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65 (a) E. V. Sayre and H. N. Lechtman, Stud. Conserv. 13, force for innovation and discovery in astronomical 161–185 (1968); (b) M. W. Ainsworth et al., Art and Autora- imaging. diography: Insights into the Genesis of Paintings by Rem- Classical astronomy — for example, the search for new brandt, Van Dyck, and Vermeer, The Metropolitan Museum solar system objects and the classification of — is of Art, NY, 1982; (c) C. O. Fischer et al., Nucl. Instrum. Meth- still largely conducted in the optical wavelength regime ods A 424, 258–262 (1999); (d) Images and supporting text (400–700 nm). This has been the case, of course, since provided by Ward Laboratory, Cornell University, Ithaca, humans first imagined the , noted the NY 14853-7701. appearance of ‘‘wandering stars’’ (planets), and recorded 66. D. L. Glackin and E. P. Korsmo, Jet Propulsion Laboratory, the appearance of transient phenomena such as comets Final Report 83-75, JPL Publications, Pasadena, 1983. and novae. During the latter half of the twentieth century, 67.J.R.Druzik,D.Glackin,D.Lynn,andR.Quiros,10th Annu. however, a revolution in astronomical imaging took Meet. Am. Inst. Conserv., 1982, pp. 71–72. place (1). This relatively brief period in recorded history 68. E. J. Wood, Textile Res. J. 60, 212–220 (1990). saw the development and rapid refinement of techniques 69. F. Heitz, H. Maitre, and C. DeCouessin, IEEE Trans. Acous., for collecting and detecting electromagnetic radiation Speech Signal Process. 38, 695–704 (1990). across a far broader wavelength range, from the radio 70. J. Sobus, B. Pourdeyhimi, B. Xu, and Y. Ulcay, Textile Res. J. through γ rays. Just as these techniques have reached 62,26–39 (1992). maturation, astronomers have also developed the means 71. K. Knox, R. Johnston, and R. L. Easton Jr., Opt. Photonics to surmount apparently fundamental physical barriers News 8,30–34 (1997). placed on image quality, such as the distorting effects 72. L. Likforman-Sulem, H. Maitre, and C. Sirat, Pattern Recog- of refraction by Earth’s atmosphere and diffraction by a nition 24, 121–137 (1991). single telescope of finite aperture. The accelerating pace of 73. E. Lang, and D. Watkinson, Conserv. News 47,37–39 (1992). these innovations has resulted in deeper understanding of, 74. F. Su, OE Reports SPIE 99,1,8–(1992). and heightened appreciation for, both the rich diversity of 75. J. L. Kirsch and R. A. Kirsch, Leonardo 21, 437–444 (1988). astrophysical phenomena and the fundamental, unsolved 76. J. F. Asmus, Opt. Eng. 28, 800–804 (1989). mysteries of the cosmos. 77. R. Sablatnig, P. Kammerer, and E. Zolda, Proc. 14th Int. Conf Pattern Recognition, 1998, pp. 172–174. OPENING THE WINDOWS: MULTIWAVELENGTH 78. L. R. Doyle, J. J. Lorre, and E. B. Doyle, Stud. Conserv. 31, IMAGING 1–6 (1986). 79. J. Asmus, Byte Magazine March (1987). For most of us, our eyes provide our first, fundamental 80. P. Clogg, M. Diaz-Andreu, and B. Larkman, J. Archaeological contact with the universe. It is interesting to ponder Sci. 27, 837 (2000). how humans would conceive of the universe if we had 81. P. Clogg and C. Caple, Imaging the Past, British Museum nothing more in the way of imaging apparatus at our Occasional Paper, London, 1996, p. 114. disposal, as was the case for astronomers before Galileo. In contrast to the complex cosmologies currently pondered in modern physics, most of which involve an expanding universe shadowed by the afterglow of the Big Bang, the ‘‘first contact’’ provided by our eyes produces a model of IMAGING SCIENCE IN ASTRONOMY the universe that is entirely limited to the Sun, Moon, and planets, the nearby stars, and the faint glow of JOEL H. KASTNER the collective background of stars in our own Milky Rochester Institute of Technology Way and a handful of other, nearby . Rochester, NY From this simple thought experiment, it is clear that the bulk of the visible radiation arriving at Earth is emitted by stars. INTRODUCTION But the apparent predominance of visible light from the Sun and nearby stars is in fact merely an accident The vast majority of information about the universe is of our particular position in the universe, combined collected via electromagnetic radiation. This radiation is with the evolutionary adaptation that gave our eyes emitted by matter distributed across tremendous ranges maximal sensitivity at wavelengths of electromagnetic in temperature, density, and chemical composition. Thus, radiation that are near the maximum of the Sun’senergy more than any other science, astronomy depends on output. The Sun provides by far the majority of the innovative methods to extend image taking to new, visible radiation arriving at Earth strictly by virtue unexplored regions of the electromagnetic spectrum. To of its proximity. The brightest in the night sky, bring sufficient breadth and depth to their studies, Sirius (in the Canis Major), actually has astronomers also require imaging capability across a vast an intrinsic luminosity about 50 times larger than that of range in spatial resolution and sensitivity, with emphasis the Sun, but is about 8.6 light years distant (a light year on achieving the highest possible resolution and signal is the distance traveled by light in one year, 9 × 1012 km; gain in a given wavelength regime. This simultaneous the Sun is about 8 light minutes from Earth). In turn, quest for better wavelength coverage and ever higher Sirius is only about one-ten-thousandth as luminous as spatial resolution and sensitivity represents the driving the star Rigel (in the neighboring constellation Orion), IMAGING SCIENCE IN ASTRONOMY 683 but Sirius appears several times brighter than Rigel because it is about 50 times closer to us. Like the Sun, which has a surface temperature of about 6,000 K, most of the brightest stars have surfaces within the range of temperatures across which hot objects radiate very efficiently (if not predominantly) in the visible region. Representative stellar surface temperatures are 3,000 K for reddish Betelgeuse, a red supergiant in Orion; 10,000 K for Sirius; and 15,000 K for the blue supergiant Rigel (Fig. 1).

Thermal Continuum Emission The tendency of objects at the temperatures of the Sun and stars to emit in the visible can be understood to first order via Planck’s Law, which describes the wavelength dependence of radiation emitted by a perfect blackbody. The peak of the Planck function lies within the visible regime for an object at a temperature of 6,000 K. This same fundamental physical principle tells us that objects much hotter or cooler than the Sun should radiate predominantly at wavelengths much shorter or longer than visible, respectively. Indeed, for a perfect blackbody, the peak wavelength of radiation is given by Wien’s displacement law (2), 0.51 λ(cm) ∼ .(1) T(K)

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

106 Figure 2. Wide-field photograph of Orion, illustrating the difference in color between the relatively cool star Betelgeuse 5 (upper left) and the hot star Rigel (lower right). The large, red 10 Rigel Betetgeuse object at the lower center of the image, just below Orion’sbelt, 104 is the Orion (see Fig. 7). (Photo credit: Till Credner, Main sequence Superglants AlltheSky.com) See color insert. 1) Spica = 103 Red glants Capella Aldebaran 102 Vega The same, simple relationship also provides powerful Arcturus insight into astrophysical processes that occur across a Sirius A 10 Pollux very wide range of energy regimes (Fig. 3). The lowest Altair Procyon A energies and hence longest (radio) wavelengths reveal 1 Sun ‘‘cold’’ phenomena, such as emission from dust and gas in optically opaque clouds distributed throughout interstellar 10−1 Intrinsic brightness (Sun White dwarts space in our galaxy. At the highest energies and hence shortest wavelengths (characteristic of X rays and γ rays), −2 10 astronomers probe the ‘‘hottest’’ objects, such as the Sirius B − explosions of supermassive stars or the last vestiges 10 3 Procyon B of superheated material that is about to spiral into a 40,000 20,000 10,000 6,000 4,000 3,000 2,000 black hole. Stars' surface temperature (K) Figure 1. The Hertzsprung–Russell diagram. The diagram Nonthermal Continuum Emission shows the main sequence (Sun-like stars that are fusing hydrogen Certain radiative phenomena in astrophysics do not to helium in the cores), red giants, supergiants, and white dwarfs. In addition, the positions of the Sun, the twelve brightest strongly depend on the precise temperature of the material stars visible from the Northern Hemisphere, and the white and are instead sensitive probes of material density and/or dwarf companions of Sirius and Procyon are indicated [Source: chemical composition (3,4). For example, the emission NASA (http://observe.ivv.nasa.gov/nasa/core.shtml.html)]. See from ‘‘jets’’ ejected from supermassive black holes at color insert. the centers of certain galaxies (Fig. 4) is said to be 684 IMAGING SCIENCE IN ASTRONOMY

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

‘‘nonthermal’’ because its source is high-velocity electrons emission at one of these specific wavelengths1 is both that orbit around magnetic field lines. Other, similar necessary and sufficient to determine the presence of examples are the emission from filaments of ionized gas that element or molecule. Hence, our knowledge of the located near the center of our own galaxy and from the origin and evolution of the elements that make up the chaotic remnant of the explosion of a massive star in universe is derived from astronomical spectroscopy (which 1054 A.D. (the ‘‘Crab Nebula’’). Such so-called ‘‘synchrotron might also be considered multiband, one-dimensional radiation’’ often dominates radiation emitted in the radio imaging). wavelength regime (Fig. 5). Indeed, if human eyes were Spectra obtained by disparate means across a very sensitive to radio rather than to visible wavelengths, the broad range of wavelengths can be used to ascertain early mariners probably would have navigated by the both chemical compositions and physical conditions (i.e., Galactic Center and the Crab because they appear from temperatures and densities) of astronomical sources Earth as the brightest stationary radio continuum sources because the emissive characteristics of a given element in the northern sky. The synchrotron emission from the depend on the physical conditions of the gas or dust Crab is particularly noteworthy; it can be detected across in which it resides. For example, cold (100 K), largely a very broad wavelength range from radio through X ray neutral hydrogen gas emits strongly in the radio at 21 cm, (Fig. 6). whereas hot (10,000 K), largely ionized hydrogen gas emits at a series of optical wavelengths (known as the Balmer series). The former conditions are typical of the Monochromatic (‘‘Line’’) Emission and Absorption gas that permeates interstellar space in our own galaxy and in external galaxies, and the latter conditions are Astronomers use Deducing Chemical Compositions. typical of gas in the proximity of very hot stars, which electronic transitions of atoms (as well as electronic, are sources of ionizing ultraviolet light. Such ionized vibrational, and rotational transitions of molecules) as gas also tends to glow brightly in the emission lines of Rosetta stones to understand the chemical makeup of heavier elements such as oxygen, nitrogen, sulfur, and gas in a wide variety of astrophysical environments. iron (Fig. 7). Because each element or molecule radiates (and absorbs radiation) at a discrete and generally well-determined set of wavelengths — specified by that element’sparticular 1 Such spectral features are called ‘‘lines,’’ because they appeared subatomic structure — detection of an excess (or deficit) of as dark lines in early spectra of the Sun. IMAGING SCIENCE IN ASTRONOMY 685

Figure 4. At a distance of 11 million light years, Centaurus A Figure 6. X-ray image of the innermost region of the Crab is the nearest example of a so-called ‘‘active galaxy.’’ This radio Nebula. This image covers a field of view about one-quarter image shows opposing ‘‘jets’’ of high energy particles blasting out that of the radio image in the previous figure. The image shows from its center [Source: National Radio Astronomy Observatory tilted rings or waves of high-energy particles that appear to have (NRAO)]. See color insert. been flung outward across a distance of a light year from the central star (Source: Chandra X-Ray Center). See color insert.

Figure 5. The Crab Nebula is the remnant of a supernova Figure 7. Color mosaic of the central part of the Great Nebula explosion that was seen from the earth in 1054 A.D. It is 6,000 in Orion, obtained by the . Light emitted light years from Earth. This radio image shows the complex by ionized oxygen is shown as blue, ionized hydrogen emission arrangement of gas filaments left in the wake of the explosion is shown as green, and ionized nitrogen emission as red. The (Source: NRAO). See color insert. sources of ionization of the nebula are the hot, blue-white stars of the young Trapezium cluster, which is embedded in nebulosity just left of center in the image (Source: NASA and C.R. O’Dell Deducing Radial Velocities from Spectral Lines. Atomic and S.K. Wong). See color insert. and molecular emission lines also serve as probes of bulk motion. If a given source has a component of velocity Doppler shifted away from the rest wavelength. The along our line of sight, then its emission lines will be absorption or emission lines of sources that approach 686 IMAGING SCIENCE IN ASTRONOMY

spent core causes the atmosphere of the star to expand, 30,000 forming a red giant. Although the extended atmospheres of red giants are ‘‘cool’’ enough (∼3,000 K) for dust grains to condense out of the stellar gas, red giant luminosities can be huge (more than 10,000 times that of the Sun). This radiant energy pushes dust away from the outer −1 20,000 atmosphere of the star at speeds of 10–20 km s .The outflowing dust then collides with and accelerates the gas away from the star, as well. Eventually enough of the atmosphere is removed so that the hot, inert stellar core is revealed. This hot core is destined to become Velocity (km/s) Velocity a fossil remnant of the original star: a white dwarf. 10,000 But before the ejected atmosphere departs the scene entirely, it is ionized by the intense ultraviolet light from the emerging white dwarf, which has cooled from core nuclear fusion temperatures (107 to 108 K) to a ‘‘mere’’ 105 K or so. The ionizing radiation from the white dwarf 0 causes the ejected gas to fluoresce, thereby producing a 0 100 200 300 400 500 . Distance (Mpc) Because the varied conditions that characterize the Figure 8. Plot of recession velocity vs. distance [in megaparsecs evolution of planetary nebulae result in a wide variety of (Mpc); 1 Mpc ≈ 3 × 1019 km] for a sample of galaxies. This figure phenomena in any given nebula, such objects demand a illustrates that, to very high accuracy, the recession velocity multiwavelength approach to imaging. A case in point is of a distant galaxy, as measured from its redshift, is directly the young planetary nebula BD +30° 3639 (Fig. 10). This proportional to its distance. This correlation was first established planetary nebula emits strongly at wavelengths ranging in 1929 by Edwin Hubble and underpins the Big Bang model for from radio through X ray. The Chandra X-ray image shows  the origin of the Universe (Figure courtesy Edward L. Wright, a region of X-ray emission that seems to fit perfectly inside 1996). the shell of ionized and molecular gas seen in Hubble Space Telescope images and in other high-resolution images us are shifted to shorter wavelengths and are said to obtained from the ground. The optical and X-ray emitting be ‘‘blueshifted,’’ whereas the lines of sources moving regions of BD +30° 3639, which lies about 5,000 light away from us are shifted to longer wavelengths and are years away, are roughly 1 million times the volume of our said to be ‘‘redshifted.’’ The observation by Hubble in solar system. The X-ray emission apparently originates in 1929 that emission lines of distant galaxies are uniformly thin gas that is heated by collisions between the ‘‘new’’ redshifted and that these redshifts increase monotonically wind blown by the white dwarf, which is seen at the as the distances of the galaxies increase, underpins center of the optical and infrared images, and the ‘‘old,’’ modern theories of the expansion of the universe2 (Fig. 8). photoionized red giant wind, which appears as a shell Images obtained at multiple wavelengths that span the of ∼10,000 K gas surrounding the ‘‘hot bubble’’ of X-ray rest wavelength of a bright spectral line can allow emission. astronomers to deduce the spatial variation of line- of-sight velocity for a source whose velocity gradients are large. Such velocity mapping, which is presently REQUIREMENTS AND LIMITATIONS feasible at wavelengths from the radio through the optical, To understand the requirements placed on spatial helps elucidate the three-dimensional structure of sources resolution and sensitivity in astronomical imaging, we (Fig. 9). must consider the angular sizes and energy fluxes of astronomical objects and phenomena of interest. In turn, Multiwavelength Astronomical Imaging: An Example there are three fundamental sources of limitation on the Planetary nebulae represent the last stages of dying, Sun- resolution and limiting sensitivity (and hence quality) of like stars. These highly photogenic nebulae are formed astronomical images: the atmosphere, the telescope, and after the nuclear fuel at the core of a Sun-like star has the detector. been spent, that is, the bulk of the core hydrogen has been convertedtohelium.Theexhaustionofcorehydrogen Spatial Resolution and the subsequent nuclear fusion, in concentric shells, of Requirements: Angular Size Scales of Astronomical hydrogen into helium and helium into carbon around the Sources. Figure 11 shows schematically typical scales of physical size and distance from Earth for representative objects and phenomena studied by astronomers. Most of 2 In practice, all astrophysical sources that emit line radia- tion — even those within our solar system — will appear Doppler the objects of intrinsically small size, like the Sun, Moon, shifted, due for example, to the Earth’s motion around the Sun. and the planets in our solar system, lie at small distances; Hence it is necessary to account properly for ‘‘local’’ sources of we can study these small objects in detail only because Doppler shifts when deducing the line-of-sight velocity component they are relatively close, such that their angular sizes are of interest. substantial. IMAGING SCIENCE IN ASTRONOMY 687

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38′′41′30′′ 21h02m19.0 21h02m18.5 21h02m18.0 R.A. (2000.0) Figure 9. Radio maps of the Egg Nebula, a dying star in the constellation , showing emission 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 CO emission from the source. One interpretation for the localized appearance of the blueshifted and redshifted CO emission is that the Egg Nebula is the source of a complex system of ‘‘molecular jets,’’ shown schematically in the top left panel. Such jets may be quite common during the dying stages of Sun-like stars [Source: Lucas et al. 2000 (5)]. See color insert.

Within our own Milky Way galaxy, we observe objects member), are detectable and resolvable by the naked eye, that span a great range of angular size scales. The whereas the Andromeda galaxy (a Local Group member angular size of a Sun-like star at even a modest that is a near-twin to the Milky Way) is detectable distance makes such stars a challenge to resolve spatially, and resolvable with the aid of binoculars. The angular even with the best available techniques. On the other sizes of intrinsically similar galaxies in more distant hand, many structures of interest in our own Milky galaxy clusters span a range similar to that of the Way galaxy, such as star-forming molecular clouds and planets in our solar system. The luminous cores of certain the expelled remnants of dying or expired stars, are distant galaxies (‘‘quasars’’) — which can outshine their sufficiently large that their angular sizes are quite large.3 host galaxies — likely have sizes only on the order of Certain giant molecular clouds, planetary nebulae, and that of our solar system; yet these are some of the most supernova remnants subtend solid angles similar to that distant objects known, and hence quasars are exceedingly of the Moon. small in angular size. Galaxy clusters themselves are Just as for stars, the angular sizes of external galaxies of relatively large angular size, simply by virtue of span a very wide range. The Magellanic Clouds, which are their enormous size scales; indeed, such clusters (and the nearest members of the Local Group of galaxies (of larger scale structures that consist of clusters of such which the Milky Way is the most massive and luminous clusters) probably represent the largest gravitationally bound structures in the universe. At still larger size scales lies the cosmic background radiation, the radiative 3 The ejected envelopes of certain dying, sun-like stars were long remnant of the Big Bang itself. This radiation encompasses ago dubbed ‘‘planetary nebulae’’ because their angular sizes and 4π steradians and has only very subtle variations in round shapes resembled the planets Jupiter and Saturn. intensity with position across the sky. 688 IMAGING SCIENCE IN ASTRONOMY

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

Of course, even within our solar system, there are on a circular aperture, in direct analogy to plane- sources of great interest (e.g., the primordial, comet-like parallel waves of wavelength λ incident on a single bodies of the Kuiper Belt) that are sufficiently small that slit of size d. The resulting intensity distribution for a they are unresolvable by present imaging techniques. point source (known as the ‘‘point-spread function’’)is Sources of large angular sizes (such as molecular clouds, in fact a classical diffraction pattern, a central disk (the planetary nebulae, supernova remnants, and galaxy ‘‘Airy disk’’) surrounded by alternating bright and dark clusters) typically show a great wealth of structural annuli. In ground-based optical astronomy using large detail when imaged at high spatial resolution. Thus, telescopes, atmospheric scintillation usually dominates our knowledge of objects at all size and distance scales over telescope diffraction (that is, the ‘‘seeing disk’’ improves with any increase in spatial resolving power at is much larger than the ‘‘Airy disk’’), and such a a given wavelength. diffraction pattern is not observed. However, in space- based optical astronomy or in ground-based infrared and Limitations radio astronomy, diffraction represents the fundamental Atmosphere. Time- and position-dependent refraction limitation on spatial resolution. by turbulent cells in the atmosphere causes astronomical Detector. Charge-coupled devices (CCDs) have been point sources, such as stars, to ‘‘scintillate’’; i.e., stars actively used in optical astronomy for more than two twinkle. Scintillation occurs when previously plane- decades. During this period, CCD pixel sizes have steadily parallel wave fronts from very distant sources encounter decreased, and array formats have steadily grown. As atmospheric cells and become distorted. Astronomers use a result, CCDs have remained small and still maintain the term ‘‘seeing’’ to characterize such atmospheric image good spatial coverage. Detector array development at distortion; the ‘‘seeing disk’’ represents the diameter of other wavelength regimes lags behind the optical, to an unresolved (point) source that has been smeared by various degrees, in number and spacing of pixels. However, atmospheric distortion. Seeing varies widely from site to almost all regimes, from X ray to radio, now employ some site, but optical seeing disks at visual wavelengths are form of detector array. Sizes range from the suite of ten typically not smaller than (that is, the seeing is not better 1, 024 × 1, 024 X-ray-sensitive CCDs aboard the orbiting than) ∼1 at most mountaintop observatories. Chandra X-Ray Observatory to the 37- and 91-element bolometer arrays used for submillimeter-wave imaging Telescope. The diameter of a telescope places a fundamental limitation on the angular resolution at by the James Clerk Maxwell Telescope on Mauna Kea. a given wavelength. Specifically, the limiting angular These devices have a common goal of achieving a balance resolution (in radians) is given by between optimal (Nyquist) sampling of the point-spread function and maximal image (field) size. λ θ ≈ 1.2 (2) d Sensitivity where θ is the angle subtended by a resolution element, Requirements: Energy Fluxes of Astronomical Sources. λ is the wavelength of interest, and d is the telescope Astronomical sources span an enormous range of intrinsic diameter. This relationship follows from consideration luminosity. Figure 12 readily shows that the least of simple interference effects of wave fronts incident luminous objects known tend to be close to Earth (e.g., IMAGING SCIENCE IN ASTRONOMY 689

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10−10 10−10 10−5 100 105 1010 Distance (light years) 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 distance traveled by light in one year (9 × 1012 km). Represented in the figure are objects within our own solar system, the nearby Sun-like star α Cen, the red supergiant Betelgeuse, the pulsar at the center of the Crab Nebula , a typical circumstellar debris disk (‘‘CS disk’’), a typical planetary nebula (the Ring Nebula), the supernova remnant Cas A, the galactic giant molecular cloud located in the direction of the constellation Cygnus (‘‘Cygnus GMC’’), the nearby Andromeda galaxy (M31), the quasar 3C 273, and the Virgo cluster of galaxies. Diagonal lines represent lines of constant angular size, and angular size decreases from upper left to lower right. small asteroids in the inner solar system), and the for sources of uniform luminosity L. Real samples (of, e.g., most luminous sources known (e.g., the central engines stars or galaxies), of course, may include a wide range of active galaxies or the primordial cosmic background of intrinsic luminosities. As a result, there tends to be radiation) are also the most distant. This tendency to strong selection bias in astronomy, such that the number detect intrinsically more luminous sources at greater and/or significance of intrinsically faint objects tends to be distances follows directly from the expression for energy underestimated in any sample of sources selected on the flux received at Earth, basis of minimum flux. For this reason in particular, astronomers require L F = ,(3) increasingly sensitive imaging systems. To calibrate 4πD2 detected fluxes properly, such systems must still retain where F is the flux, L is luminosity, and D is distance. good dynamic range, so that the intensities of faint sources Thus an astronomical imaging system that has a limiting can be accurately referenced to the intensities of bright, sensitivity F ≥ Fl penetrates to a limiting distance, well-calibrated sources. In addition, because a given source of extended emission may display a wide variation in L surface brightness, a combination of high sensitivity and Dl ≤ ,(4) 4πFl good dynamic range frequently is required to characterize 690 IMAGING SCIENCE IN ASTRONOMY

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10−15 10−10 10−5 100 105 1010 Distance (light years) Figure 12. Intrinsic luminosities vs. distances (from Earth) for representative astronomical sources; 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 apparent brightness, and apparent brightness decreases from upper left to lower right. source morphology adequately and, hence, deduce intrinsic most of the blackbody radiation of the atmosphere source structure. emerges. This background radiation tends to limit the signal-to-noise ratio of infrared observations for which Limitations other noise sources (such as detector noise) are minimal. Atmosphere. The Earth’s atmosphere attenuates the Elimination of thermal radiation from the atmosphere signals of most astronomical sources. Signal attenuation provides a primary motivation for the forthcoming Space is a function of both the path length through the Infrared Telescope Facility (SIRTF), the last in NASA’s atmosphere between the source and telescope and the line of Great Observatories. atmosphere’s intrinsic opacity at the wavelength of interest. Atmospheric attenuation tends to be smallest Telescope. Sensitivity (or image signal-to-noise ratio) is at optical and longer radio wavelengths, at which the directly proportional to the collecting area and efficiency atmosphere is essentially transparent. Attenuation is of the telescope optical surfaces (‘‘efficiency’’ here refers to largest at very short (γ ray, X ray and UV) wavelengths, the fraction of photons incident on the telescope optical where the atmosphere is essentially opaque; attenuation surface that are transmitted to the camera or detector).4 is also large in the infrared. In the infrared regime Reflecting telescopes supplanted refracting telescopes at especially, atmospheric transparency depends strongly the beginning of the twentieth century because large on wavelength because the main source of opacity is primary mirrors could be supported more easily than large absorption by molecules (in particular, water vapor). Theatmospherealsoisasourceof‘‘background’’ radiation at most wavelengths, particularly in the thermal 4 The product of telescope collecting area and efficiency is referred infrared and far-infrared (2 µm ≤ λ ≤ 1 mm), at which to as the effective area of the telescope. IMAGING SCIENCE IN ASTRONOMY 691 objective lenses and the aluminized surface of a mirror exceeds that of optical imaging by the Hubble Space provides nearly 100% efficiency at optical wavelengths. Telescope. Recently, however, several optical and infrared Furthermore, unlike lenses, paraboloid mirrors provide interferometers have been developed and successfully images that are free of spherical or chromatic aberrations. deployed; examples include the Navy Prototype Optical These same mirrors provide excellent efficiency and image Interferometer at Anderson Mesa and the Infrared Optical quality in the near-infrared, as well. Parabolic reflectors Telescope Array on Mt. Hopkins, both in Arizona, and the are also used as the primary radiation collecting surfaces optical interferometer operated at Mt. Wilson, California, in the radio regime, where the requirements of mirror by the Center for High Angular Resolution Astronomy. figure are less stringent (due to the relatively large wavelengths of interest). Beating the Limitations of Materials: Mirror Fabrica- tion. The sheer weight of monolithic, precision-ground Detector. The photon counting efficiency of a detector mirrors and the difficulty of maintaining the requisite and sources of noise within the detector also dictate the precise figures renders them impractical for constructing image signal-to-noise ratio. Photon counting efficiency is telescope apertures larger than about 8 meters in dia- usually referred to as detector quantum efficiency (QE). meter. Hence, during the late 1980s and early 1990s, two Detector QEs at or higher than 80% are now feasible competing large mirror fabrication technologies emerged: in many wavelength regimes; however, such high QE spin-cast and segmented mirrors (Fig. 13). Both methods often comes at the price of the introduction of noise. have yielded large mirrors whose apertures are far lighter Typical image noise sources are read noise, the inherent and more flexible than previously feasible. The former uncertainty in the signal readout of the detector, and dark method has yielded the 8-meter-class mirrors for facilities signal, the signal registered by the detector in the absence such as the twin Gemini telescopes, and the latter method of exposure to photons from an external source. has yielded the largest mirrors thus far, for the twin 10-meter Keck telescopes on Mauna Kea. It is not clear, Surmounting the Obstacles however, that either technique can yield optical-quality mirrors larger than about 15 meters in diameter. Beating the Limitations of the Atmosphere: Adaptive An entirely different mirror fabrication approach is Optics and Space-Based Imaging. Adaptive optics tech- required at high energies because, for example, X rays niques have been developed to mitigate the effects of are readily absorbed (rather than reflected) by aluminized atmospheric scintillation. In such systems, the image glass mirrors when such mirrors are used at near-normal of a fiducial point source — either a bright star or a incidence. The collection and focusing of X-ray photons laser-generated artificial ‘‘star’’ — is continuously moni- instead requires grazing incidence geometry to optimize tored, and these data are used to drive a quasi-real-time efficiency and nested mirrors to optimize collecting image correction system (typically a deformable or steer- surface (Fig. 14). The challenge now faced by high-energy able mirror). Naturally — as has been demonstrated by astronomers is to continue to increase the effective the spectacular success of the refurbished Hubble Space area of such optical systems while meeting the strict Telescope — placement of the telescope above the Earth’s weight requirements imposed by space-based observing atmosphere provides the most robust remedy for the effects platforms. It is not clear that facilities larger than the of atmospheric image distortion. present Chandra and XMM-Newton observatories are practical given present fabrication technologies; indeed, Beating the Limitations of Aperture: Interferometry. The diffraction limit of a single telescope can be surmounted Chandra was the heaviest payload ever launched aboard by using two or more telescopes in tandem. This a NASA Space Shuttle. technique is referred to as ‘‘interferometry’’ because it uses the interference patterns produced by combination THE SHAPE OF THINGS TO COME of light waves from multiple sources. Therefore, the angular resolution of such a multiple telescope system, Projects in Progress at least in one dimension, is limited by the longest At this time, several major new astronomical facilities separation between telescopes, rather than by the aperture are partially or fully funded and are either in design or of a single telescope. However, it is generally not under construction. All are expected to accelerate further possible to ‘‘fill in’’ the gaps between two telescopes at the steady progress in our understanding of the universe. large separation by using many telescopes at smaller A comprehensive list is beyond the scope of this article; separation. As a result, interferometry is generally however, we mention a few facilities of note. limited to relatively bright sources, and interferometric image reconstruction techniques necessarily sacrifice • The Space Infrared Telescope Facility (SIRTF): information at low spatial frequencies (i.e., large-scale SIRTF is a modest-aperture (0.8 m) telescope structure) in favor of recovering information at high equipped with instruments of extraordinary spatial frequency (fine spatial structure). Interferometry sensitivity for observations in the 3 to 170 µm has long been employed at radio wavelengths because wavelength regime. SIRTF features a powerful recombination of signals from multiple apertures is combination of sensitive, wide-field imaging and relatively easy at long wavelengths. Indeed, the angular spectroscopy at low to moderate resolution over this resolution achieved routinely at centimeter wavelengths wavelength range. It is well equipped to study (among by NRAO’s Very Large Array in New Mexico rivals or many other things) primordial galaxies, newborn 692 IMAGING SCIENCE IN ASTRONOMY

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.

Field-of-view —5°

Doubly reflected Four nested hyperboloids Focal X rays surface

Doubly reflected X rays

10 meters

X rays

X rays Four nested paraboloids

Mirror elements are 0.8 m long and from 0.6 m to 1.2 m in diameter 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 these its ability to surmount most of Earth’s atmosphere, phenomena emit strongly in the mid- to far-infrared. SOFIA will make infrared observations that are SIRTF has a projected 5-year lifetime and is expected impossible for even the largest and highest ground- to be deployed into its Earth-trailing orbit in 2002. based telescopes. The observatory is being developed • The Stratospheric Observatory for Infrared Astron- and operated for NASA by a consortium led by omy (SOFIA): SOFIA will consist of a 2.5-meter the Universities Space Research Association (USRA). telescope and associated cameras and spectrometers SOFIA will be based at NASA’s Ames Research Cen- installed aboard a Boeing 747 aircraft. SOFIA will ter at Moffett Federal Airfield near Mountain View, be the largest airborne telescope in the world. Due to California. It is expected to begin flying in the year IMAGING SCIENCE IN BIOCHEMISTRY 693

2004 and will remain operational for two decades. objects (including near-Earth asteroids and some of Like SIRTF, SOFIA is part of NASA’sOriginsPro- the most distant, undiscovered objects in the solar gram, and hence its science goals are similar and system); and complementary to those of SIRTF. • the Terrestrial Planet Finder, a NASA mission • The Atacama Large Millimeter Array (ALMA): designed to discover and study Earth-like planets ALMA will be a large array of radio telescopes around other stars. optimized for observations in the millimeter wave- length regime and situated high in the Atacama BIBLIOGRAPHY desert in the Chilean Andes. Using a collecting area of up to 10,000 square meters, ALMA will feature 1. A. Sandage, Ann. Rev. Astron. Astrophys. 37, 445–486 (1999). roughly 10 times the collecting area of today’slargest 2. K. R. Lang, Astrophysical Formulae, 3rd ed., Springer-Verlag, millimeter-wave telescope arrays. Its telescope-to- Berlin, 1999. telescope baselines will extend to 10 km, providing 3. G. B. Rybicki and A. P. Lightman, Radiative Processes in angular resolution equivalent to that of a diffraction- Astrophysics, John Wiley & Sons, Inc., NY, 1979. limited optical telescope whose diameter is 4 meters. 4. D. Osterbrock, Astrophysics of Gaseous Nebulae and Active ALMA observations will focus on emission from Galactic Nuclei, University Science Books, Mill Valley, 1989. molecules and dust from very compact sources, such 5. R. Lucas, P. Cox, and P. J. Huggins, in J. H. Kastner, N. Soker, as galaxies at very high redshift and solar systems in and S. Rappaport, eds., Asymmetrical Planetary Nebulae II: formation. From Origins to Microstructures, vol. 199, Astron. Soc. Pac. Conf. Ser., 2000, p. 285. Recommendations of the Year 2000 Decadal Review 6. J. H. Kastner, N. Soker, S. Vrtilek, and R. Dgani, Astrophys. The National Research Council, the principal operat- J. (Lett.) 545,57–59 (2000). ing arm of the National Academy of Sciences and 7. C. W. Allen and A. N. Cox, Astrophysical Quantities,4thed., the National Academy of Engineering, has mapped Springer Verlag, Berlin, 2000. out priorities for investments in astronomical research 8. C. McKee et al., Astronomy and Astrophysics in the New during the next decade (8). The NRC study should Millennium, National Academy Press, Washington, 2001. not 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, IMAGING SCIENCE IN BIOCHEMISTRY the National Science Foundation, and the Keck Foun- dation does offer insight into some potential ground- NICOLAS GUEX breaking developments in multiwavelength astronomical TORSTEN SCHWEDE imaging. MANUEL C. PEITSCH Highest priority in the NRC study was given to the GlaxoSmithKlimeResearch& Next Generation Space Telescope (NGST). This 8-meter- Development SA class, infrared-optimized telescope will represent a major Geneva, Switzerland improvement on the Hubble Space Telescope in both sensitivity and spatial resolution and will extend space- INTRODUCTION based infrared imaging into the largely untapped 2–5 µm wavelength regime. This regime is optimal for studying Research in biology, aimed at understanding the funda- the earliest stages of star and galaxy formation. NGST mental processes of life, is both an experimental and an presently is scheduled for launch in 2007. observational science. During the last century, all classes Several other major initiatives were also deemed crucial of biomolecules relevant to life have been discovered and to progress in astronomy by the NRC report. Development defined. Consequently, biology progressed from cataloging of the ground-based Giant Segmented Mirror Telescope species and their life styles to analyzing their underlying was given particularly high priority. This instrument has molecular mechanisms. Among the molecules required by as its primary scientific goal the study of the evolution life, proteins represent certainly the most fascinating class of galaxies and the intergalactic medium. Other projects because they are the actual ‘‘working molecules’’ involved singled out by the NRC report include in both the processes of life and the structure of living beings. Proteins carry out diverse functions, including sig- • Constellation-X Observatory, a next-generation naling and chemical communication (for example kinases X-ray telescope designed to study the origin and and hormones), structure (keratin and collagen), trans- properties of black holes; port of metabolites (hemoglobin), and transformation of • a major expansion of the Very Large Array radio metabolites (enzymes). telescope in New Mexico, designed to improve on its In contrast to modeling and simulation, observation and already unique contributions to the study of distant analysis are the main approaches used in biology. Early galaxies and the disk-shaped regions around stars biology dealt only with the observation of macroscopic where planets form; phenomena, which could be seen by the naked eye. The • a large ground-based survey telescope, designed to development of microscopes permitted observation of the perform repeated imaging of wide fields to search smaller members of the living kingdom and hence of the for both variable sources and faint solar-system cells and the organelles they contain (Fig. 1). Observing