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Abstract. The James Webb Space Telescope (JWST) was conceived as the scientific successor to the. Hubble Space Telescope (HST) and Spitzer Space ...
Experimental Astronomy (2005) 19: 149–162 DOI: 10.1007/s10686-005-9015-0

C 

Springer 2006

THE JAMES WEBB SPACE TELESCOPE AND ITS INFRARED DETECTORS BERNARD J. RAUSCHER1 and MICHAEL E. RESSLER2 1

NASA Goddard Space Flight Center; 2 Jet Propulsion Laboratory

(Received 15 September 2005; accepted 6 December 2005)

Abstract. The James Webb Space Telescope (JWST) was conceived as the scientific successor to the Hubble Space Telescope (HST) and Spitzer Space Telescope. The instrument suite provides broad wavelength coverage and capabilities aimed at four key science themes: 1) The end of the dark ages: first light and reionization, 2) The assembly of galaxies, 3) The birth of stars and protoplanetary systems, and 4) Planetary systems and the origins of life. To accomplish these ambitious goals, JWST’s detectors provide state-of-the-art performance spanning the λ = 0.6–28 μm wavelength range. In this paper, we describe JWST with an emphasis on its infrared detectors. Keywords: detector, infrared, James Webb Space Telescope

1. Introduction The James Webb Space Telescope (JWST) was conceived as the scientific successor to the Hubble Space Telescope (HST), and to the Spitzer Space Telescope.1 Answering scientific questions beyond the grasp of these previous missions requires a large and flexible infrared observatory. In turn, these considerations motivated the selection of a deployable, segmented primary mirror, and an L2 orbit far from the earth. JWST has four Scientific Instruments (SIs). These are as follows: Near-Infrared Camera (NIRCam), Near-Infrared Spectrograph (NIRSpec), Mid-Infrared Instrument (MIRI), and Fine Guidance Sensor with Tunable Imager (FGS/TI). The four SIs are located in the Integrated Science Instruments Module (ISIM), which is located behind the primary mirror in Figure 1. In total, the SIs incorporate 16 Rockwell HAWAII-2RG mercury-cadmiumtelluride (HgCdTe) Sensor Chip Assemblies (SCAs), and 3 Raytheon SB-305 Si:As SCAs (see Figure 2). In this paper, we briefly summarize JWST’s science goals to provide context, and then discuss the mission and each of the SIs with an emphasis on detectors. Readers who are more interested in JWST’s science goals may wish to see one of the more recent articles on the subject (Rieke, 2005).

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Figure 1. Fully deployed JWST showing major components.

Figure 2. JWST detector summary.

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Figure 3. JWST will explore the dark ages.

2. Science themes 1) First Light: JWST’s top science goal is finding the light from the first objects to coalesce after the Universe cooled following the Big Bang (see Figure 3). The potential of JWST for studying distant sources has prompted a number of theoretical studies predicting the properties of the first stars, which are thought to be quite different from stars forming today because of the lack of any elements heavier than helium. Ultra-deep imaging and spectroscopic surveys using JWST are expected to detect the first super star clusters, or proto-galactic objects, to form at redshifts as high as z = 15–20. Appropriate timing of the observations may enable the detection of individual supernovae of super-massive population III stars. These surveys, along with spectroscopy of the highest redshift quasars, will trace the evolution of the first light objects through the epoch of reionization. 2) Assembly of Galaxies: The same observational drivers that define the search for first light also support detailed study galaxy assembly. Specifically, how do galaxies evolve from small, sub-galaxy sized fragments into the suite of morphologies and galaxy types (the Hubble Sequence) that we see today? For example, there is a significant body of literature describing how smaller mass condensations might hierarchically cluster (and evolve) to build clusters of galaxies, and eventually individual galaxies that we see today (see Figure 4). Areas requiring additional work include the details of how the mass condensations predicted by Cold Dark Matter

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Figure 4. Hierarchical clustering models suggest that structure in the Universe begins to form on small scales, which then aggregate to build the superclusters and clusters of galaxies that we see today. The middle pane illustrates the merger process. Although hierarchical clustering has had considerable success in building up mass aggregations, work is still needed, particularly with regard to how the first stars form in low-metalicity mass aggregations.

(CDM) models form stars and become luminous. This, of course, relates to the first light theme, although the full history of star formation is of interest from the perspective of the assembly of galaxies. Understanding how today’s galaxies evolved from first light objects requires more knowledge than can be provided by imaging alone. Multi-object spectroscopy is required for a statistically significant sample of galaxies spanning the redshift range over which galaxies formed. A spectral resolution, R∼1000, is needed to separate diagnostic spectral lines and for estimating heavy element content and star formation rates. Moreover, NIRSpec’s Integral Field Unit (IFU) will provide R∼ 3000, 3-dimensional spectra of individual galaxies at moderate and high redshift. The IFU will provide important dynamical information on targets including active and merging galaxies. For studying galaxies, JWST’s MIRI, which also incorporates an IFU, will provide a more complete picture of galaxy structure and evolution than is possible using only near-IR wavelengths (see Figure 5). By looking through dust (see Figure 6) that can obscure our view of star forming regions, and imaging warm and hot dust in dense, star forming cores, MIRI will facilitate mapping the regions of galaxies that are undergoing the most intense star formation. Moreover, MIRI IFU spectroscopy will be used to diagnose the role of active galactic nuclei in the evolution of galaxies. 3) The Birth of Stars and Protoplanetary Systems: A good framework is in place for understanding how stars form and how they may form planetary systems, but a number of key steps in the process are not yet understood. JWST will work to unravel the birth and early evolution of stars, from infall onto dust-enshrouded protostars to the genesis of planetary systems. JWST needs to provide high sensitivity and high spatial resolution over a span of wavelengths that can penetrate the

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Figure 5. JWST’s near-IR and mid-IR instruments, working together, provide essential and complementary astrophysical information. This is illustrated by the above near-IR and mid-IR composite view of the spiral galaxy M81. The 3.6 μm (left; near-IR) image is dominated by older, evolved stars. The 8 μm (middle; mid-IR) image emphasizes warm dust in the spiral arms. Finally, the 24 μm (right; mid-IR) image highlights the hot dust associated with deeply embedded star forming regions. The composite image, at bottom, clearly shows where the older, well established stars that dominate the mass are found, as well as fine structure in the spiral arms and a relatively un-obstructed view of the regions of active star formation. Credit: NASA/JPL-Caltech/K. Gordon (U. Arizona), S. Willner (Harvard/CfA), & N.A. Sharp (NOAO/AURA/NSF).

clouds enveloping the early stages of star formation and that can penetrate large column densities along the lines of sight through our galaxy. 4) Planetary Systems and the Origins of Life: Recent discoveries of many exosolar planets lends impetus to programs designed to characterize them. Equally important is developing an understanding of how planetary systems form, and what determines the numbers and arrangements of planets in a system. Spitzer Space Telescope discoveries of how debris disks, the likely remnants from planetary system formation, decay with time are a good illustration of how the observation of these disks tie in to the solar system and hence to planetary systems in general. JWST will be an excellent platform for studying these low surface brightness objects with high sensitivity MIRI coronagraphy. JWST will also provide a wealth of information on the surface compositions and albedos for Kuiper Belt Objects in the solar system, which will facilitate comparing what happens in debris disks with what is seen very locally. NIRCam imaging and NIRSpec spectroscopy are needed to measure absorption features from a variety of ices, while MIRI observations are needed to image the disks.

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Figure 6. Infrared wavelengths cut through dust that obscures our view of the visible Universe. (left) An optical wavelength image of the Eagle Nebula, M16, taken by HST. (right) A composite near-IR image (J, H, and Kshort bands) taken using the ISAAC camera on the ESO VLT ANTU telescope. Credit: (left) NASA, ESA, STScI, J. Hester & P. Scowen (Arizona State University) & (right) Mark McCaughrean & Morton Andersen, Astrophysical Institute Potsdam (AIP), and the European Southern Observatory (ESO).

2.1. LAUNCH

AND DEPLOYMENT

JWST will be launched using an Ariane V rocket provided by the European Space Agency. Prior to launch, the primary mirror and other deployable elements (incl. sunshade & secondary mirror) are folded into a shroud. After launch, JWST and its sunshield will be deployed while the spacecraft is still relatively warm (see Figure 7 for the deployment sequence). During the cruise phase to L2, the telescope and instruments will cool to reach their eventual operating temperature of ∼35 K. The detectors, apart from MIRI’s λco = 27 μm Si:As SCAs, are cold-biased to operate a few degrees K above the temperature of the SI. 2.2. I NSTRUMENTS &

DETECTORS

The high-level instrument capabilities are summarized in Figure 8. In the following paragraphs, we describe the instruments and their detectors. For more detailed technical information and test results, interested readers should see, for example, Figer et al. (2004) for NIRCam, NIRSpec, and FGS/TI. For MIRI’s Si:As SCAs, Hoffman, et al. (2005) presents some more recent information. With the exception of MIRI’s three actively cooled λco = 27 μm Si:As SCAs, which are operated at T