Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.eso.org/~rfosbury/tmp/NASdecadesurvey.pdf
Äàòà èçìåíåíèÿ: Tue Jun 22 14:10:44 1999
Äàòà èíäåêñèðîâàíèÿ: Sat Dec 22 09:37:02 2007
Êîäèðîâêà:

Ïîèñêîâûå ñëîâà: arp 220

Table of Contents

5 7 13 17 21 25 29 30 31 32

Introduction Science Goals Missions Concepts Key Trades and Discover y Potential Technology Development Management Approach Beyond NGST Acronyms References Who's Involved

For more information on NGST:

www.ngst.nasa.gov
For an online version of this document:

www.ngst.nasa.gov/cgi-bin/ pubdownload?Id=325

3


· NGST a key component of Origins · 8 m radiatively-cooled telescope at L2 · High resolution, large field, deep sensitivity, low background · 10-years of observing to meet astronomers' needs · Opens door to future affordable observatories

1.0
INTRODUCTION

The Next Generation Space Telescope (NGST) is a key component of NASA's Origins Program. Reflecting major current astrophysics research themes as restated in NASA's Space Science Enterprise strategic plan [1], Origins responds directly to the questions:
·

AT A GLANCE
Primary Mirror 8m Wavelength R ange .6-10+ µm Mission Lifetime 5 years, 10-year goal Passively Cooled <50 K Orbit L2

How did the Universe, galaxies, stars, and planets evolve? How can our exploration of the Universe and our Solar System revolutionize our understanding of physics, chemistry, and biology? Does life in any form -- however simple or complex, carbon-based or other -- exist elsewhere in the Universe? Are there Earth-like planets beyond our Solar System?
including launch, operations, grants, technology development, and inflation, will be around $2B (in real year dollars). This sum represents about onequarter the amount invested in HST. NGST will be a unique scientific tool, with excellent angular resolution over a large field of
continued page 6

·

GST has been under study since 1995 and is planned to be launched around 2008, nearly 400 years after Galileo discovered the moons of Jupiter, over 60 years after Lyman Spitzer proposed space telescopes, and 50 years after the National Space Act created NASA. The mission is a logical successor to the Hubble Space Telescope (HST), and fits in the context of the other Origins missions: FUSE, SOFIA, SIRTF, SIM, and the Terrestrial Planet Finder and Planet Imager, which are planned or under construction. The schedule for these missions is shown in Fig. 1.1. NGST logically depends on technology developed by SIRTF and HST, and, in turn, future missions will use NGST technology to search for terrestrial-sized planets. NGST will be an 8 m class deployable, radiatively cooled telescope, optimized for the 1 ­ 5 µm band, with background limited sensitivity from 0.6 to 10 µm or longer, operating for 10 years near the Earth-Sun second Lagrange point (L2), 1.5 million km from Earth. It will be a generalpurpose observatory, operated by the Space Telescope Science Institute (STScI) for competitively selected observers from the international astronomy community. NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA) will build NGST, with construction to start in 2003. The planned NASA part of the construction budget is $500 M (FY96), but the combined total of NASA, ESA, and CSA contributions,

N

Figure 1.1: The Origins Mission Plan. Each mission builds upon the science and the technologies of its predecessors.

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Atmospheric Background
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Figure 1.2 NGST takes advantage of the lower infrared background of space. The upper cur ve shows a model of the atmospheric and telescope backgrounds for Mauna Kea (1 mm water). The two lower cur ves show the zodiacal background at 1 and 3 AU respectively, with a contribution at longer wavelengths from the 50 K optics.
from page 5

view, deep sensitivity and a low-infrared (IR) background. As a cold space telescope, NGST will achieve far better sensitivities than ground-based telescopes. Figure 1.2 [1.2] shows the background levels from Mauna Kea and in space. They differ by one to six orders of magnitude, depending on wavelength. NGST will have diffraction limited resolution at 2 µm or better, and will achieve much higher Strehl ratios and wider fields of view than anticipated from ground-based telescopes using adaptive optics. NGST's aperture is an order of magnitude larger than SIRTF's, with a factor of 100 better sensitivity. NGST will be able to observe the first generations of stars and galaxies, including individual starburst regions, protogalactic fragments, and supernovae out to redshifts of z = 5 - 20. NGST will resolve individual stars in nearby galaxies, penetrate dust clouds around local star-forming regions, and discover thousands of isolated substellar and Kuiper Belt objects. In 2008, it will be NASA's premier general-purpose observatory, serving the needs of thousands of astronomers and pushing frontier knowledge far beyond the currently known Universe. The NGST design also opens the door for an affordable "product line" of observatories for the future.

6


· HST & Beyond Committee defined need for NGST · Observe the origin & evolution of galaxies · Study structure & chemical enrichment of the Universe · Limitless potential to study star & planet formation--a key Origins topic · Design Reference Mission includes 5 strategic themes enabled by NGST

2.0
SCIENCE GOALS

2.1 The Dressler "Core" Mission: The Origin and Evolution of Galaxies

M

otivated by the spectacular success of HST in pioneering the exploration of high-redshift (z > 1) galaxies and clusters, the HST & Beyond Committee [2.1] foresaw the enormous potential of a scientific successor to HST, optimized for the near infrared (1 - 5 µm), that would

· The far-infrared (FIR > 100 µm) extragalactic background has been found to be comparable in strength to that of the visible and NIR (1-5 µm). The Submillimeter Common User Bolometer Array (SCUBA) has unveiled a population of faint sub-mm sources that may be dust-covered regions of intense star formation or Active Galactic Nuclei (AGN) at z ~ 1. [2.20-2.23] · Observations with HST/NICMOS have established the importance of small and faint galaxies at high redshift and the early creation of galactic spheroids at z > 2. [2.24-2.27] · Collaborative and independent ground and HST observations of Type 1a supernovae suggest that the Universe is accelerating, = 0.8, and hence has an age consistent with the ages of the oldest stars. [2.28-2.29]
continued page 8

"...be an essential tool in an ambitious program of study in many areas of astronomy; it will be especially powerful in studying the origin and evolution of galaxies. By making detailed studies of these distant galaxies, whose light is shifted into the infrared portion of the spectrum, we will be able to look back in time to study the process of galaxy formation as it happened." In fact, the NGST Project has many technical and scientific antecedents. Among them are the Edison proposal to ESA [2.2], the High-Z and MIROS proposals to NASA [2.3-2.4], and a number of workshops concerning missions to follow the Great Observatories [2.5]. However, the report by the HST & Beyond Committee made the key scientific case and created the initial momentum for NGST. Since 1996, when the report was written, many of the advances foreseen by the HST & Beyond Committee as well as several unanticipated ones have occurred: · Distant star-forming galaxies have been observed and studied down to magnitudes as faint as B ~ 29 and z ~ 5.6 by HST and groundbased 8 and 10 m telescopes. [2.6-2.12] · The low but detectable metallicity of the Lyman-alpha forest and damped Lyman-alpha systems have provided evidence for the early formation of stars at z > 4. [2.13-2.16] · Observations of the ultraviolet luminosity density at high redshift by Keck and HST have placed a lower limit on the early chemical history of the Universe. [2.17-2.19]

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Figure 2.1: The sensitivity of an NGST deep field (106 s in 30% bandwidths, 10-sigma detection). Also indicated are the spectra of starburst regions (106 solar masses in 106 years) and established populations (108 solar masses at 1 Gyr) at various redshifts (m = 0.2). Comparable sensitivities also are shown for the HDF using NICMOS. (Lilly, University of Toronto)

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from page 7

We will see many more such dramatic advances over the next nine years leading up to the launch of NGST in 2008. With more than a dozen large, IR-optimized ground-based telescopes, with HST and its Advanced Camera for Surveys (ACS) and possibly an NIR channel in the Wide Field Camera 3, and especially with SIRTF, we expect that by 2008 [2.30]: · Morphological and spectroscopic surveys of galaxies with 0 < z < 1 will be reasonably complete -- likely showing that the assembly of many galaxies has occurred before this relatively modern epoch. · Large samples of luminous galaxies with redshifts 1 < z < 4 will be obtained by HST, SIRTF, and the 8 to 10 m diameter ground-based telescopes. We may know when large, mature galaxies first appear but not how. · Redshift records will continue to be broken. However, we will likely not know whether the redshifts of the first, crucial episodes of "galactogenesis" have been reached. Rare, bright, J-band dropouts (high-redshift galaxies with z > 9) will be detected and confirmed. However, without NGST, the samples will remain too small and of little use in constraining models of early galaxy formation.

In 2008, NGST will be poised to build upon these foundations to complete our understanding of the formation and early evolution of galaxies such as the Milky Way. Other facilities will have exploited their capabilities and have "hit the wall" as they attempt to reach fainter, redder targets. Although ground-based NIR observations will benefit from adaptive optics, their "wall" will be the dramatic increase in atmospheric emission at NIR wavelengths. For HST, it will be the thermal emission from the entire telescope at > 1.8 µm; for SIRTF, it will be the inherent limitation of a 0.8 m aperture. The NGST Ad Hoc Science Working Group (ASWG) and oversight bodies such as the Science Oversight Committee (1996-97) and the NGST External Science Review (NESR, 1998) concur that understanding the origins and evolution of galaxies is NGST's primary science goal. To do that, we need attributes that only NGST will possess, capabilities that will make it the premier observatory at the end of the next decade, enabling such studies as: · Detecting the earliest phases of star and galaxy formation -- the end of the "dark ages" (Fig. 2.1) [2.31]. This requires superb NIR sensitivity (< 1 nJy, 1-4 µm) in deep broadband imaging (~ 105 s).
continued page 9

8

Figure 2.2: Simulated NGST spectr um of the nearby starburst galaxy NGC 7714, obser ved at z = 6. (Kennicutt, Steward Obser vator y) [2.33]


from page 8

· Resolving the first galactic substructures larger than individual star clusters (~ 300 pc for 0.5 < z < 5.0). This requires HST-like resolution in the NIR (~ 0.060" at 2 µm) [2.32]. · Quantitatively measuring the fundamental properties of individual galaxies. This will be enabled by emission-line and absorption-line spectroscopy, with broad spectral coverage and low-to-moderate spectral resolution (R = /): R ~ 300 (0.6-5.0 µm) for redshift confirmation, cluster membership, and ages of stellar populations; R ~ 1000 (0.6-5.0 µm or longer) for star formation rates, metallicity, and reddening (Fig. 2.2); R ~ 3000 (1.0-10 µm) for dynamics (mass). · Statistically analyzing high-redshift galaxy properties, clustering, and rates of interaction. This will be accomplished with wide field (~ 4' x 4') imaging and spectroscopic surveys. This angular size corresponds to restframe scales ~ 1 Mpc x 1 Mpc (for 0.5 < z < 5.0 and all reasonable cosmologies) and will include all likely progenitor substructures within galactic regions comparable to the Local Group, as well as the central regions of distant clusters of galaxies. (Fig. 2.3) · Detecting and diagnosing dust-enshrouded regions hiding massive star formation or active galactic nuclei during the epoch of greatest star formation to a minimum of z ~ 2. Resolving the mid-infrared (MIR) and far-infrared (FIR) backgrounds would be enabled with the NGST stretch goal of MIR imaging and spectroscopy (5-28 µm). (Fig. 2.4) 2.2 The Structure and Chemical Enrichment of the Universe The geometry and structure of the Universe, as well as its history of element formation, is intimately related to the formation of galaxies. In the coming decade, the MAP and Planck missions will measure the power spectrum of the Cosmic Microwave Background (CMB) at z ~ 1300 and, using standard models, will provide or constrain key cosmological constants. NGST will play a powerful complementary role in determining the distribution of mass and light on small scales. Large microlensing imaging surveys will use the wide field, superb angular resolution, and excellent 0.6-5.0 µm sensitivity of NGST to measure the mass structure of the Universe at z = 1 - 5 on scales smaller than those probed by CMB measurements from space or possible from the ground or HST.

Anticipated science programs include: · The dark matter halos of galaxies to redshifts of z ~ 5 will be weighed statistically by deep imaging of selected fields. · The growth of galaxy clusters to redshifts of z ~1-3 will be measured using multi-color deep imaging of selected high-redshift clusters and proto-clusters discovered by AXAF, Planck, and ground-based surveys. · The statistical properties of the distribution of matter on scales of 1-10 Mpc can be found from wide-area, high-resolution NGST imaging surveys (>1 deg2). These scales are larger than those of galaxy clusters and smaller than those probed by the CMB satellites and ground-based surveys. [2.35] These imaging programs are comparable in depth and required field of view to those used for the study of galaxy evolution. Such surveys also provide an excellent method for discovering Type 1a and Type 2 supernovae (SNe) at redshifts between 1 < z < 5. Supernovae at even higher redshifts could be confirmed by NGST and followed, using ground-based survey telescopes, to detect their brief but luminous ultraviolet precursor transients. Measuring the rates and galactic associations of Type 1a and Type 2 supernovae will provide an independent assessment of the history of element production. We
continued page 10

Figure 2.3: A simulated image of the HDF (right panel) and a simulated 20 orbit, 0.08-1.6 µm, R ~ 300 HST multi-object spectr um of the same field with a limiting magnitude of JAB ~ 25.1. A comparable spectrograph for NGST would have twice the field of view and be an order of magnitude more sensitive. (Stiavelli, STScI) [2.34]

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from page 9

expect that NGST will be crucial in extending the observations of Type 1a supernovae beyond z ~ 0.9 to z ~ 5. Only at the higher redshifts is it possible to distinguish between the behavior of Type 1a supernovae with cosmologies involving only H0, m and , and models with significant SNe evolution or smoothly distributed gray obscuration (Fig. 2.5). Such data will provide measurements of the cosmological parameters, which are independent of and complementary to those derived from the CMB missions. These science programs will require coordinated preparation and data analysis efforts by the community to optimize the science return: · The microlensing surveys will require well-characterized, high-resolution, point-spread functions over the entire wide field of view and wavelength range (0.6-5 µm). This can be accomplished either by special calibrations or some form of continuous figure sensing of the individual primary segments. · To follow the light curves of supernovae, the NGST science operations must respond to new supernovae discovered in NGST fields within ~ 1 week. The time dilation at high redshift

helps relax this requirement compared with that needed for nearby supernovae. NGST also must return to the same field, perhaps in a different orientation, over periods of two weeks to six months (for the highest redshift supernovae). 2.3 The Pr ocesses of Star and Planet Formation This key Origins topic will be addressed by many ground and space observatories and over a broad range of wavelengths (Angstroms - millimeters). Nevertheless, NGST, with an extended MIR wavelength coverage (5-28 µm), will have a unique role in this area, comparable in importance to MMA/LSA. The potential studies in this arena are essentially limitless and depend crucially on the available spectral resolutions and MIR wavelength coverage. We foresee the following examples: · Characterizing the infall and outflow processes through which stars are built and their final masses determined. MIR spectroscopy will diagnose the accretion shocks in protostellar systems, while NIR imaging will reveal outflow shocks and jets near their source, with a resolution of ~ 2 AU.
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0.1

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Figure 2.4: The imaging sensitivity of NGST compared with other facilities in the MIR for a 1 arcsec2 tar get. We include a spectr um similar to that of Arp 220 at z = 2 and with 1% intrinsic luminosity. Only the Millimeter Array/Large Southern Ar ray (MMA/LSA) will have a comparable sensitivity to such dust-enshrouded star-for mation regions. (Lilly, University of Toronto)

Figure 2.5: The deviations of obser ved brightness of Type 1a SNe at maximum light compared with the prediction for an empty Universe (m = 0.0) and those with m = 0.2) and dif fering acceleration parameters ( = 0.0, 0.2, 0.4, 0.6, 0.8). We also indicate the trend expected for = 0.0 and an evolutionar y ef fect that causes high-redshift supernovae to be ~ 0.5 magnitudes fainter at z > 1. (STScI)

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from page 10

· Tracing the structure and evolution of circumstellar material, from the massive envelopes of Class 0 protostars to the protoplanetary disks of pre-main sequence stars, and finally to the dissipation of these disks into mature debris disks of main sequence stars. NIR and MIR spectroscopy of gas and dust features, their excitation, and their radial variation within the circumstellar region will permit study of the growth of dust grains toward planetesimals, the chemical processing of disk gas, and the disk dissipation mechanisms that define the time available for planet formation. High resolution NIR and MIR imaging with NGST will be a powerful probe of the distribution of cool material in dense circumstellar regions, allowing the resolution of AU-scale structures. Images in thermal emission, and perhaps also in reflected light if a coronagraph is provided, will enable direct study of central holes and radial gaps in massive protoplanetary and tenuous debris disks. Such dynamically driven internal structures provide indirect evidence of the presence of planets. · Detecting and characterizing substellar objects. Ground-based sky surveys and adaptive optics programs are now beginning to discover significant numbers of isolated and companion brown dwarf stars. However, these observations will be limited to the bright (high mass/low age) end of the substellar luminosity function and to wide binary companions. Only NGST will have the needed combination of high-angular resolution, high sensitivity, and a stable PSF for high-contrast imaging of faint substellar companions in planetary orbits. By observing at 5 µm with a graded-mask coronagraph, the baseline NGST configuration will be able to directly detect planets with Jupiter 's mass, age, and orbital semi-major axis in more than 90% of the single stars within 8 pc of the Sun (> 50 systems). By detecting planetary photons directly, NGST will provide the first opportunity to spectrally characterize exoplanet atmospheres. In conjunction with mass determinations for the companions from astrometric surveys, NGST observations will allow the theoretical cooling curves for substellar objects to be checked against actual luminosity and temperature measurements. Finally, by taking the first direct images of planets orbiting other stars, NGST will make a strong impact on the minds of the general public.

2.4 The Design Reference Mission The NGST science described above is part of the Design Reference Mission (DRM), a set of science programs enabled by NGST [2.35]. The goals of the DRM are to: · Provide examples of NGST science to stimulate further inputs from the astronomy community. · Provide descriptions of science programs in sufficient detail to derive secondary requirements/capabilities of the observatory. · Provide a semi-quantitative basis for trade studies (e.g., sensitivity versus field of view). These science programs have been assembled by the NGST Ad Hoc Science Working Group under five themes and can be accessed through the NGST science website [2.36]. During the Formulation phase (Phase A/B), we will continue to solicit programs for the DRM from our international partners and the astronomy community.

Cosmology and Structure of the Universe Origin and Evolution of Galaxies History of the Local Universe Birth of Stars Origin and Evolution of Planetary Systems

2.5 Summar y: Addressing Strategic Science Goals NGST science goals address key objectives in NASA's Strategic Plan as well as major research themes articulated by the astronomy community. These investigations are crucial to our understanding of the early formation of galaxies and their subsequent evolution. The statistical analyses of deep fields and the observations of distant supernovae and star-formation regions will unveil clues to the underlying structure and chemical evolution of the Universe. Investigations of star and planetary systems through observations in the MIR will clarify formation processes in new systems and discover the fossil evidence of formation in stars like the Sun. The key science goals detailed in the Design Reference Mission clarify the technical requirements for NGST and represent a broad range of investigations. With its high resolution, large field of view, deep sensitivity and low background, NGST will be the next major step in enabling Origins research.

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· Three mission architectures studied; NASA "Yardstick" used as reference · Three-mirror anastigmat provides accessible pupil and fast primary · 8 m segmented primary -- central mirror with eight petals · Instrument complement of NIR camera, multi-object NIR spectrometer, MIR camera/spectrometer · Passive cooling uses multi-layer shield, baffles, stops, large surface area · 40% immediate sky accessibility; full-sky in 6 months · Deployed by next generation medium launcher or Atlas IIAS. · STScI as Science and Operations Center

3.0
MISSION CONCEPT

T

he science goals for NGST require a telescope with high sensitivity covering the wavelength range from 0.6 to 10 µm, with capability out to 28 µm, and with NIR angular resolution comparable to that of HST. Ball Aerospace, TRW, and NASA studied three mission architectures during pre-Phase A [3.1]. For simplicity, the NASA architecture, referred to as the Yardstick, is presented here. The other concepts are similar, responding to the same high level requirements. The Yardstick architecture established the technical and financial feasibility of the mission, and serves as a reference design to which proposed architectures and instruments can be compared. Figure 3.1 shows the observatory and its main components: the Optical Telescope Assembly (OTA), the Integrated Science Instruments Module (ISIM) and the Spacecraft Support Module (SSM).

Secondar y Mirror Spacecraft Support Module

Optical Telescope Assembly

Sunshield

Primar y Mirror Integrated Science Instrument Module

Fig.3.1: The Elements of the Yardstick Concept.

3.1 The Yar dstick Optical Telescope Assembly The Yardstick optical configuration is a threemirror anastigmat that provides a real, accessible pupil and permits a relatively fast primary mirror to minimize telescope length. This design provides excellent imaging over a field of more than 20 arcminutes with achievable alignment tolerances. A real pupil permits the use of a deformable mirror (DM) for wavefront correction, and a fast-steering mirror for fine pointing using image compensation. The primary mirror is a compact 8 m diameter segmented aperture. It is composed of a central mirror segment, with a diameter of 3.3 meters, surrounded by eight petals. The petals are folded alternately up and down and deployed after launch (Fig. 3.2). The Yardstick mirror is made of beryllium, thermally controlled with very low power heaters (20 mW total) so that its figure remains insensitive to rapid or large positioning slews. The areal density of the primary mirror assembly (mirror, actuators and backup structure) is 13 kg/m2. The DM provides a design margin for figure errors in the primary mirror, including those due to gravity release, thermal gradients, or edge effects. The DM will correct the wavefront so that the system will be diffraction limited below 2 µm. Unlike telescopes such as HST that are launched fully assembled, NGST must be able to compensate for errors in deployment position, long-term dimensional changes, and on-orbit thermal variations. Optics are aligned and phased by observing the image of a star and deriving mirror position corrections. Wavefront errors are determined by obtaining defocused star images and analyzing the image with a "phase retrieval" computer algorithm [3.2]. Simulation of typical wavefront errors due to polishing, thermal gradients, etc., and diffraction effects due to aperture notches, gaps and obstruction of the secondary mirror support, indicate that
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the final image will have a Strehl ratio of about 81% at 2 µm, and 60% at 0.6 µm without additional DM correction. 3.2 The Yar dstick Integrated Science Instrument Module The Integrated Science Instrument Module (ISIM) consists of a cryogenic instrument module integrated with the OTA, and processors, software, and other electronics located in the Spacecraft Support Module. The ISIM provides the structure, environment, and data handling for several modular science instruments as well as components of the OTA system - the tertiary mirror, DM, and fast-steering mirror. This ISIM design is illustrative and is not intended to define NGST's final complement of instruments. A wide range of pre-Formulation phase (pre-Phase A) studies of ISIM architecture and individual science instruments are being conducted by science community teams in the US, Europe, and Canada [3.3]. Procurement responsibility for the science instruments will be allocated among NASA, ESA, and CSA during the Formula-

tion phase (Phase A/B), and instrument proposals solicited by those agencies following selection of the flight NGST architecture. The Yardstick instrument suite includes: · A NIR camera covering 0.6 to 5 µm, critically sampled at 2 µm. Efficient surveying capability, as well as guiding requirements, set the field at about 4' x 4', apportioned over four subcameras each covering a field of 2' x 2'. The NIR detectors (InSb or thinned HgCdTe) are radiatively cooled to 30 K. · A NIR multi-object spectrometer, with spectral resolutions of 300 and 3000 and a spatial resolution of 100 mas, covering a field of 3' x 3'. Multi-object capability is enabled by an array of 20482 micro-mirrors used to form a reflective slit mask, directing light into or away from the spectrometer. · A MIR camera/spectrometer covering a field of 2' x 2' with a spectral range of 5 - 28 µm using a 1K x 1K Si:As array as detector, and a long slit cross-dispersed grism. Its spectral resolution is ~103. The camera employs a selection of slits and a no-slit option to enable direct imaging with filters. The MIR detector is cooled to 6 K by a miniaturized reverse turbo-Brayton cooler; open cycle solid hydrogen cooling has been identified as a viable alternative. Following the 1996 NGST study [3.4], the NASA Project undertook a detailed design study of the ISIM [3.5] to demonstrate engineering feasibility of the mission's science goals, assess the required technologies, and revisit the cost estimates. This study concluded that all engineering requirements of the baseline instrument complement including detector, thermal, and data system requirements are feasible with technology that is expected to be mature in 2003 at the beginning of the Implementation phase (Phase C/D). In addition, this study suggested that a highly modular approach for the ISIM is possible, enabling procurement of individual instruments from science community teams. 3.3 Passive Cooling and Thermal Contr ol All NGST designs solve the problem of cooling to the cryogenic temperatures required for NIR and MIR operation passively by: · Protecting the observatory from the Sun with a multi-layer shield; · Using a heliocentric orbit to decrease the Earth's thermal input; and
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Secondar y Mirror

For ward-Folded Primar y Mirror Petals

Deployable Optical Bench and Fixed Conical Baffle Tube

Aft-Folded Primar y Mirror Petals (4)

SI Module

Sunshield Solar Array

Fig. 3.2: The Yardstick Launch Configuration.

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3.5 Launch and Orbit The overall mass of the Yardstick NGST is approximately 3300 kg, within the capability of an Atlas IIAS or the next generation of medium launchers (EELV Medium). The launch sequence is shown schematically in Fig. 3.4. Deployment of the OTA occurs soon after launch, before the sunshield is deployed, while all the mechanisms are still relatively warm. Optics alignment can then begin, followed by science calibration as the telescope cools. The halo orbit at L2 is reached about 3 months later. 3.6 Science and Mission Operations NGST's Science and Mission Operations will be simple and efficient because of the telescope's location at L2. Operations for NGST will be based upon an optimized long-range observing plan that consists of sequenced science programs ranging from large imaging surveys, often with preplanned spectroscopic follow-ups, to intensive studies of individual objects. This long-range optimization is required since the amount of time needed at some target positions