Opto-mechanical design of the near infrared spectrograph NIRSpec

Abstract

The NIRSpec instrument on the James Webb Space Telescope (JWST) is a multi-object spectrograph capable of measuring the near infrared spectrum of at least 100 objects simultaneously at various spectral resolutions. It operates under cryogenic conditions (T∼ 35 K). NIRSpec is part of the JWST science instruments suite. Its main purpose is to provide low (R=100), medium (R=1000) and high resolution (R=2700) spectroscopic observations over the wavelength range 0.6 µm -5.0 µm in support of the four JWST science programs. The NIRSpec instrument is being developed by the European Space Agency with EADS Astrium Germany GmbH as the prime contractor The Optical Telescope Element of JWSTThe four JWST science instruments in the Integrated Science Instruments Module (ISIM) pick-off their input beam from the focal surface of the JWST Optical Telescope Element (OTE). The OTE is a "Korsch" type, all-reflective, on-axis, three-mirror anastigmat (TMA), with the addition of a fine steering mirror (FSM) near the exit pupil. As with most on-axis TMA systems, it is used off-axis in field to enable beam clearance after the tertiary mirror. The Field of View (FOV) offset equals -0.13° (or -7.8'). The primary, secondary and tertiary mirrors have conic contours (ellipse, hyperbola, ellipse), while the FSM is a flat. The focal surface is curved, with its radius matched to the FSM to focal surface distance. This feature ensures no defocus when the FSM is operated. The telescope has a primary mirror made out of 18 hexagonal segments with a overall, flat-to-flat diameter of ∼6.6 m. The output f-number is f/20. NIRSpec will assume a maximum Wave-Front Error (WFE) at its optical entrance field plane of 145 nm RMS for all field points, equivalent to a diffraction limited performance at 1.93 µm (Strehl ratio = 0.80). The Optical Design of NIRSpec 2.

Constraining Exoplanet Metallicities and Aerosols with the Contribution to ARIEL Spectroscopy of Exoplanets (CASE)

“…Thus the presumed noise floors of 20, 30, and 50ppm adopted for JWST's NIRISS SOSS, NIRCam grism, and MIRI LRS by Greene et al (2016) could prove to be conservative. Given that NIRCam, NIRISS, and NIRSpec also use a HgCdTe detector like WFC3 (Beichman et al 2012;Doyon et al 2012;te Plate et al 2005) and JWST's larger aperture size, we thus scale the currently best WFC3 precision (15 ppm; Tsiaras et al 2016a) by the ratio of JWST's to Hubble's primary mirror sizes to estimate that the noise floor for NIRISS SOSS could be as low as 5.3ppm (assuming photon-noise-limited observations), 6.0ppm (assuming 1.14×the photon noise), or 7.0ppm (assuming 1.33×the photon noise). Assuming similar reductions in the noise floors of JWST's other instruments, we predict that NIRCam grism could have a noise floor of 8.0ppm (assuming photon-noise-limited observations), 9.1ppm (assuming 1.14×the photon noise), or 10.6ppm (assuming 1.33×the photon noise) and MIRI LRS could have a noise floor of 13.3ppm (assuming photon-noise-limited observations), 15.2ppm (assuming 1.14× the photon noise), or 17.6ppm (assuming 1.33×the photon noise).…”

In-orbit Performance of the Near-infrared Spectrograph NIRSpec on the James Webb Space Telescope

“…The full photometric calibration of NIRSpec requires three major steps in the reduction pipeline, with their associated reference files: (i) the detector flat (D-Flat) to capture the pixel-topixel response variations, and derived from component-level ground test data of the two NIRSpec SCAs, (ii) the spectrograph flat (S-Flat) to correct the throughput variations of the spectrograph optics, and measured from exposures using the internal calibration lamps in the CAA, and (iii) the FORE optics flat (F-Flat) to characterize any fieldand wavelength-dependent effects caused by the OTE and the NIRSpec FORE Optics. For the last step, observations of spectro-photometric standard stars are necessary to verify the integrity of the entire NIRSpec optical system, in particular the pick-off and FORE optics (including the FWA), neither of which can be illuminated with the CAA lamps (see te Plate et al 2005, for a detailed description of the light path from the CAA). For a detailed overview of the NIRSpec flat field strategy we refer to Rawle et al (2016).…”

Development of the MICADO Flat-field and Wavelength Calibration Unit: From Design to Prototyping

“…A challenging factor in the realization of the MCA is related to the size of the MICADO focal plane ∼200 mm×200 mm and its segmentation in 3×3 detectors. The use of conventional integrating spheres to collect and homogenize the light from the calibration sources (te Plate et al 2005;Wildi et al 2009;Kelz et al 2012) is discouraged for their impractical size if rescaled to the MICADO context. The implementation of a dome flatfield strategy (Verdoes Kleijn et al 2013) is also unfeasible due to the size of the ELT (39 m diameter).…”

“…Many of the instruments operating in the 8 m class telescopes rely on Integrating Spheres (IS) to achieve the flatfield calibration (te Plate et al 2005;Wildi et al 2009;Kelz et al 2012). An IS homogenizes the injected light by multiple Lambertian scattering processes and reflections onto the spherical walls of its cavity that are coated with highly reflective material (Spectralon).…”