Download - Figure 3. · 684 IMAGING SCIENCE IN ASTRONOMY Figure 3. Schematic diagram showing various regimes of the electromagnetic spectrum in terms of temperatures corresponding to emission

682 IMAGING SCIENCE IN ASTRONOMY

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IMAGING SCIENCE IN ASTRONOMY

JOEL H. KASTNER

Rochester Institute of TechnologyRochester, NY

INTRODUCTION

The vast majority of information about the universe iscollected via electromagnetic radiation. This radiation isemitted by matter distributed across tremendous rangesin temperature, density, and chemical composition. Thus,more than any other science, astronomy depends oninnovative methods to extend image taking to new,unexplored regions of the electromagnetic spectrum. Tobring sufficient breadth and depth to their studies,astronomers also require imaging capability across a vastrange in spatial resolution and sensitivity, with emphasison achieving the highest possible resolution and signalgain in a given wavelength regime. This simultaneousquest for better wavelength coverage and ever higherspatial resolution and sensitivity represents the driving

force for innovation and discovery in astronomicalimaging.

Classical astronomy — for example, the search for newsolar system objects and the classification of stars — isstill largely conducted in the optical wavelength regime(400–700 nm). This has been the case, of course, sincehumans first imagined the constellations, noted theappearance of ‘‘wandering stars’’ (planets), and recordedthe appearance of transient phenomena such as cometsand novae. During the latter half of the twentieth century,however, a revolution in astronomical imaging tookplace (1). This relatively brief period in recorded historysaw the development and rapid refinement of techniquesfor collecting and detecting electromagnetic radiationacross a far broader wavelength range, from the radiothrough γ rays. Just as these techniques have reachedmaturation, astronomers have also developed the meansto surmount apparently fundamental physical barriersplaced on image quality, such as the distorting effectsof refraction by Earth’s atmosphere and diffraction by asingle telescope of finite aperture. The accelerating pace ofthese innovations has resulted in deeper understanding of,and heightened appreciation for, both the rich diversity ofastrophysical phenomena and the fundamental, unsolvedmysteries of the cosmos.

OPENING THE WINDOWS: MULTIWAVELENGTHIMAGING

For most of us, our eyes provide our first, fundamentalcontact with the universe. It is interesting to ponderhow humans would conceive of the universe if we hadnothing more in the way of imaging apparatus at ourdisposal, as was the case for astronomers before Galileo.In contrast to the complex cosmologies currently ponderedin modern physics, most of which involve an expandinguniverse shadowed by the afterglow of the Big Bang, the‘‘first contact’’ provided by our eyes produces a model ofthe universe that is entirely limited to the Sun, Moon,and planets, the nearby stars, and the faint glow ofthe collective background of stars in our own MilkyWay galaxy and a handful of other, nearby galaxies.From this simple thought experiment, it is clear that thebulk of the visible radiation arriving at Earth is emittedby stars.

But the apparent predominance of visible light fromthe Sun and nearby stars is in fact merely an accidentof our particular position in the universe, combinedwith the evolutionary adaptation that gave our eyesmaximal sensitivity at wavelengths of electromagneticradiation that are near the maximum of the Sun’s energyoutput. The Sun provides by far the majority of thevisible radiation arriving at Earth strictly by virtueof its proximity. The brightest star in the night sky,Sirius (in the constellation Canis Major), actually hasan intrinsic luminosity about 50 times larger than that ofthe Sun, but is about 8.6 light years distant (a light yearis the distance traveled by light in one year, 9 × 1012 km;the Sun is about 8 light minutes from Earth). In turn,Sirius is only about one-ten-thousandth as luminous asthe star Rigel (in the neighboring constellation Orion),

IMAGING SCIENCE IN ASTRONOMY 683

but Sirius appears several times brighter than Rigelbecause it is about 50 times closer to us. Like the Sun,which has a surface temperature of about 6,000 K, mostof the brightest stars have surfaces within the rangeof temperatures across which hot objects radiate veryefficiently (if not predominantly) in the visible region.Representative stellar surface temperatures are 3,000 Kfor reddish Betelgeuse, a red supergiant in Orion; 10,000 Kfor Sirius; and 15,000 K for the blue supergiant Rigel(Fig. 1).

Thermal Continuum Emission

The tendency of objects at the temperatures of the Sunand stars to emit in the visible can be understood to firstorder via Planck’s Law, which describes the wavelengthdependence of radiation emitted by a perfect blackbody.The peak of the Planck function lies within the visibleregime for an object at a temperature of 6,000 K. This samefundamental physical principle tells us that objects muchhotter or cooler than the Sun should radiate predominantlyat wavelengths much shorter or longer than visible,respectively. Indeed, for a perfect blackbody, the peakwavelength of radiation is given by Wien’s displacementlaw (2),

λ(cm) ∼ 0.51T(K)

. (1)

This relationship between the temperatures of objects andthe wavelengths of their emergent radiation allows us tounderstand why Betelgeuse appears reddish and Rigelappears blue (Fig. 2).

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Figure 1. The Hertzsprung–Russell diagram. The diagramshows the main sequence (Sun-like stars that are fusing hydrogento helium in the cores), red giants, supergiants, and whitedwarfs. In addition, the positions of the Sun, the twelve brighteststars visible from the Northern Hemisphere, and the whitedwarf companions of Sirius and Procyon are indicated [Source:NASA (http://observe.ivv.nasa.gov/nasa/core.shtml.html)]. Seecolor insert.

Figure 2. Wide-field photograph of Orion, illustrating thedifference in color between the relatively cool star Betelgeuse(upper left) and the hot star Rigel (lower right). The large, redobject at the lower center of the image, just below Orion’s belt,is the Orion Nebula (see Fig. 7). (Photo credit: Till Credner,AlltheSky.com) See color insert.

The same, simple relationship also provides powerfulinsight into astrophysical processes that occur across avery wide range of energy regimes (Fig. 3). The lowestenergies and hence longest (radio) wavelengths reveal‘‘cold’’ phenomena, such as emission from dust and gas inoptically opaque clouds distributed throughout interstellarspace in our galaxy. At the highest energies and henceshortest wavelengths (characteristic of X rays and γ rays),astronomers probe the ‘‘hottest’’ objects, such as theexplosions of supermassive stars or the last vestigesof superheated material that is about to spiral into ablack hole.

Nonthermal Continuum Emission

Certain radiative phenomena in astrophysics do notstrongly depend on the precise temperature of the materialand are instead sensitive probes of material density and/orchemical composition (3,4). For example, the emissionfrom ‘‘jets’’ ejected from supermassive black holes atthe centers of certain galaxies (Fig. 4) is said to be


Figure 3. Schematic diagram showing various regimes of the electromagnetic spectrum in termsof temperatures corresponding to emission in that regime. The diagram also illustrates thewavelength ‘‘niches’’ of NASA’s four orbiting ‘‘Great Observatories.’’ [Source: NASA/ChandraX-Ray Center (http://chandra.harvard.edu)]. See color insert.

‘‘nonthermal’’ because its source is high-velocity electronsthat orbit around magnetic field lines. Other, similarexamples are the emission from filaments of ionized gaslocated near the center of our own galaxy and from thechaotic remnant of the explosion of a massive star in1054 A.D. (the ‘‘Crab Nebula’’). Such so-called ‘‘synchrotronradiation’’ often dominates radiation emitted in the radiowavelength regime (Fig. 5). Indeed, if human eyes weresensitive to radio rather than to visible wavelengths, theearly mariners probably would have navigated by theGalactic Center and the Crab because they appear fromEarth as the brightest stationary radio continuum sourcesin the northern sky. The synchrotron emission from theCrab is particularly noteworthy; it can be detected acrossa very broad wavelength range from radio through X ray(Fig. 6).

Monochromatic (‘‘Line’’) Emission and Absorption

Deducing Chemical Compositions. Astronomers useelectronic transitions of atoms (as well as electronic,vibrational, and rotational transitions of molecules) asRosetta stones to understand the chemical makeup ofgas in a wide variety of astrophysical environments.Because each element or molecule radiates (and absorbsradiation) at a discrete and generally well-determined setof wavelengths — specified by that element’s particularsubatomic structure — detection of an excess (or deficit) of

emission at one of these specific wavelengths1 is bothnecessary and sufficient to determine the presence ofthat element or molecule. Hence, our knowledge of theorigin and evolution of the elements that make up theuniverse is derived from astronomical spectroscopy (whichmight also be considered multiband, one-dimensionalimaging).

Spectra obtained by disparate means across a verybroad range of wavelengths can be used to ascertainboth chemical compositions and physical conditions (i.e.,temperatures and densities) of astronomical sourcesbecause the emissive characteristics of a given elementdepend on the physical conditions of the gas or dustin which it resides. For example, cold (100 K), largelyneutral hydrogen gas emits strongly in the radio at 21 cm,whereas hot (10,000 K), largely ionized hydrogen gasemits at a series of optical wavelengths (known as theBalmer series). The former conditions are typical of thegas that permeates interstellar space in our own galaxyand in external galaxies, and the latter conditions aretypical of gas in the proximity of very hot stars, whichare sources of ionizing ultraviolet light. Such ionizedgas also tends to glow brightly in the emission lines ofheavier elements such as oxygen, nitrogen, sulfur, andiron (Fig. 7).

1 Such spectral features are called ‘‘lines,’’ because they appearedas dark lines in early spectra of the Sun.


Figure 4. At a distance of 11 million light years, Centaurus Ais the nearest example of a so-called ‘‘active galaxy.’’ This radioimage shows opposing ‘‘jets’’ of high energy particles blasting outfrom its center [Source: National Radio Astronomy Observatory(NRAO)]. See color insert.

Figure 5. The Crab Nebula is the remnant of a supernovaexplosion that was seen from the earth in 1054 A.D. It is 6,000light years from Earth. This radio image shows the complexarrangement of gas filaments left in the wake of the explosion(Source: NRAO). See color insert.

Deducing Radial Velocities from Spectral Lines. Atomicand molecular emission lines also serve as probes of bulkmotion. If a given source has a component of velocityalong our line of sight, then its emission lines will be

Figure 6. X-ray image of the innermost region of the CrabNebula. This image covers a field of view about one-quarterthat of the radio image in the previous figure. The image showstilted rings or waves of high-energy particles that appear to havebeen flung outward across a distance of a light year from thecentral star (Source: Chandra X-Ray Center). See color insert.

Figure 7. Color mosaic of the central part of the Great Nebulain Orion, obtained by the Hubble Space Telescope. Light emittedby ionized oxygen is shown as blue, ionized hydrogen emissionis shown as green, and ionized nitrogen emission as red. Thesources of ionization of the nebula are the hot, blue-white starsof the young Trapezium cluster, which is embedded in nebulosityjust left of center in the image (Source: NASA and C.R. O’Delland S.K. Wong). See color insert.

Doppler shifted away from the rest wavelength. Theabsorption or emission lines of sources that approach


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Figure 8. Plot of recession velocity vs. distance [in megaparsecs(Mpc); 1 Mpc ≈ 3 × 1019 km] for a sample of galaxies. This figureillustrates that, to very high accuracy, the recession velocityof a distant galaxy, as measured from its redshift, is directlyproportional to its distance. This correlation was first establishedin 1929 by Edwin Hubble and underpins the Big Bang model forthe origin of the Universe (Figure courtesy Edward L. Wright, 1996).

us are shifted to shorter wavelengths and are said tobe ‘‘blueshifted,’’ whereas the lines of sources movingaway from us are shifted to longer wavelengths and aresaid to be ‘‘redshifted.’’ The observation by Hubble in1929 that emission lines of distant galaxies are uniformlyredshifted and that these redshifts increase monotonicallyas the distances of the galaxies increase, underpinsmodern theories of the expansion of the universe2 (Fig. 8).Images obtained at multiple wavelengths that span therest wavelength of a bright spectral line can allowastronomers to deduce the spatial variation of line-of-sight velocity for a source whose velocity gradientsare large. Such velocity mapping, which is presentlyfeasible at wavelengths from the radio through the optical,helps elucidate the three-dimensional structure of sources(Fig. 9).

Multiwavelength Astronomical Imaging: An Example

Planetary nebulae represent the last stages of dying, Sun-like stars. These highly photogenic nebulae are formedafter the nuclear fuel at the core of a Sun-like star hasbeen spent, that is, the bulk of the core hydrogen has beenconverted to helium. The exhaustion of core hydrogenand the subsequent nuclear fusion, in concentric shells, ofhydrogen into helium and helium into carbon around the

2 In practice, all astrophysical sources that emit line radia-tion — even those within our solar system — will appear Dopplershifted, due for example, to the Earth’s motion around the Sun.Hence it is necessary to account properly for ‘‘local’’ sources ofDoppler shifts when deducing the line-of-sight velocity componentof interest.

spent core causes the atmosphere of the star to expand,forming a red giant. Although the extended atmospheresof red giants are ‘‘cool’’ enough (∼3,000 K) for dust grainsto condense out of the stellar gas, red giant luminositiescan be huge (more than 10,000 times that of the Sun).This radiant energy pushes dust away from the outeratmosphere of the star at speeds of 10–20 km s−1. Theoutflowing dust then collides with and accelerates thegas away from the star, as well. Eventually enough ofthe atmosphere is removed so that the hot, inert stellarcore is revealed. This hot core is destined to becomea fossil remnant of the original star: a white dwarf.But before the ejected atmosphere departs the sceneentirely, it is ionized by the intense ultraviolet light fromthe emerging white dwarf, which has cooled from corenuclear fusion temperatures (107 to 108 K) to a ‘‘mere’’105 K or so. The ionizing radiation from the white dwarfcauses the ejected gas to fluoresce, thereby producing aplanetary nebula.

Because the varied conditions that characterize theevolution of planetary nebulae result in a wide variety ofphenomena in any given nebula, such objects demand amultiwavelength approach to imaging. A case in point isthe young planetary nebula BD +30° 3639 (Fig. 10). Thisplanetary nebula emits strongly at wavelengths rangingfrom radio through X ray. The Chandra X-ray image showsa region of X-ray emission that seems to fit perfectly insidethe shell of ionized and molecular gas seen in Hubble SpaceTelescope images and in other high-resolution imagesobtained from the ground. The optical and X-ray emittingregions of BD +30° 3639, which lies about 5,000 lightyears away, are roughly 1 million times the volume of oursolar system. The X-ray emission apparently originates inthin gas that is heated by collisions between the ‘‘new’’wind blown by the white dwarf, which is seen at thecenter of the optical and infrared images, and the ‘‘old,’’photoionized red giant wind, which appears as a shellof ∼10,000 K gas surrounding the ‘‘hot bubble’’ of X-rayemission.

REQUIREMENTS AND LIMITATIONS

To understand the requirements placed on spatialresolution and sensitivity in astronomical imaging, wemust consider the angular sizes and energy fluxes ofastronomical objects and phenomena of interest. In turn,there are three fundamental sources of limitation on theresolution and limiting sensitivity (and hence quality) ofastronomical images: the atmosphere, the telescope, andthe detector.

Spatial Resolution

Requirements: Angular Size Scales of AstronomicalSources. Figure 11 shows schematically typical scales ofphysical size and distance from Earth for representativeobjects and phenomena studied by astronomers. Most ofthe objects of intrinsically small size, like the Sun, Moon,and the planets in our solar system, lie at small distances;we can study these small objects in detail only becausethey are relatively close, such that their angular sizes aresubstantial.


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R.A. (2000.0)21h02m18.0

Figure 9. Radio maps of the Egg Nebula, a dying star in the constellation Cygnus, showingemission from the carbon monoxide molecule. At the lower left is shown blueshifted CO emission,and at the lower right redshifted emission; the upper right panel shows the total intensity of COemission from the source. One interpretation for the localized appearance of the blueshifted andredshifted CO emission is that the Egg Nebula is the source of a complex system of ‘‘molecularjets,’’ shown schematically in the top left panel. Such jets may be quite common during the dyingstages of Sun-like stars [Source: Lucas et al. 2000 (5)]. See color insert.

Within our own Milky Way galaxy, we observe objectsthat span a great range of angular size scales. Theangular size of a Sun-like star at even a modestdistance makes such stars a challenge to resolve spatially,even with the best available techniques. On the otherhand, many structures of interest in our own MilkyWay galaxy, such as star-forming molecular clouds andthe expelled remnants of dying or expired stars, aresufficiently large that their angular sizes are quite large.3

Certain giant molecular clouds, planetary nebulae, andsupernova remnants subtend solid angles similar to thatof the Moon.

Just as for stars, the angular sizes of external galaxiesspan a very wide range. The Magellanic Clouds, which arethe nearest members of the Local Group of galaxies (ofwhich the Milky Way is the most massive and luminous

3 The ejected envelopes of certain dying, sun-like stars were longago dubbed ‘‘planetary nebulae’’ because their angular sizes andround shapes resembled the planets Jupiter and Saturn.

member), are detectable and resolvable by the naked eye,whereas the Andromeda galaxy (a Local Group memberthat is a near-twin to the Milky Way) is detectableand resolvable with the aid of binoculars. The angularsizes of intrinsically similar galaxies in more distantgalaxy clusters span a range similar to that of theplanets in our solar system. The luminous cores of certaindistant galaxies (‘‘quasars’’) — which can outshine theirhost galaxies — likely have sizes only on the order ofthat of our solar system; yet these are some of the mostdistant objects known, and hence quasars are exceedinglysmall in angular size. Galaxy clusters themselves areof relatively large angular size, simply by virtue oftheir enormous size scales; indeed, such clusters (andlarger scale structures that consist of clusters of suchclusters) probably represent the largest gravitationallybound structures in the universe. At still larger sizescales lies the cosmic background radiation, the radiativeremnant of the Big Bang itself. This radiation encompasses4π steradians and has only very subtle variations inintensity with position across the sky.


Figure 10. Optical (left), infrared (center), and X-ray (right) images of the planetary nebulaBD +30° 3639 [Source: Kastner et al. 2000 (6)]. The optical image was obtained by the WideField/Planetary Camera 2 aboard the Hubble Space Telescope in the light of doubly ionizedsulfur at a wavelength of 9,532 A. The infrared image was obtained by the 8-meter Gemini Northtelescope at a wavelength of 2.2 µm (also referred to as the infrared K band). The X-ray image wasobtained by the Advanced CCD Imaging Spectrometer aboard the Chandra X-Ray Observatory,and covers the wavelength range from ∼7 A to ∼30 A. Images are presented at the same spatialscale. See color insert.

Of course, even within our solar system, there aresources of great interest (e.g., the primordial, comet-likebodies of the Kuiper Belt) that are sufficiently small thatthey are unresolvable by present imaging techniques.Sources of large angular sizes (such as molecular clouds,planetary nebulae, supernova remnants, and galaxyclusters) typically show a great wealth of structuraldetail when imaged at high spatial resolution. Thus,our knowledge of objects at all size and distance scalesimproves with any increase in spatial resolving power ata given wavelength.

Limitations

Atmosphere. Time- and position-dependent refractionby turbulent cells in the atmosphere causes astronomicalpoint sources, such as stars, to ‘‘scintillate’’; i.e., starstwinkle. Scintillation occurs when previously plane-parallel wave fronts from very distant sources encounteratmospheric cells and become distorted. Astronomers usethe term ‘‘seeing’’ to characterize such atmospheric imagedistortion; the ‘‘seeing disk’’ represents the diameter ofan unresolved (point) source that has been smeared byatmospheric distortion. Seeing varies widely from site tosite, but optical seeing disks at visual wavelengths aretypically not smaller than (that is, the seeing is not betterthan) ∼1′′ at most mountaintop observatories.

Telescope. The diameter of a telescope places afundamental limitation on the angular resolution ata given wavelength. Specifically, the limiting angularresolution (in radians) is given by

θ ≈ 1.2λ

d(2)

where θ is the angle subtended by a resolution element,λ is the wavelength of interest, and d is the telescopediameter. This relationship follows from considerationof simple interference effects of wave fronts incident

on a circular aperture, in direct analogy to plane-parallel waves of wavelength λ incident on a singleslit of size d. The resulting intensity distribution for apoint source (known as the ‘‘point-spread function’’) isin fact a classical diffraction pattern, a central disk (the‘‘Airy disk’’) surrounded by alternating bright and darkannuli. In ground-based optical astronomy using largetelescopes, atmospheric scintillation usually dominatesover telescope diffraction (that is, the ‘‘seeing disk’’is much larger than the ‘‘Airy disk’’), and such adiffraction pattern is not observed. However, in space-based optical astronomy or in ground-based infrared andradio astronomy, diffraction represents the fundamentallimitation on spatial resolution.

Detector. Charge-coupled devices (CCDs) have beenactively used in optical astronomy for more than twodecades. During this period, CCD pixel sizes have steadilydecreased, and array formats have steadily grown. Asa result, CCDs have remained small and still maintaingood spatial coverage. Detector array development atother wavelength regimes lags behind the optical, tovarious degrees, in number and spacing of pixels. However,almost all regimes, from X ray to radio, now employ someform of detector array. Sizes range from the suite of ten1, 024 × 1, 024 X-ray-sensitive CCDs aboard the orbitingChandra X-Ray Observatory to the 37- and 91-elementbolometer arrays used for submillimeter-wave imagingby the James Clerk Maxwell Telescope on Mauna Kea.These devices have a common goal of achieving a balancebetween optimal (Nyquist) sampling of the point-spreadfunction and maximal image (field) size.

Sensitivity

Requirements: Energy Fluxes of Astronomical Sources.Astronomical sources span an enormous range of intrinsicluminosity. Figure 12 readily shows that the leastluminous objects known tend to be close to Earth (e.g.,


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Figure 11. Physical radii vs. distances (from Earth) for representative astronomical sources (7).One astronomical unit (AU) is the Earth–Sun distance (1.5 × 108 km). A light year is the distancetraveled by light in one year (9 × 1012 km). Represented in the figure are objects within our ownsolar system, the nearby Sun-like star α Cen, the red supergiant Betelgeuse, the pulsar at thecenter of the Crab Nebula supernova remnant, a typical circumstellar debris disk (‘‘CS disk’’),a typical planetary nebula (the Ring Nebula), the supernova remnant Cas A, the galactic giantmolecular cloud located in the direction of the constellation Cygnus (‘‘Cygnus GMC’’), the nearbyAndromeda galaxy (M31), the quasar 3C 273, and the Virgo cluster of galaxies. Diagonal linesrepresent lines of constant angular size, and angular size decreases from upper left to lower right.

small asteroids in the inner solar system), and themost luminous sources known (e.g., the central enginesof active galaxies or the primordial cosmic backgroundradiation) are also the most distant. This tendency todetect intrinsically more luminous sources at greaterdistances follows directly from the expression for energyflux received at Earth,

F = L4πD2

, (3)

where F is the flux, L is luminosity, and D is distance.Thus an astronomical imaging system that has a limitingsensitivity F ≥ Fl penetrates to a limiting distance,

Dl ≤√

L4πFl

, (4)

for sources of uniform luminosity L. Real samples (of, e.g.,stars or galaxies), of course, may include a wide rangeof intrinsic luminosities. As a result, there tends to bestrong selection bias in astronomy, such that the numberand/or significance of intrinsically faint objects tends to beunderestimated in any sample of sources selected on thebasis of minimum flux.

For this reason in particular, astronomers requireincreasingly sensitive imaging systems. To calibratedetected fluxes properly, such systems must still retaingood dynamic range, so that the intensities of faint sourcescan be accurately referenced to the intensities of bright,well-calibrated sources. In addition, because a given sourceof extended emission may display a wide variation insurface brightness, a combination of high sensitivity andgood dynamic range frequently is required to characterize


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Figure 12. Intrinsic luminosities vs. distances (from Earth) for representative astronomicalsources; symbols are the same as in the previous figure. Luminosities are expressed in solar units,where the solar luminosity is 4 × 1033 erg s−1. Diagonal lines represent lines of constant apparentbrightness, and apparent brightness decreases from upper left to lower right.

source morphology adequately and, hence, deduce intrinsicsource structure.

Limitations

Atmosphere. The Earth’s atmosphere attenuates thesignals of most astronomical sources. Signal attenuationis a function of both the path length through theatmosphere between the source and telescope and theatmosphere’s intrinsic opacity at the wavelength ofinterest. Atmospheric attenuation tends to be smallestat optical and longer radio wavelengths, at which theatmosphere is essentially transparent. Attenuation islargest at very short (γ ray, X ray and UV) wavelengths,where the atmosphere is essentially opaque; attenuationis also large in the infrared. In the infrared regimeespecially, atmospheric transparency depends stronglyon wavelength because the main source of opacity isabsorption by molecules (in particular, water vapor).

The atmosphere also is a source of ‘‘background’’radiation at most wavelengths, particularly in the thermalinfrared and far-infrared (2 µm ≤ λ ≤ 1 mm), at which

most of the blackbody radiation of the atmosphereemerges. This background radiation tends to limit thesignal-to-noise ratio of infrared observations for whichother noise sources (such as detector noise) are minimal.Elimination of thermal radiation from the atmosphereprovides a primary motivation for the forthcoming SpaceInfrared Telescope Facility (SIRTF), the last in NASA’sline of Great Observatories.

Telescope. Sensitivity (or image signal-to-noise ratio) isdirectly proportional to the collecting area and efficiencyof the telescope optical surfaces (‘‘efficiency’’ here refers tothe fraction of photons incident on the telescope opticalsurface that are transmitted to the camera or detector).4

Reflecting telescopes supplanted refracting telescopes atthe beginning of the twentieth century because largeprimary mirrors could be supported more easily than large

4 The product of telescope collecting area and efficiency is referredto as the effective area of the telescope.


objective lenses and the aluminized surface of a mirrorprovides nearly 100% efficiency at optical wavelengths.Furthermore, unlike lenses, paraboloid mirrors provideimages that are free of spherical or chromatic aberrations.These same mirrors provide excellent efficiency and imagequality in the near-infrared, as well. Parabolic reflectorsare also used as the primary radiation collecting surfacesin the radio regime, where the requirements of mirrorfigure are less stringent (due to the relatively largewavelengths of interest).

Detector. The photon counting efficiency of a detectorand sources of noise within the detector also dictate theimage signal-to-noise ratio. Photon counting efficiency isusually referred to as detector quantum efficiency (QE).Detector QEs at or higher than 80% are now feasiblein many wavelength regimes; however, such high QEoften comes at the price of the introduction of noise.Typical image noise sources are read noise, the inherentuncertainty in the signal readout of the detector, and darksignal, the signal registered by the detector in the absenceof exposure to photons from an external source.

Surmounting the Obstacles

Beating the Limitations of the Atmosphere: AdaptiveOptics and Space-Based Imaging. Adaptive optics tech-niques have been developed to mitigate the effects ofatmospheric scintillation. In such systems, the imageof a fiducial point source — either a bright star or alaser-generated artificial ‘‘star’’ — is continuously moni-tored, and these data are used to drive a quasi-real-timeimage correction system (typically a deformable or steer-able mirror). Naturally — as has been demonstrated bythe spectacular success of the refurbished Hubble SpaceTelescope — placement of the telescope above the Earth’satmosphere provides the most robust remedy for the effectsof atmospheric image distortion.

Beating the Limitations of Aperture: Interferometry. Thediffraction limit of a single telescope can be surmountedby using two or more telescopes in tandem. Thistechnique is referred to as ‘‘interferometry’’ because ituses the interference patterns produced by combinationof light waves from multiple sources. Therefore, theangular resolution of such a multiple telescope system,at least in one dimension, is limited by the longestseparation between telescopes, rather than by the apertureof a single telescope. However, it is generally notpossible to ‘‘fill in’’ the gaps between two telescopes atlarge separation by using many telescopes at smallerseparation. As a result, interferometry is generallylimited to relatively bright sources, and interferometricimage reconstruction techniques necessarily sacrificeinformation at low spatial frequencies (i.e., large-scalestructure) in favor of recovering information at highspatial frequency (fine spatial structure). Interferometryhas long been employed at radio wavelengths becauserecombination of signals from multiple apertures isrelatively easy at long wavelengths. Indeed, the angularresolution achieved routinely at centimeter wavelengthsby NRAO’s Very Large Array in New Mexico rivals or

exceeds that of optical imaging by the Hubble SpaceTelescope. Recently, however, several optical and infraredinterferometers have been developed and successfullydeployed; examples include the Navy Prototype OpticalInterferometer at Anderson Mesa and the Infrared OpticalTelescope Array on Mt. Hopkins, both in Arizona, and theoptical interferometer operated at Mt. Wilson, California,by the Center for High Angular Resolution Astronomy.

Beating the Limitations of Materials: Mirror Fabrica-tion. The sheer weight of monolithic, precision-groundmirrors and the difficulty of maintaining the requisiteprecise figures renders them impractical for constructingtelescope apertures larger than about 8 meters in dia-meter. Hence, during the late 1980s and early 1990s, twocompeting large mirror fabrication technologies emerged:spin-cast and segmented mirrors (Fig. 13). Both methodshave yielded large mirrors whose apertures are far lighterand more flexible than previously feasible. The formermethod has yielded the 8-meter-class mirrors for facilitiessuch as the twin Gemini telescopes, and the latter methodhas yielded the largest mirrors thus far, for the twin10-meter Keck telescopes on Mauna Kea. It is not clear,however, that either technique can yield optical-qualitymirrors larger than about 15 meters in diameter.

An entirely different mirror fabrication approach isrequired at high energies because, for example, X raysare readily absorbed (rather than reflected) by aluminizedglass mirrors when such mirrors are used at near-normalincidence. The collection and focusing of X-ray photonsinstead requires grazing incidence geometry to optimizeefficiency and nested mirrors to optimize collectingsurface (Fig. 14). The challenge now faced by high-energyastronomers is to continue to increase the effectivearea of such optical systems while meeting the strictweight requirements imposed by space-based observingplatforms. It is not clear that facilities larger than thepresent Chandra and XMM-Newton observatories arepractical given present fabrication technologies; indeed,Chandra was the heaviest payload ever launched aboarda NASA Space Shuttle.

THE SHAPE OF THINGS TO COME

Projects in Progress

At this time, several major new astronomical facilitiesare partially or fully funded and are either in design orunder construction. All are expected to accelerate furtherthe steady progress in our understanding of the universe.A comprehensive list is beyond the scope of this article;however, we mention a few facilities of note.

• The Space Infrared Telescope Facility (SIRTF):SIRTF is a modest-aperture (0.8 m) telescopeequipped with instruments of extraordinarysensitivity for observations in the 3 to 170 µmwavelength regime. SIRTF features a powerfulcombination of sensitive, wide-field imaging andspectroscopy at low to moderate resolution over thiswavelength range. It is well equipped to study (amongmany other things) primordial galaxies, newborn


Figure 13. Photo of the segmented primary mirror of the 10-meter Keck telescope (Photo credit:Andrew Perala and W.M. Keck Observatory). See color insert.

Four nested hyperboloids

Doubly

reflected

X rays

Doubly

reflected

X rays

Focal

surface

Field-of-view

—5°

Four nested paraboloidsX rays

X rays

Mirror elements are 0.8 m long and from 0.6 m to 1.2 m in diameter

10 meters

Figure 14. Geometry of the nested mirrors aboard the orbiting Chandra X-Ray Observatory[Source: NASA/Chandra X-Ray Center (http://chandra.harvard.edu)]. See color insert.

stars and planets, and dying stars because all of thesephenomena emit strongly in the mid- to far-infrared.SIRTF has a projected 5-year lifetime and is expectedto be deployed into its Earth-trailing orbit in 2002.

• The Stratospheric Observatory for Infrared Astron-omy (SOFIA): SOFIA will consist of a 2.5-metertelescope and associated cameras and spectrometersinstalled aboard a Boeing 747 aircraft. SOFIA willbe the largest airborne telescope in the world. Due to

its ability to surmount most of Earth’s atmosphere,SOFIA will make infrared observations that areimpossible for even the largest and highest ground-based telescopes. The observatory is being developedand operated for NASA by a consortium led bythe Universities Space Research Association (USRA).SOFIA will be based at NASA’s Ames Research Cen-ter at Moffett Federal Airfield near Mountain View,California. It is expected to begin flying in the year

IMAGING SCIENCE IN BIOCHEMISTRY 693

2004 and will remain operational for two decades.Like SIRTF, SOFIA is part of NASA’s Origins Pro-gram, and hence its science goals are similar andcomplementary to those of SIRTF.

• The Atacama Large Millimeter Array (ALMA):ALMA will be a large array of radio telescopesoptimized for observations in the millimeter wave-length regime and situated high in the Atacamadesert in the Chilean Andes. Using a collecting areaof up to 10,000 square meters, ALMA will featureroughly 10 times the collecting area of today’s largestmillimeter-wave telescope arrays. Its telescope-to-telescope baselines will extend to 10 km, providingangular resolution equivalent to that of a diffraction-limited optical telescope whose diameter is 4 meters.ALMA observations will focus on emission frommolecules and dust from very compact sources, suchas galaxies at very high redshift and solar systems information.

Recommendations of the Year 2000 Decadal Review

The National Research Council, the principal operat-ing arm of the National Academy of Sciences andthe National Academy of Engineering, has mappedout priorities for investments in astronomical researchduring the next decade (8). The NRC study shouldnot be used as the sole (or perhaps even pri-mary) means to assess future directions in astron-omy, but this study, which was funded by NASA,the National Science Foundation, and the Keck Foun-dation does offer insight into some potential ground-breaking developments in multiwavelength astronomicalimaging.

Highest priority in the NRC study was given to theNext Generation Space Telescope (NGST). This 8-meter-class, infrared-optimized telescope will represent a majorimprovement on the Hubble Space Telescope in bothsensitivity and spatial resolution and will extend space-based infrared imaging into the largely untapped 2–5 µmwavelength regime. This regime is optimal for studyingthe earliest stages of star and galaxy formation. NGSTpresently is scheduled for launch in 2007.

Several other major initiatives were also deemed crucialto progress in astronomy by the NRC report. Developmentof the ground-based Giant Segmented Mirror Telescopewas given particularly high priority. This instrument hasas its primary scientific goal the study of the evolutionof galaxies and the intergalactic medium. Other projectssingled out by the NRC report include

• Constellation-X Observatory, a next-generationX-ray telescope designed to study the origin andproperties of black holes;

• a major expansion of the Very Large Array radiotelescope in New Mexico, designed to improve on itsalready unique contributions to the study of distantgalaxies and the disk-shaped regions around starswhere planets form;

• a large ground-based survey telescope, designed toperform repeated imaging of wide fields to searchfor both variable sources and faint solar-system

objects (including near-Earth asteroids and some ofthe most distant, undiscovered objects in the solarsystem); and

• the Terrestrial Planet Finder, a NASA missiondesigned to discover and study Earth-like planetsaround other stars.

BIBLIOGRAPHY

1. A. Sandage, Ann. Rev. Astron. Astrophys. 37, 445–486 (1999).2. K. R. Lang, Astrophysical Formulae, 3rd ed., Springer-Verlag,

Berlin, 1999.3. G. B. Rybicki and A. P. Lightman, Radiative Processes in

Astrophysics, John Wiley & Sons, Inc., NY, 1979.4. D. Osterbrock, Astrophysics of Gaseous Nebulae and Active

Galactic Nuclei, University Science Books, Mill Valley, 1989.5. R. Lucas, P. Cox, and P. J. Huggins, in J. H. Kastner, N. Soker,

and S. Rappaport, eds., Asymmetrical Planetary Nebulae II:From Origins to Microstructures, vol. 199, Astron. Soc. Pac.Conf. Ser., 2000, p. 285.

6. J. H. Kastner, N. Soker, S. Vrtilek, and R. Dgani, Astrophys.J. (Lett.) 545, 57–59 (2000).

7. C. W. Allen and A. N. Cox, Astrophysical Quantities, 4th ed.,Springer Verlag, Berlin, 2000.

8. C. McKee et al., Astronomy and Astrophysics in the NewMillennium, National Academy Press, Washington, 2001.

IMAGING SCIENCE IN BIOCHEMISTRY

NICOLAS GUEX

TORSTEN SCHWEDE

MANUEL C. PEITSCH

Glaxo Smith Klime Research &Development SAGeneva, Switzerland

INTRODUCTION

Research in biology, aimed at understanding the funda-mental processes of life, is both an experimental and anobservational science. During the last century, all classesof biomolecules relevant to life have been discovered anddefined. Consequently, biology progressed from catalogingspecies and their life styles to analyzing their underlyingmolecular mechanisms. Among the molecules required bylife, proteins represent certainly the most fascinating classbecause they are the actual ‘‘working molecules’’ involvedin both the processes of life and the structure of livingbeings. Proteins carry out diverse functions, including sig-naling and chemical communication (for example kinasesand hormones), structure (keratin and collagen), trans-port of metabolites (hemoglobin), and transformation ofmetabolites (enzymes).

In contrast to modeling and simulation, observation andanalysis are the main approaches used in biology. Earlybiology dealt only with the observation of macroscopicphenomena, which could be seen by the naked eye. Thedevelopment of microscopes permitted observation of thesmaller members of the living kingdom and hence of thecells and the organelles they contain (Fig. 1). Observing

Download - Figure 3. · 684 IMAGING SCIENCE IN ASTRONOMY Figure 3. Schematic diagram showing various regimes of the electromagnetic spectrum in terms of temperatures corresponding to emission

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