OPTICAL ASTRONOMY

by Ann Finkbeiner

By the time astronomers got a big telescope into orbit, they had figured out ways to make it less necessary. That the future, in space or on the ground, appears to be up in the air may be as literal as it is figurative.

12 MOSAIC Volume 22 Number 4 Winter 1991

Virtually the only way to learn anything directly about the uni­verse is through the wave­

lengths of light: Everything visi­ble shines by its own or by reflected light. Unfortunately, light keeps as many secrets as it reveals: It leaves stars and galaxies and radiates outward for mil­lions or hundreds of millions or billions of years. By the time it reaches the earth, the image it carries of a star or galaxy has dimmed drastically, sometimes to near imperceptibility. And before light can reach an astronomer's telescope, it encounters earth's shimmering atmo­sphere and the image is either blocked out completely or scrambled. As a result, astronomers on earth are trying to see

objects whose light can be no more than a few photons per second per square meter and whose images are distorted, blurred, and dancing around.

For nearly 400 years astronomers have worked around the problems, building larger and larger mirrors to catch more of the light, siting their tele­scopes up higher and higher mountains in thinner, stiller air. In 1990 astrono­mers tried what was billed as the best-yet solution, a telescope above the atmo­sphere, in space.

But the Hubble Space Telescope is not the best yet One reason is human error in its construction and the near-impossibility of making repairs. Another reason is that, in the 30 years between

the first studies for the Hubble and its launch, ground-based astronomers learned to build even larger mirrors to gather even more light, invented detec­tors that catch nearly every photon, and attached to their telescopes devices that compensate for the effects of the atmo­sphere and restore images to precision and stability. By the time space astron­omers got the Hubble in place, ground-based astronomers were giving it a run for its money.

Catching photons

The things astronomers want to look at are faint That is, though the things themselves are often uproariously bright, their light, like all electromag-

MOSAIC Volume 22 Number 4 Winter 1991 13

netlc radiation, falls off as the square of their distance. So an object at twice the distance delivers a quarter of the light and an object at ten times the distance, a hundreth of the light.

An extreme case, a star-like quasar with the light of ten thousand galaxies at a distance of ten billion light-years, might send into a mirror four meters across only two photons a second. Hu­mans cannot see two photons. Although neurons in the retina will fire on impact from one photon, the brain does not think it sees anything until the neurons have detected 100 photons. The retina, moreover, does not record photons and does not accumulate them, so to see faint objects, astronomers attach cam­eras to the telescopes and expose pho-tographic plates for hours. Photographic plates build up images and improve on eyesight 100-fold.

But astronomers are always, in their words, photon-hungry. In the 1970s they began using solid-state detectors, called charge-coupled devices, that were more sensitive than photographic plates. From any given object photographic plates mounted on telescopes could de­tect approximately one percent of im­pinging photons. Charge-coupled de­vices, says Sidney Wolff, director of the National Optical Astronomy Observato­ries, "detect 70 or 80 or 90 percent. That means we're detecting all the light there is." In fact, Wolff adds, "we're coming to the limit of what we can do by improving detectors."

Fortunately, capturing faint light can be done not only with sensitive detectors but also with larger mirrors. Seventy years after Galileo built his first tele­scope in 1609, Isaac Newton built a ver­sion that replaced the curved lens with a curved mirror. Since Newton's time, mirrors have replaced lenses in most telescopes. The mirrors are curved in such a way that they reflect all the pho­tons hitting them to a single focus point. By the late 1980s large telescopes usu­ally had mirrors four meters across, "whose surfaces are so controlled," says Roger Angel, mirror maker at the Uni­versity of Arizona, "that all the rays from

Finkbeiner is a Baltimore-based free­lance science journalist and coauthor with John Bartlett of The Guide to Living with HIV Infection. Her most recent arti­cle for Mosaic was "Mapmaking on the Cosmic Scale" in Volume 21 Number 3 Fall 19m

an undistorted star are aimed at one spot: a few microns across."

Moreover, the larger the mirror's sur­face, the more photons it can capture; a four-meter mirror can routinely see ob­jects as dim as 25th magnitude. (Magni­tude, an arbitrary measure of an object's brightness, is on a logarithmic scale on which each step up in magnitude is two-and-a-half times as bright as the previous step. Objects of 25th magnitude are 50 million times as faint as the faintest stars the naked eye can register.)

But, like detectors, mirrors are reach­ing the limits of the technology. Though the celebrated Hale telescope on Mount Palomar is five meters across, for a num­ber of reasons a four-meter mirror is about as big as a conventionally built mirror can realistically get. A larger mir­ror built the same way as the usual four-meter would weigh so much more that gravity would affect the perfection of its surface. "No further scaling up of the standard design is practical," wrote Roger Angel.

In short, as of the late 1980s, the de­tectors and mirrors of telescopes on the ground were capturing all the photons they could.

Seeing through the atmosphere

A second fundamental problem that prevents astronomers from see O V - O l l l i ;

everything they want to is the hundred-odd-mile depth of the earth's atmo­sphere. The atmosphere does unspeak­able things to light.

The things astronomers want to look at emit light in all wavelengths; the at­mosphere blocks out most of them. It blocks out all wavelengths shorter than about 0.3 millionth of a meter, or 0.3 micron (far-ultraviolet wavelengths, x rays, and gamma rays). It lets through wavelengths between a few millimeters and tens of meters (the so-called radio window) and blocks out everything above 100 meters (long radio waves). The tiny fraction that does get through and that human eyes can see (between 0.3 micron and 10 microns, the near-in­frared, visible, and near-ultraviolet wave­lengths) is named for the Greek word for eye, ops, for optical.

The atmosphere is not only choosy; it glows. It has what Angel calls "a sort of phosphorescence that goes on all the time." Various molecules give off energy in infrared wavelengths, and extracting

14 MOSAIC Volume 22 Number 4 Winter 1991

a star's signal from the atmosphere's noise is often impossible. In addition, much of the light that cities emit gets scattered off the atmosphere so that, near a city, the seemingly dark night sky can be a hundred times as bright as the faintest galaxies.

The wavelengths that the atmosphere does not block or swamp with noise are scrambled. To an optical astronomer the atmosphere is a collection of blobs or patches of warmer and colder air, each about ten centimeters across. Light try­ing to get through the atmosphere is deflected off each patch differently. The amount of deflection depends on the

patch's temperature. Cold patches are denser than hot ones and accordingly deflect light more. And if all that were not enough, the patches flit through the light's path and change the deflection with time. A light wave from a star hits a hot patch and deflects one way; the patch scoots out of the light path. A colder patch scoots into the path, and the light deflects another way. The up­shot is that in a telescope images twin­kle. They shimmer, jitter, and change hundreds of times a second, stretching an image out by 10 to 20 times.

Astronomers call the effect on the image of those changes in size seeing.

Any given night can have seeing that changes little or fluctuates dramatically. Astronomers say that looking through the atmosphere is like looking through a river or the air rising from a bonfire. Images have no clear edges and some­times no clear distinction between neighboring objects. That is, they have far less resolution.

Resolution

Resolution is fundamentally a mea­sure of the sharpness of detail in an image; the higher the resolution, the greater the detail. Astronomers define resolution in various ways, one of which

MOSAIC Volume 22 Number 4 Winter 1991 15

is the distance over which one can see that two points are separate. (Human eyes can resolve two points three centi­meters apart at 100 meters.)

Astronomers measure resolution in angles, in units called minutes of arc or seconds of arc; 60 seconds is one min­ute, and 60 minutes is one degree. In these units human eyes can resolve two points separated by one arc minute. For years astronomers thought that the res­olution of which an optical telescope was capable was limited by that ten-centime­ter patch on the sky, an arc second across, or two points five millimeters apart at a distance of one kilometer. "People have thought of it as a one-arc-second atmosphere," says Wolff.

One arc second of resolution lets as­tronomers see things that are either rel­atively nearby or intrinsically large: Jupi-ter is 40 arc seconds across; globular star clusters around the Andromeda gal­axy are one or two arc seconds across. A quasar, however, is about 0.0001 arc second across. Quasars are extremely old, appear to be pointlike, like stars, and can have the brightness of ten thou­sand galaxies. Astronomers would like to find out how quasars work, but the 0.0001 arc second image gets smeared over one arc second. "A whole lot of de­tail is lost," says Holland Ford of the Space Telescope Science Institute and Johns Hopkins University. "You know if s in there and you can't see it."

Telescopes in space

One solution to the problem caused by the atmosphere has been to put tele­scopes in the still air at the tops of high mountains located in the middle of oceans or at the edges of continents. The seeing on Mauna Kea, Hawaii, can be as good as 0.4 arc second.

A better solution ought to be to put a telescope above the atmosphere, in space. Satellites in space have been suc­cessfully detecting wavelengths other than the optical for years. The Hubble Space Telescope, with a 2.4-meter mir­ror, operates in optical (and ultraviolet) wavelengths at an intended resolution of better than 0.1 arc second. The observa­tions planned included the shapes of gal­axies ten billion light-years away, black holes in the centers of galaxies, the clouds out of which stars form, and the evidence for a scale by which to measure distances in the universe, known now only to within a factor of two.

Among other problems, unfortu­

nately, the Bubble's mirror was curved wrong; instead of 70 percent of the light hitting a focal point of 0.1 arc second, only 15 percent of the light does. The other 85 percent of the light is spread in a smudge three arc seconds across. A star seen through the Hubble is a large bright cloud with a small brighter core. Few of the planned observations depen­dent on sharp resolution can be done. For the time being astronomers can work around some of the Bubble's prob­lem, observing bright objects and recti­fying images by computer. They hope in a few years to be able to correct the problem by adding 'a mirror that curves with the equal but opposite wrongness.

The Bubble's problems—in control mechanisms and materials as well as in optics - were engineering and manage­ment problems; they do not reflect on the feasibility of telescopes in space. What may limit the feasibility of space telescopes, besides accessibility, is the time they take from planning to launch. The Hubble was originally suggested in 1946. Studies were done between 1962 and 1971; the first call for design of the detectors on the telescope came in 1977, and the telescope was launched in June 1990. During the 15 years between the Bubble's design and launch, ground-based detectors became more sensitive and larger, computers became smarter, astronomers figured out how to make mirrors larger, and a technology devel­oped for military purposes went a long way toward canceling the atmosphere's shimmer. "I think the ground has caught up with a lot of the things the Hubble was supposed to do," says Wolff. "Not all of the things—but our technol­ogy has come to the point where we are pushing the Hubble hard."

Pushing the Hubble

One of the most dramatic changes in optical telescopes on the ground has been in the mirrors. Until the late 1980s each larger mirror was scaled up from its predecessor until mirrors were rou­tinely half a meter thick, three and four meters in diameter, and weighed in at around 15 tons.

But these mirrors, if scaled up to eight meters, would weigh 120 tons—so much, says Angel, that gravity would cause them to "droop." Warping of any kind messes up the resolution.

Other than gravity, says Angel, the enemy is temperature change. Massive mirrors heat up and cool down slowly;

the air around them changes tempera­ture much more quickly. So inevitably the mirrors and the air surrounding them will have different temperatures. The difference causes much the same shimmer in images as atmospheric see­ing does. For each difference of one de­gree Celsius between mirror and air, the shimmer adds to the image a blurring of 0.4 arc second.

The solutions to the problems with relative temperature and gravity have been ingenious new designs, and for the first time mirrors eight and ten meters across have become feasible. With eight-meter mirrors, astronomers can see objects four times as faint and take spectra on objects 10 to 20 times as faint as they can with a four-meter mirror.

The solution for the heat problem has been to keep a telescope at the tempera­ture of the air around it. In the New Technology Telescope, or NTT—a three-and-a-half-meter telescope the European Southern Observatory built in La Silla, Chile, to test several new technolo­gies—the temperature of the building housing the telescope is kept as cool as the ambient air, as are the temperature of the floor and the air around the mir­ror.

16 MOSAIC Volume 22 Number 4 Winter 1991

The solution to the gravity problem is to make mirrors lighter. Astronomers have several ways of doing this. Angel builds one-piece mirrors not as thick slabs but as glass sandwiches whose "bread" Is two sheets of glass, each 25 to 28 millimeters thick, and whose "fill­ing" is a lightweight glass honeycomb up to 80 centimeters thick. An eight-meter mirror built the traditional way would weigh 120 tons and sag four times as much as a four-meter mirror does. An eight-meter mirror built from Angel's honeycombs would weigh only 14 tons and sag as little as would a four-meter.

Angel's mirror laboratory will cast two eight-meter mirrors for telescopes owned in part by NOAG, the National Op­tical Astronomical Observatories, among the largest single mirrors ever to be cast. One NOAO eight-meter mirror, sited high on Hawaii's Mauna Kea for its excellent seeing, will be specialized to work in both optical and near-infrared.

The other, sited on Cerro Pachon in Chile, will be for observing the skies, historically neglected, above the south­ern hemisphere. Together, the twin eight-meter telescopes, being called the Gemini, will provide coverage of the whole sky. 'The telescope on Mauna Kea ought to be the best Imaging ground-based telescope in the world," says Wolff. Gemini is an international project, funded by the National Science Foundation, Great Britain's Science and Engineering Research Council, and other International partners.

Another way to make mirrors lighter is to make them in pieces. The Keck telescope on Mauna Kea will have a mir­ror pieced together out of 36 hexagons, each 1.8 meters across. The segments are positioned to act as a single mirror ten meters across. The Keck, a private gift from the W. M. Keck Foundation to the University of California and the California Institute of Technology, is

planned to go into operation In 1992. Still another way to make mirrors

lighter Is to make them thinner. The mir­ror for the planned Japan National Large Telescope, or JNLT, to be sited on Mauna Kea by 1999, will be eight meters across and 200 millimeters thick. The four mir­rors of the Very Large Telescope, or VLT, planned by the European Southern Ob­servatory to be sited in Chile by 1998, will each be eight meters across and 175 millimeters thick. (The ratio of the di­ameter to the thickness of the mirror in a traditional telescope Is around six to one; that ratio for the VLT mirrors is 46 to one.) On the JNLT and the VLT, the mirrors if unsupported would lop.

The floppy mirrors and the Keek's segmented mirrors are not only lighter, they can also be adjusted to counter gravity's effect on the mirror. The tech­nology for adjusting mirrors is called ac­tive optics. The Keek's segments are in­dependently controlled: Behind each

MOSAIC Volume 22 Number 4 Winter 1991 17

segment are three little rods, or actua­tors, that move the segment side to side, front to back, or up and down. "Each motion of the actuators is five to ten nanometers," says Keek's designer, Jerry Nelson, an astronomer at the Uni­versity of California at Berkeley, "and the whole system is updated twice a sec­ond." The JNLT and the VLT are also backed with actuators; the VLT'S technol­ogy is being tested on the NTT, which also has an unusually thin mirror.

Active optics works. The NTT on its first night in 1989 achieved a resolution of 0.33 arc second, three times that of a conventional telescope of the same size at the same site on the same night. In its first year of operation the NTT re­solved for the first time the individual stars in a cluster in the nearby Fornax galaxy and found at optical wavelengths some objects that had been seen only in

the radio. These may be a dense cluster of stars and a black hole at the center of the Milky Way. The Keek's resolution in the near-infrared should be even bet­ter, around 0.25 arc second. (See "Build­ings That Behave Like Machines" by Kendrick Frazier, Mosaic Volume 11 Number 1 January/February 1980, and "Astronomy From the Ground Up" by Sandra Blakeslee, Mosaic Volume 17 Number 2 Summer 1986.)

Sharpening resolution

Mirrors are not the only technology enabling ground-based telescopes to challenge space telescopes. The VLT has four mirrors because it is meant to be used as an interferometer. An interfer­ometer is an array of two or more tele­scopes, positioned so that the light from all mirrors converges precisely into one image. Interferometers, which have so

Galileo Galilei's spyglass

Galileo Galilei, born in Florence, Italy, taught mathe­matics at the University of Pisa and supplemented a faculty income by making scientific instruments. In 1610, after he built the first telescope, he wrote about the instrument and its applications in a brief classic, Siderius Nuncius, translated as The Sidereal (or Starry) Messenger. His pre­occupations and methods sound remarkably familiar.

Early in 1609, Galileo wrote, he had heard rumors of a spyglass "by means of which visible objects, though far removed from the observer, were distinctly perceived as though nearby." The spyglass had been made of eyeglass lenses by a Dutch spectacle maker named Hans Lipper-hey, whose patent application was denied because the spyglass was too easy to copy. Meanwhile word spread and other spectacle makers throughout Europe made and sold other spyglasses, probably of poor quality. Using the rumors, the "science of refraction," and "inspired by divine grace," Galileo wrote, he re-invented the spyglass.

With his first instrument objects were three times as close and nine times as large; with his second, 60 times as large. "Finally, sparing no labor or expense," he wrote, "I progressed so far that I constructed for myself an in­strument so excellent that things seen through it appear about a thousand times larger and more than thirty times closer than when observed with the natural faculty only."

The following year, with his spyglass, Galileo detailed the surface of the moon, found around faint stars "such a crowd of others that escape natural sight that it is hardly believable," and announced that the Milky Way is "noth­ing else than a congeries of innumerable stars. To what­ever region of it you direct your spyglass, an immense number of stars immediately offer themselves to view, of which... the multitude of small ones is truly unfathom­able." Most important, he said, he found four "little stars" circling Jupiter, "never seen from the beginning of the

world right up to our day." With fine grantsmanship, Galileo had given the doge

and senate of Venice the rights to manufacture the spy­glass, and in return Galileo's contract at the University of Padua was to be renewed for life and his salary increased. When he found out the contract also precluded any further salary increases, he changed allegiance and named the moons of Jupiter the Medicea Sidera after the ruler of Florence, Cosimo II de Medici. Cosimo gave Galileo a court position and moved him to the nearby University of Pisa as principal mathematician, a job with no duties. Gali­leo's spyglass was first called a telescope at a banquet given in his honor by the Florentine Academia dei Lyncei.

Galileo understood almost immediately the implications of what he observed through the telescope, and subse­quent observations of Venus confirmed it: that Copernicus was right, and the earth is not the center of the solar system. In 1632 he wrote up the argument in Dialogue on the Two Great World Systems. The two world systems were the Copernican and the non-Copernican.

In those days the church regarded astronomy as a branch of theology, and the theology was non-Copernican. Galileo was tried by the Roman Inquisition for contradict­ing the scriptures, was forced to recant, and was exiled to his own home in Arcetri on the hills above Florence.

But back In 1610 Johannes Kepler had written a letter to Galileo, published as "Conversations with the Sidereal Messenger." Galileo's discoveries, Kepler wrote, led him to suspect that God gradually leads man "step by step from one stage of knowledge to another." The "smug philosophers" who think nothing is new under the sun, Kepler wrote, should look back and reflect: "How far has the knowledge of nature progressed, how much is left, and what may the men of the future expect?" Optical as­tronomers could not agree more. • A. F.

18 MOSAIC Volume 22 Number 4 Winter 1991

MOSAIC Volume 22 Number 4 Winter 1991 19

Tony Redhead, Palomar Observatory

far been used primarily as radio tele­scopes, have one particularly important advantage: Resolution goes way up. The larger the mirror compared with the wavelength of light, the smaller the size of the image. Because the size of the image is another definition of resolution, larger mirrors have better resolution. The four mirrors of the VLT will be spread out over 100 meters and will have the resolution of a 100-meter mirror. The VLT mirrors, used as an interferom­eter, should have a resolution measured in thousandths of an arc second. (See "Optical Interferometry" by Marcia Bartusiak, Mosaic Volume 14 Number 2; "Mapping the Sky" by Derral Mulhol-land, Mosaic Volume 20 Number 1 Spring 1989; and "Opening Another Window" by Frederic Golden, Mosaic Volume 21 Number 2 Summer 1990.)

"We've only recently started thinking about optical interferometry seriously," says Robert Wilson of AT&T Bell Labora­tories at Holmdel, New Jersey. One

early example is the Multi-Mirror Tele­scope (MMT) on Mount Hopkins in Ari­zona, built in the early 1980s. The MMT has six 1.8-meter mirrors designed to be used together as an interferometer. But because the MMT'S effective diameter is only around six meters, its owners, the University of Arizona and the Smith­sonian Institution, are replacing it with a single, 6.4-meter honeycomb mirror that will collect more light and have the same resolution. Other optical interfer­ometers have been tested on bright ob­jects using the amateur's telescopes: In­terferometers built out of off-the-shelf Questar telescopes, says Wilson, "have mirrors a few inches in diameter placed 10 meters apart, and you get information corresponding to a 10-meter mirror in resolution."

Interferometry is one of optical astronomy's two highest priorities for

the next ten years. The decade report on astronomy's goals by a National Re­search Council panel, published in March 1991, recommended that, of the moderately priced ground-based tech­nologies, optical interferometry be given second-highest priority.

Interferometry with the VLT awaits the end of the century. Interferometry with a second Keck, funded partly by the Na­tional Aeronautics and Space Adminis­tration, is in the works for some time even later. "Well copy Keck I to make maintenance easy," says Nelson, "and a few years later, well do interferometry with both of them." Also in the works is a telescope called Columbus, an inter­ferometer built of two eight-meter mir­rors and so far funded by the University of Arizona, through the Arcetri Astro-physical Observatory, and the govern­ment of Italy. Columbus will be built on Mount Graham in Arizona and should "see first light" in 1996. "Optical inter­ferometers with resolutions of 0.001 arc

20 MOSAIC Volume 22 Number 4 Winter 1991

second," says Wolff. "It would blow your mind. You ought to be able to see the structure around black holes at the cen­ters of quasars."

Compensating for the atmosphere

The NRC'S report accords its highest priority to a new technology called adap­tive optics—which, like interferometry, drastically improves resolution; unlike interferometry, it is a device that can be attached to any optical telescope. While active optics compensates for the effects of gravity, adaptive optics compensates for the shimmering atmosphere.

A wavefront of light, just before it hits the atmosphere, is a plane wave, a nice, lat wave. As it moves through the at­mosphere and encounters hotter and colder patches of air, the flat wavefront develops tilts in different directions and at different angles. The wavefront that hits the telescope mirror, then, is a se­ries of tilts, each of which changes every hundredth of a second.

Adaptive optics sends this messy wavefront from the mirror to a device that senses the tilts in the wavefront. That sensor in turn sends the pattern of tilts to a computer. The computer con­trols the positions of a group of actuators attached to a second, or adaptive, mir­ror, "a little tiny guy, at most several inches across and an eighth of an inch thick," says Angel. Depending on the pattern of tilts in the wavefront, the ac­tuators change the' curvature of the adaptive mirror so that it reflects the equal but opposite pattern of tilts. As a

result the wavefront that the adaptive mirror sends on to the astronomer is again a flat plane wave. The whole pro­cess repeats a hundred times every sec­ond. "The mirrors move with the re­sponse of a loudspeaker," says Angel.

The wavefront sensor, however, needs a lot of light to make its compu­

tations and can therefore work only with unusually bright objects. To observe faint objects astronomers would need to choose those that are near some unusu­ally bright object, make adaptive correc­tions based on the bright object, then apply the corrections to the faint object. That means that the faint object needs to be within a few arc seconds of a bright object. "The chances of this working out," says Sam Durrance, adaptive op­tics researcher at Johns Hopkins Univer­sity, "are probably infinitesimal, and cer­tainly impractical."

The solution is straight out of tech­nology developed for missile defense: Make a bright star artificially. Do this by shooting a laser beam into the-upper, less-turbulent layers of the atmosphere. There it reflects off molecules or makes atoms fluoresce. In either case it makes an artificial star. If it works the laser flashes, the artificial star shines, the adaptive mirror vibrates, and the wave-front straightens out, all hundreds of times a second. The astronomer sees a sharp, distinct, steady star.

How well adaptive optics works in practice is still a little unclear. The Stra­tegic Defense Initiative Office of the De­partment of Defense has built the only complete system-—wavefront sensor, adaptive mirror, and laser. The purpose was military; the specifics of the system have since been released to NSF for use by astronomers. The system SDIO tested had a laser that placed an artificial star six kilometers high and corrected an adaptive mirror 3,000 times a second.

MOSAIC Volume 22 Number 4 Winter 1991 21

Optical astronomy and the human eye

The visible colors, from red to violet, are a tiny fraction of all wavelengths of the electromagnetic spectrum. Opti­cal astronomy includes the visible and those wavelengths just beyond the visible and a little way into the infrared and ultraviolet. It is still a tiny fraction, but it has an out-sized importance in astronomy.

Optical astronomy is disproportionately important for three reasons: physical, historical, and psychological. The physical reason, says Garth Illingworth, astronomer at the University of California at Santa Cruz, "is that a substantial fraction of the matter in the universe—stars, galaxies, gas—emits radiation in optical wavelengths." That in­cludes the sun. Humans evolved to see in wavelengths that the sun emits and earth admits and that, happily, cany much of the information about the rest of the universe. "If you could set up only one telescope," says Illingworth, "your best choice would be [the optical]."

The historical reason is that the optical region has what Illingworth calls a huge historical database. From 1.609 and Galileo's first observations until the 1940s, when the first radio telescopes were built, astronomers took most of their data in the optical.

The psychological reason, says Sidney Wolff, director of the National Optical Astronomical Observatories, is that we have "an obvious prejudice." As a result of the preju­dice, says Illingworth, "if people make observations in x rays or radio waves, they don't talk about identification of the source until they go to an optical map to find out what the hell it is." Wolff quotes Sandra Faber, Illingworth's colleague at Santa Cruz: "You can have unidentified ob­jects in other wavelengths, but never in the optical."

None of this is to say that observations at nonoptical wavelengths are not important or fascinating, only that somehow, to people, optical makes more sense. A. F.

22 MOSAIC Volume 22 Number 4 Winter 1991

When tested on a telescope with a 60-centimeter mirror, the setup took pic­tures in visible wavelengths of bright stars with resolutions (0.2 arc second) limited only by the size of the mirror.

Before SDIO systems can be retrofitted routinely to working telescopes, the sys­tems will have to work on dimmer ob­jects. That means they would need to be used with three- or four-meter mirrors, which in turn means thousands of adap­tive actuators instead of SDIO'S 241. The systems would also need to place artifi­cial stars above more of the turbulence, nearer the top of the atmosphere 90 ki­lometers up. Researchers at the Massa­chusetts Institute of Technology's Lin­coln Laboratory who helped develop SDIO'S system say the problem is not technology but who will pay for devel­oping the technology. Aram Mooradian at Lincoln Laboratory says he has al­ready made an artificial star with a laser tuned to make sodium atoms fluoresce and has placed it in a sodium-rich layer of the atmosphere 90 kilometers high.

Meanwhile astronomers have been tinkering with their own adaptive optics systems. Because most astronomers lack SDIO'S resources, they have had to make compromises. One compromise is that most of their systems have wave-front sensors and mirrors but no lasers

and so can be used only on bright stars. Another is that their systems can correct images taken only in infrared wave­lengths, which are longer than visible wavelengths. (The atmosphere affects longer wavelengths less, so infrared im­ages are easier to correct.) The Euro­pean Southern Observatory's prototype adaptive optics system recorded images of bright stars with a resolution of 0.18 in infrared wavelengths.

Compromises notwithstanding, some of the astronomers' systems are inge­nious. Sam Durrance is building a sys­tem whose adaptive mirror is a celluloid film and is moved not mechanically but by changing voltages across it. Francois Roddier at the University of Hawaii is working on a system that may not need a laser, because the wavefront sensor itself is sensitive enough to detect faint objects. Angel has tested on the MMT a system that substitutes for a wavefront sensor a neural network computer pro­gram trained to relate the distortion in the image directly to the corrections made to the adaptive mirror. (See 'The Brain as Template" by Ann Finkbeiner, Mosaic Volume 19 Number 2.)

But how good will the systems be? "Five years ago there was a lot of skep­ticism about adaptive optics," says Durr­ance. "Now people just assume it's going to happen, and it is." But no one thinks that adaptive optics is the final solution. Even when the systems work at visible wavelengths, have sodium lasers and

more actuators, and cost a reasonable fraction of the whole telescope, they will still be able to correct only the patch of sky near the artificial star. That means that astronomers wanting to observe ex­tended objects, like clusters of galaxies, with high resolution will have to resort to telescopes in space. "I'm not in the foregone-conclusion camp about adap­tive optics," says Sandra Faber, astron­omer at the University of California at Santa Cruz. "At the very least, it will be maybe 20 years before they're a routine, trouble-free part of the arsenal."

in space or on the ground?

Do bigger mirrors with fancier de­signs, better light-gathering capabilities, and better resolution then mean the foreseeable future of optical astronomy is probably on the ground? The answer is unanimous, obvious, and unspectac­ular: yes and no. What can be done on the ground should be; what cannot should be done in space.

The advantage of ground-based tele­scopes turns out to be a negative: They're not in space. In principle ground telescopes can do no observations better than the space telescopes can. "But space is going to be given a real ran for its money by the ground-based obser-vators," says Riccardo Giacconi, director of the Space Telescope Science Insti­tute. "Because," he observes, "in space we have problems."

One problem is that space telescopes,

MOSAIC Volume 22 Number 4 Winter 1991 23

once launched, are all but inaccessible: "Gradually, it's dawned on everyone that when it's up there, it's up there," says Garth Illingworth, astronomer at Santa Cruz. Correcting the mirror would have been an annoying but easily fixable problem in a ground telescope; in the Hubble, correction will wait three-and-a-half years, unless more pressing re­pairs—to gyros and solar panels—take precedence. Another problem is that telescopes in space cost 100 times as much as comparable telescopes on the ground. The reason, says John Bahcall, astronomer at the Institute for Advanced Study in Princeton, New jersey, who chaired the NRC'S decade report, is "the necessity of making things so well that you never need to take a screwdriver or Scotch tape to them and that they can withstand the rigors of the launch."

The overriding problem is that space telescopes, regardless of wavelength,

24 MOSAIC Volume 22 Number 4 Winter 1991

take so much time to design and build. In 1978 Giacconi chaired the science working group planning an x-ray space telescope called AXAF and, he says, "we are now planning launch in '98." That 20 years has affected the whole field of x-ray astronomy, not the least because it is a significant fraction of an astrono­mer's research lifetime. "We have burned a whole generation," says Giac­coni. In the x-ray region, astronomers, Giacconi says, "have been living off the archives of Einstein [x-ray satellite], which finished flying in '81, or going around begging a little data from the Japanese or now the Germans.

"In this country, if you're not in AXAF, you're not in x-ray astronomy," he con­tinues." On the other hand, if you're in AXAF, you're not in x-ray astronomy ei­ther. It's virtual science, virtual data; no­thing is actually happening. Today, if a student asked me whether to go into

space astronomy, I would say no. Be­cause the time schedules are so long, it doesn't make any sense." By compari­son, Illingworth points out, "the Keck has come from nowhere to being virtu­ally operational in a decade."

That 20 years also means, says Wolff, that "what you launch into space is usu­ally ten-year-old technology." On the ground, says Illingworth, "you can readily take advantage of new instru­mentation. We will be able to put things on Keck you can't even buy now. We will do upgrades that aren't available now." The upshot is, what with ground astronomy's new technologies and space astronomy's impracticalities, says Faber, "if the future of optical astronomy is anywhere, it's on the ground."

The NRC'S decade report recom­mended some changes that would make optical astronomy in space more feasi­ble. One recommendation was that NASA,

Access: a nagging problem

Most of the observing that optical astronomers want to do must be done on one or another of the 15 major (three meters and up) working telescopes in the world. Access to these telescopes is a problem, one way or the other, for the whole astronomical community.

The foundation of the problem is that telescopes are expensive. New ones are costing anywhere from $80 mil­lion for the one Gemini on Mauna Kea, to $94 million for the Keck, to $250 million for the Very Large Telescope, or VLT. The United States, through the National Optical Astronomical Observatories, or NOAO, built two telescopes in the 1970s and will fund two more (the Gemini).

The national telescopes are open to all U.S. astrono­mers. (National telescopes can give time to only one of four astronomers who apply. Of the 4,200 professional astronomers in the United States, roughly three-quarters may be dependent on publicly owned telescopes.)

The rest of the country's optical telescopes are privately owned, by universities or university consortia, which re­strict access pretty much to their own scientists. (See 'The instruments: whose and where" accompanying this arti­cle.) The private telescopes allot roughly 90 percent of observing time to astronomers in their universities and 10 percent to outsiders.

An astronomer with access to a private telescope can observe somewhere in the range of 3 to 15 nights a year. An astronomer with access to a national telescope, says Sidney Wolff, director of NOAO, typically gets three nights per semester. Nights are counted as 10 or 11 hours long. "If your typical exposure time is a couple of hours, which is common," says Wolff, "you can see you're not going to get many observations [at a public instrument] or much data. Or it could be cloudy and you're out of luck until the next time." Faber. Telescope time—a thorny issue.

which backs all space astronomy, should increasingly back smaller, less-complex projects, put those projects on a shorter time scale, and launch the proj­ects with expendable launch vehicles, not the often-delayed and risky shuttle. In the meantime, says Giacconi, "on the ground, the Keck, the Keck II, and the Gemini are going to keep [astronomers] busy for quite a while."

What cannot be done on the ground

The observations that can be done only from space are those the atmo­sphere blocks out entirely: long radio waves, long infrared waves, the far-ultra­violet, x rays, and gamma rays. 'You can't do those problems on the ground," says Sidney Wolff. "Space will always give you wavelengths you'll never reach from the ground."

In addition, the atmosphere, earth, and even the telescope all glow in the

infrared, making observations from the ground in those wavelengths difficult. "Infrared telescopes in space," says Wolff, "would be as cold as outer space/' Telescopes for all blocked-out wave­lengths are either now in space or are going there soon.

And for the further future, some as-tronomers think that a telescope on the ground will never have the resolution or the wide field of view of the same tele­scope in space: "If you want to look at faint, fuzzy stuff," says Bahcall, "you'll always have to do that from space." Illingworth agrees: "Ground-based tele­scopes allow you to meet some goals, but you don't get 100 percent there. These new techniques are only a half­way step to where you really want to be." The ideal telescope, they say, is a large telescope in space.

Astronomers are already planning for telescopes in space that sound fairly

blue-sky. Illingworth and others are do­ing the preliminary planning for a ten-meter telescope in space, called the Next Generation Space Telescope. The NGST could see things as faint as 32d magnitude, and could resolve things as small as 0.007 arc second. "We could resolve structures in galaxies at any red shift," Illingworth writes, "with the res­olution with which we now study the nearest clusters of galaxies." Angel wants a 16-meter mirror in space: uWe asked ourselves what to put in space to see if solar systems have planets like earth," Angel says. Infrared satellites, looking back at the earth's atmosphere, clearly see evidence of ozone in the at­mosphere's spectrum; and the ultimate origin of ozone, says Angel, is life. "A 16-meter in space would be capable of finding that kind of evidence of life on planets around other stars."

Angel also wants to build on the

MOSAIC Volume 22 Number 4 Winter 1991 25

Although many private telescopes allot observing time for outsiders, most outside astronomers either do not get time or do not even apply for it. "It's a tough business to be an astronomer in a university with no telescope," says Wolff. "It's even tougher to run graduate programs in astronomy where graduate students need access to com­plete their work."

For all astronomers the result is a split in the commu­nity. Says Sandra Faber of the University of California at Santa Cruz (who will have access to the Keck), "any grad­uate who's good, serious, and committed will fight tooth and nail to get into a university with a telescope.

"Of all the astronomers who are making the greatest contributions, how many rely on public telescopes? I'd say a quarter," says Faber.

One solution: The National Science Foundation, which funds NOAO, has also begun to cosponsor (and secure ac­cess to) private telescopes. The first such is a 3.5- meter telescope on Kitt Peak called WIYN telescope for its partic­ipants: Wisconsin, Indiana, Yale, NOAO. The NOAO'S two new telescopes may or may not ease the problem; Bahcall says new telescopes cause "population explosions."

Another solution: Astronomers get out of optical astron­omy and into space astronomy at other wavelengths.

A third solution: Many astronomers use data in archives collected at NOAO or by space astronomers. But only data collected at public installations is archived; data collected by astronomers using private telescopes can remain pri­vate property. Archiving data collected privately sounds like a solution and was recommended by the National Research Council; but because each instrument must have its separate archive, says Peter Boyce of the American Astronomical Society, "archiving for private instruments is unrealistic."

" I t s a tough business, a very tough business," says Wolff. Vera Rubin of the Carnegie Institution of Washington, which owns telescopes, agrees: "Can you imagine doing physics with access to a lab five days a year?" • A. F.

Current instruments: whose and where?

/ elescope

Keck

Bolshoi Alt-Azimuth

Hale

Multi-Mirror Telescope

William Herschel

NOAO Cerro Tololo

Anglo-Australian

NOAO Mayall

U.K. Infrared

Canada/France/Hawaii

European Southern Observatory

New Technology Telescope

Max Planck Institute

Lick Observatory

Mirror Size (meters)

10

6

5

4.5

4.2

4

3.9

3.8

3.8

3.6

3.6

3.6

3.5

3

Country

USA

USSR

USA

USA

Britain

USA

Britain, Australia

USA

Britain

Canada, France, USA

ESO**

ESO**

Germany

USA

Access* public private

X

X

X

X

X

X

X

X

X

X

X

X

X

X

*No telescope is internationally public—that is, open al! the time to any astronomer in the world. Both U.S. and European public telescopes are open to astronomers from the countries that own the telescopes, but they may or may not offer small amounts of telescope time to foreign astronomers. The private telescopes in the United States are open primarily to the institutions that own them, but do open small amounts of time to any astronomer in the world. "Italy, Germany, Switzerland, Belgium, Denmark, France, Sweden, Netherlands

ground a 32-meter telescope that relies on adaptive optics. Present adaptive mir­rors are small enough to have their sur­faces changed quickly. Angel would build his 32-meter like the Keck, only out of smaller pieces. 'They might be a half-inch thick," he says, "and ten inches or so across." The pieces would be sup­ported by actuators but would be small enough that they could be moved not on the long time scale of active optics but on the minuscule time scales of adaptive optics. "If you made a big mir­ror out of little, little pieces," he says, "you could move them fast enough to make [adaptive] corrections. Nobody's done that"

Probably the airiest of blue-sky space projects is an interferometer on the moon, NASA plans ultimately to station astronauts on the moon. Astronomers say that if NASA does create a lunar sta­tion, the moon would be a great place to do astronomy, particularly interfer-ometry. But most astronomers, and the NRC'S decade report, agree with Wolff: "I don't think astronomy is a reason to go to the moon. It's difficult to imagine a problem I want to solve so badly it's

26 MOSAIC Volume 22 Number 4 Winter 1991

A change of pace

The pace at which new telescopes are being built on the ground appears to be slowing. Such telescopes are usually built by national science-funding agencies or part­nerships of universities. Since the spring of 1991, three different partners, citing money problems, have pulled out of three different telescope projects. The future of ground-based optical astronomy still looks promising, but the promise will take longer to fulfill

In the spring of 1991, Johns Hopkins University backed out of a $10 million pledge to the eight-meter Magellan telescope. The following June the National Research Council of Canada announced it would not add its $44 million to the two eight-meter Gemini telescopes, one each in Hawaii and Chile. And in September Ohio State Univer­sity abruptly reneged on a promise of $15 million for the two eight-meter mirrors of the Columbus interferometer. (See main article).

The defaults leave the telescopes alive but behind sche­dule and maybe a little maimed. The partners left on the Magellan—the University of Arizona and the Carnegie Institution of Washington—will proceed with the tele­scope, but they may scale it down to 6.5 meters and in any case are looking for another partner. The partners left on the Gemini—the National Optical Astronomy Observ­atories, or NOAO, and Great Britain's Science and Engi­neering Research Council—will build the telescope on Mauna Kea as planned and are looking for another partner or two before building the companion instrument in Chile.

The partners left on the Columbus—the University of Arizona and Arcetri Astrophysical Observatory of Flor­ence, Italy—face a project in stasis, at least for now. Even with Ohio State, the partnership had enough money for only one mirror and needed a fourth partner to build a working interferometer. According to Roger Angel, whose mirror laboratory at the University of Arizona at Tucson was to build the mirrors, "Columbus is now on hold." If more money is not forthcoming the options are to build a one-mirror Columbus, usable as a conventional tele­scope until a second mirror can be added, or to redesign the instrument entirely as a conventional single-mirror telescope. The remaining partners, says Angel, are under­standably reluctant to see that happen.

The obvious solution is to raise the number of partners per telescope so that each partner puts up, say, $5 million and receives, say, 15 percent of the observing time. But the whole point of owning a private telescope is to own a large amount of observing time. Anyone wanting only a small amount of time is likely to get it on NOAO'S national telescopes without cost. For an institution wanting to com­pute the trade-offs on a $60 million telescope like Colum­bus, the break-even point is probably between $5 million and $7 million.

The need to find new partners to replace the defaulting ones will inevitably slow ail three projects. 'There will certainly be some reshuffling," says Angel, "but no doubt in the long term, well get there." • A. F.

worth the money it would take to estab­lish an observatory on the moon." But, "if you're going anyway, take along the experiments," Wolff adds. "You could do fascinating astronomy along the way." With an optical interferometer on the moon, says Giacconi, "ultimately you could hope to do microarc seconds."

A microarc second is 0.000001 arc sec­ond, enough for Vera Rubin, astronomer at the Carnegie Institution of Washing­ton, to test what she says is "a crazy idea" she's been thinking about for the past year: "to find out whether the uni­verse has a shear." Astronomers have known for a long time that the universe expands away from the solar system be­cause galaxies are traveling away, along the line of sight. They do not know whether the universe is also moving in other directions, and for that they would need to see galaxies moving across the line of sight as well. "In the nearest large galaxy, Andromeda," says Rubin, "the nucleus is moving [across the line of sight] probably at 200 kilometers a sec­ond, or one second of arc every 20,000 years. If we could get to microarc second resolution, we could measure this in a year. Lots of room for bright ideas here."

The future

The future of optical astronomy, like the future of everything else, is unknow­able. No one knows with certainty how effective the new ground-based technol­ogies will be, what the Hubble will or will not be capable of, or to what vagaries funding for astronomy will be subject. But astronomers, like everyone else, must construct the future anyway, so they must balance ground against space. Telescopes on the ground cost less, are easier to maintain, and take a more rea­sonable amount of time to build. Tele­scopes in space can detect a much wider range of wavelengths and are capable of higher resolution and a wider field of view. Telescopes on the ground are more practical; telescopes in space may allow better science.

Astronomers seem to agree that the balance for now favors the ground. For the near future, says Faber, astronomers need to "work hard in the next couple of years to test the promising new ground-based technologies." The fruits of this work, she continues, "will be needed in a little while, when planning for the next generation of space instru­ments begins in earnest."

The far future may belong to space or

may not. Astronomers are making plans in case. In the last analysis, the tele­scopes astronomers want are those that see fainter and In more detail, wherever they are. "We need bigger and better telescopes, always," says Wallace Sar­gent, of Caltech. "When you produce new capabilities, you discover more."

This is not greed. The only way to get at the universe is through telescopes, and when the limits of a telescope are reached, so are the limits of knowledge. "You don't just go out and arbitrarily build bigger and better telescopes," says Illingworth. "You do it for a reason. We're not here to build monuments. We're here to do science."

And when astronomers are asked what science they want to do, they get a little vague and give the same answers they gave when asked that question about the Hubble: They want to study quasars, faint galaxies, star formation, other solar systems. The reason for the vagueness Is that the universe is both big and inaccessible.

"With the universe," says Sargent, "we're nowhere near defining problems narrowly. It's like you're a Victorian ex­

plorer looking for the source of the Nile. It's a large body of water bringing fertil­ity to all Egypt, and you naturally want to know where it came from. And when you ran across the Pyramids, if you have any sense at all, you'll investigate them. And when you notice a new kind of croc­odile which seems more plentiful down a side stream, you'll look for more of them." In short, astronomers truly do not know what they might find.

"I point the damn telescope at the sky," Sargent says, "and see what's there." Rubin answers the question an­other way: "I've always believed if we had ten times as many instruments, we'd know ten times as much." Does Sargent agree? Yes, he does, he says, "My point exactly." •

The National Science Foundation has contributed to the support of the research described in this article principally through its National Optical Astronomy Observatories and Astronomical Instru­mentation and Development programs. The National Aeronautics and Space Ad­ministration is the United States' princi­pal supporter of space astronomy.

MOSAIC Volume 22 Number 4 Winter 1991 27