi, S. (2013). "The epoch of reionization". In Wiklind, T., Mobasher, B. and Bromm, V., 'The First Galaxies - Theoretical Predictions and Observational Clues', Springer, Astrophysics and Space Science Library, 396.
  10. ↑ Kutyrev, A.S. et al. (2008). "Microshutter arrays: high contrast programmable field masks for JWST NIRSpec". Proceedings of SPIE. Space Telescopes and Instrumentation 2008: Optical, Infrared, and Millimeter 7010: 70103D. Bibcode:2008SPIE.7010E..99K. doi:10.1117/12.790192.
  11. ↑ "JWST NIRSpec Press Conference". Astrium GmbH, Ottobrunn. 2013.
  12. ↑ Posselt, W. et al. (2004). "NIRSpec - Near Infrared Spectrograph for the JWST". Proceedings of SPIE. Optical, Infrared, and Millimeter Space Telescopes 5487: 688–697. Bibcode:2004SPIE.5487..688P. doi:10.1117/12.555659.

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