The Astronomer's New Tools.
Looking at the stars isn't what it used to be. The dedicated astronomer observing alone on a cold, dark mountaintop and taking photos that will be developed, tediously analyzed by human eye, and archived in plate libraries is mostly a forgotten creature. Astronomers today are still dedicated, but their star images are recorded in digital format, analyzed by computer software, and distributed over the Internet. For space-based observatories, remote operation is obligatory, and even a growing number of ground-based observatories offer the option of remote control by researchers thousands of miles away. Astronomers are as likely to be part of a globally distributed team as they are to be working alone.
Today, when a short burst of gamma rays that has been traveling through space for millions or even billions of years approaches Earth, it is first detected by a gamma ray observatory in orbit, then located more precisely by an orbiting X-ray telescope. Soon thereafter, observatories from many parts of the world and orbital space, tuned to diverse frequencies of the electromagnetic spectrum, focus their sights on the same tiny pinpoint of the cosmos, thanks to global E-mail delivery of the gamma burst's location. Spectrographs on the world's biggest telescopes obtain the redshift and determine, if it is a typical gamma burst, that it is a colossal explosion in a very distant galaxy.
Astronomy has become a global, multifrequency, multicultural enterprise as it has pursued its aim of unraveling the secrets of the universe by mapping and modeling it. Even in the nine years since the Hubble Space Telescope was launched, the proliferation of new instruments for observing the distant reaches of the cosmos has been mind-boggling; the upcoming decade promises even greater advances in the field. What has happened? Why, at the end of the twentieth century, have we seen such an outburst of astrophysical technologies?
The answer lies in the remarkable and yet very often underplayed synergy between the pure science of astronomy and the most advanced technologies of the day. Among the astronomers' new tools are sophisticated electronic detection devices for the entire electromagnetic spectrum, powerful high- speed computers, lasers, optical fibers, deformable mirrors, and new methods for building gigantic telescopes on the ground and in space.
An observational science
Astronomy is an observational science, quite unlike a laboratory science that conducts experiments by changing the conditions of its research subject. Historically, astronomy has been an optical science because human beings have built-in optical sensors--our eyes--that detect electromagnetic waves of a certain wavelength we call visible light. Not until the 1940s did the birth of radio wave astronomy begin to open the multispectral windows to our universe that have since then all been opened by the development of new technologies.
Apart from a few space missions that have sampled the surface of other worlds and the occasional good luck in retrieving meteorite samples, our knowledge of the universe is based on collecting all the energy forms that happen to reach Earth from the depths of space. We can intercept and study the following: electromagnetic radiation (ranging from gamma rays to radio waves); cosmic rays, which are extremely energetic charged particles; neutrinos, tiny neutral particles that barely interact with matter; and gravitational waves, disturbances in a gravitational field. Predicted by Einstein's general theory of relativity, gravitational waves have yet to be discovered, but cosmic ray and cosmic neutrino experiments have already been developed successfully. All that we know about the universe must be extracted from measurements of these energy forms.
Among these sources of cosmic information, electromagnetic (EM) radiation remains the dominant resource. EM radiation is characterized by oscillations of electric and magnetic fields such that the higher the frequency of oscillation, the greater the energy and the shorter the length of the wave. Specific intervals of the frequency (or wavelength) spectrum have been given different names. At the shortest wavelengths are gamma rays and X rays, while the longest waves are called radio waves. Light, ultraviolet, and infrared waves are in the middle of this huge spectrum. All the different frequencies of EM energy are transported at the same speed, the speed of light, which is a little less than 300,000 kilometers per second (186,000 miles/second). The stream of energy is quantized into packets called photons.
Astronomers have always been quick to embrace new technologies. Many times over, from the Renaissance to the present day, astronomers have been at the forefront of the development and application of each new technology, whether it is relevant to telescope design or to detecting EM radiation from beyond Earth. Astronomers have always wanted to enhance their capabilities of observing, recording, and analyzing. Thus, in parallel with advancing technology, they have wanted bigger and better telescopes, more sensitive detectors of light and other energy forms, and faster computers to process the deluge of data.
In the last quarter of the twentieth century, in particular, astronomers have eagerly claimed computers and microelectronics as gifts that open the way to realizing their dreams. Even classical astronomy, using conventional-looking telescopes and ordinary visible light, has benefited immensely from all this modern electronic and computer-based technology.
We can make sense of the profound changes transforming observational astronomy by keeping in mind that as astronomers pursue deeper understanding of the universe, they want to be able to "see" farther and in greater detail. To achieve this, they always want to maximize (1) angular resolution, or capacity to see fine detail; (2) energy-gathering capacity; and (3) efficiency in converting electromagnetic energy into something measurable.
As diverse technologies undergirded by computers and microelectronics deliver a torrent of improvements in each of these areas, observational astronomy has emerged as a dynamic frontier science. Ground-based observatories are beginning to challenge the orbiting Hubble Telescope's once unassailable lead in the area of angular resolution. In terms of energy-gathering capacity, which is directly proportional to the area of the telescope mirror, sophisticated mirror-making technologies have vaulted telescopes past the 5-meter diameter threshold that has prevailed for more than 40 years. On the business end of telescopes, where detectors convert EM radiation into something measurable, a whole world of changes has opened broad expanses of the EM spectrum to astronomical observation.
From eyes to electronic sensors
After construction in 1949 of the famed 5-meter (200-inch) Hale Telescope on Mount Palomar in Southern California, the trend was for somewhat smaller telescopes loosely described as "4-meter class." In the early seventies, all these telescopes received a huge boost with the invention of the remarkable charge-coupled device (or CCD) by two researchers, Bill Boyle and George Smith, at the Bell Labs research facility in New Jersey. Today, after nearly 30 years of development, these near-perfect electronic imaging devices, with almost 100 times the sensitivity of photographic plates, are used throughout astronomy for detecting X rays, ultraviolet, visible, and near-infrared light.
Spurred in part by the success of CCDs and aided by military developments requiring infrared or "thermal" imaging systems, yet another astonishing revolution in astronomical detectors occurred in the early eighties with development of infrared detectors called arrays. Based on more exotic semiconductors than silicon, these infrared imaging devices opened up a new and critically important window on the universe. Compared to other frequencies of the EM spectrum, infrared light travels most easily through the dusty clouds of gas that permeate the interstellar medium. These clouds dim and shroud from view newborn stars and their planetary systems. Objects much cooler than the Sun, such as "failed stars" called brown dwarfs, emit profusely in the infrared but only weakly at normal visible- light wavelengths. Moreover, the expansion of the universe means that light emitted as normal visible and ultraviolet waves will be stretched or "redshifted" into the infrared region for the most distant objects in the cosmos. Interestingly, since infrared waves can be collected by normal telescopes and infrared signals are much less affected by light pollution from nearby cities, many older observatories received a new lease of life with the advent of infrared array detectors that can be retrofitted onto the older observatories.
Similarly, new types of detectors introduced in the last decade offer greatly increased sensitivity to radio waves. The two key types are the SIS (superconductor-insulator-superconductor) device and HEMTs (high electron-mobility transistors).
Without doubt, a new era in astronomy was born almost a decade ago with the launch of the 2.5-meter Hubble Space Telescope. Although this unique telescope was initially plagued with an optical problem, it received corrective optics and has gone on to demonstrate magnificently the benefits of its most overwhelming asset: its location above the turbulence of Earth's atmosphere.
The Hubble's forte is angular resolution. Orbiting silently, high above Earth's disruptive atmosphere, the Hubble Telescope achieves the greatest possible angular resolution, that which is "diffraction limited," or limited only by the intrinsic properties of light itself. This means that for a given wavelength of light, the only way to achieve a sharper focus is to make the telescope's diameter bigger.
While space looked like the undisputably best locale for maximizing angular resolution when the Hubble Telescope was launched in 1990, a new technology--adaptive optics--has since emerged that promises to thrust ground-based observation far along on the road toward the diffraction limit already claimed by Hubble. With adaptive optics, it is theoretically possible to eliminate the effects of atmospheric turbulence and allow large ground-based telescopes to achieve the same ultimate limit of performance--the diffraction limit--that the Hubble achieves in space. The basic idea is as follows: Successive very-fast exposure snapshots of a star demonstrate the effects of atmospheric turbulence--the position of the star image dances around. A long, continuous exposure therefore averages over all these positions and produces a much larger, blurred image. If the image motion could be detected fast enough, a responsively changing mirror could continually redirect the light back to the central position. Such a mirror, known as "deformable," is so thin that the shape of its surface can be changed by the force of hundreds of small pistons distributed over the back surface. By changing the surface's shape at a rate of perhaps 1,000 times per second, the blurring caused by the atmosphere can be eliminated--provided there is a suitably bright "reference" star in the field of study. Often, unfortunately, no sufficiently bright reference star is nearby.
By a remarkable coincidence it turns out that Spaceship Earth comes outfitted with a thin layer of sodium particles at an altitude of about 92 kilometers (55 miles). This layer reflects back a laser beam tuned to the yellow-orange wavelength of the sodium D lines. Thus, given enough power in the laser beam, it is possible to "turn on" a sufficiently bright artificial star at any location in the sky toward which the telescope can point.
With the technology at hand for maximizing angular resolution even on Earth's surface, astronomers' dreams of making bigger telescopes to enhance energy-gathering capacity took on a greater urgency. Bigger ground-based telescopes promised to outperform smaller space-based telescopes. But building telescopes larger than 4--5 meters in diameter was by no means trivial.
Three successful new approaches to making telescope mirrors are now coming to fruition. In the vanguard of the new era of large telescopes are the world's largest telescopes, the twin, 10-meter diameter Keck Telescopes on the 14,000-foot summit of Mauna Kea on the big island of Hawaii. Since the amount of energy collected increases as the area of the mirror, or its diameter squared, each Keck telescope is 16 times more effective than the Hubble Telescope in this respect. Each of the primary mirrors is constructed with 36 hexagonal-shaped segments only 7.5 centimeters thick. Each element is actively controlled by computer to be "bent" into just the correct shape for its location in the overall structure. This jigsaw approach, the brainchild of Jerry Nelson at the University of California at Santa Cruz, was totally radical when it was proposed a decade ago. Making mirrors about 1 meter in size was no problem, but fitting them together and controlling them so they behaved like a single smooth dish was only possible with the help of high-speed computers.
An alternative technique is to make the mirror out of a single very thin piece of low expansion glass whose meniscus shape is determined by the active computer control of hundreds of actuator mechanisms exerting forces on the rear surface. This approach was favored by many of the large optical firms and is the basis for the mirrors of the European Southern Observatory's Very Large Telescope on Cerro Paranal in Chile, Japan's 8.2- meter Subaru telescope on Mauna Kea, and a similar pair of telescopes for the International Gemini Telescopes Project on Mauna Kea and Chile's Cerro Pachon.
Yet another approach employed by the University of Arizona Mirror Lab is to melt large chunks of normal glass in a gigantic spinning furnace. The spinning liquid glass assumes a parabolic shape that it retains if the molten mass is allowed to cool while still spinning. The liquid glass also flows around a mold to produce a honeycomb structure. Pioneered by Roger Angel, spin-casting eliminates the need to remove large amounts of glass by polishing. Once again, telescopes using these mirrors can only be controlled by computers. Such mirrors are cheaper to make but are much more rigid as a single large lens. Thus the computer control is different than for the meniscus or segmented mirrors.
Mixing up the signals
What's next? Can more be done with ground-based facilities? The answer is yes. Even greater resolution can be obtained when two or more telescopes are linked up, using a technique called interferometry.
NASA's Origins Program, for example, is supporting a project for linking the two large Keck telescopes plus four smaller "outrigger" telescopes into an interferometer network. Simultaneous with this upgrade, both of the large telescopes also need to be retrofitted with adaptive optics systems to enable them to be diffraction-limited. The primary goals of the Keck interferometer are to look for planets around other stars and for light similar to that in our solar system caused by sunlight scattering from small dust grains. This ground-based work will aid future space-based missions such as the Space Interferometry Mission, scheduled for launch in 2005, and the proposed Planet Finder Mission.
Telescopes used for ultraviolet or infrared astronomy look very similar to the familiar visible-light telescopes, but UV and IR telescopes are distinctive inside. UV telescopes require higher-quality mirrors, while IR telescopes often use gold- or silver-coated mirrors and avoid black baffle tubes because these structures glow at infrared wavelengths. Infrared telescopes are best sited on high, dry mountaintops, since absorption of IR by water vapor is a major impediment at these wavelengths. One way to get even higher is to mount a telescope on an airplane and fly it into the stratosphere at 45,000 feet up. This is precisely the goal of NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA) project, in which a modified Boeing 747-SP jumbo jet will carry a 2.5-meter infrared telescope aloft. At the longest IR wavelengths, however, there are big gains in going into space, simply because there is no atmosphere, it is a vacuum, and therefore the entire telescope can be cooled to cryogenic temperatures, not just the instrument and its detector. Joint NASA/European Space Agency missions such as IRAS (Infrared Astronomical Satellite) and ISO (Infrared Space Observatory) have paved the way for planned Hubble-style observatories such as SIRTF, the Space Infrared Telescope Facility.
X-ray telescopes look rather different. Because X-ray photons can penetrate the reflective coatings of a normal telescope, a special arrangement is necessary in which the X-ray photon just "grazes" the surface. Only at such grazing angles of incidence will the X ray actually be reflected. There have been several very successful X-ray telescopes in space, including the Einstein observatory and the Roentgen satellite (ROSAT). The next generation of X-ray telescopes will have better imaging properties, including the use of silicon CCDs.
On the other hand, the Compton Gamma Ray Observatory (GRO) bears little resemblance to any of the other space telescope facilities. Gamma rays cannot be focused in the normal sense, but their direction of travel can be triangulated to within a small patch of sky, called an "error box." As soon as the first gamma ray telescopes were placed in orbit they discovered an unusual phenomenon, which became one of the greatest enigmas in astronomy for a long time. Very strong and very short bursts (lasting just a few minutes) of gamma rays appeared to be coming from random locations all over the sky, yet no obvious counterparts could be identified after the fact using optical or radio telescopes to probe the error boxes.
Part of the problem was that the data processing needed to get the positions for the error boxes took a long time and so any optical counterpart would have faded away. Until it was possible for computerized instrumentation to provide almost instant notification of a gamma ray burst, follow-up observations were always too late to catch the "smoking gun." Now, orbiting X-ray imaging telescopes, which have better position- determination properties, can follow up the gamma ray burst quickly and achieve good enough positions to enable large ground-based observatories to respond within 24 hours. In this way we now know that gamma ray bursts are cosmological and are associated with an event occurring in very distant galaxies. What the "event" is, however, is still hotly debated.
A golden age
After all the new large telescopes with diameters ranging from 6.5 meters to 10 meters are completed, what's next? Well, even more ambitious plans are already being discussed. One of these is the Overwhelmingly Large telescope project, which would have an effective mirror composed of 2,000 smaller ones each about as big as the Hubble Telescope mirror! For space, the big push is toward the Next Generation Space Telescope, an 8-meter telescope.
Other, very different ways of doing astronomy are now becoming a reality, too. For example, giant underground chambers have been constructed to detect neutrinos from supernova and other catastrophic cosmic events. LIGO, the Laser Interferometer Gravitational Observatory, is a fundamentally new observatory designed to search for gravitational waves, ripples in the fabric of space-time predicted by Einstein.
So what about the old observatories with their aging stacks of photographic plates? Actually, many are in the forefront of change. Most converted to CCD cameras as soon as they became practical, and many of the special series of photographic plates, such as the Palomar Observatory Sky Survey, have long since been digitized. One widely available collection of digitized photographic images is the Hubble Telescope Guide Star Catalog. The digitized sky survey is available to all on the World Wide Web.
Thanks to the "silicon age" of microchips and computers, this is a golden age for discovery about the universe.n
Ian S. McLean is professor of physics and astronomy at the University of California, Los Angeles (UCLA), and director of the department's Infrared Imaging Detector Laboratory. He is currently president of International Astronomical Union Commission 9 on astronomical instrumentation and is the author of Electronic Imaging in Astronomy: Detectors and Instrumentation.