THIS MONTH, a star will come out of retirement. When it was first built 78 years ago, the Mount Wilson Observatory鈥檚 Hooker Telescope was the biggest and best in the world, and it was there that Edwin Hubble revolutionised astronomy with his discovery that the Universe is expanding. But it was gradually overtaken by a new generation of large telescopes, and finally retired nine years ago, overwhelmed by light pollution from the metropolis of Los Angeles, which sprawls at the foot of the mountain.
Now, a combination of new technology and enthusiastic astronomers is bringing the telescope a new lease of life. The Hooker Telescope鈥檚 palatial dome, 2000 metres above Los Angeles, still smells of old concrete and cold air. The original brass fixtures, ancient light bulbs, and giant, hand-carved gears give the observatory a picturesque, gothic quality. The rickety chair used by Hubble still sits at the telescope鈥檚 old eyepiece, as if awaiting his ghostly return. But juxtaposed with the historic is the distinctly modern: superfast computers, solid-state electronics and shiny new motors to control and monitor the renovated machine.
The telescope鈥檚 most significant new aspect by far lies on an unremarkable looking optical table mounted on the telescope鈥檚 side. This state-of-the-art technology is known as adaptive optics, and it removes one of the most irksome obstacles in astronomy 鈥� the blurring of images caused by the Earth鈥檚 atmosphere.
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Nearly a century ago, Mount Wilson was famous among astronomers for its superior atmosphere. The same feature of Los Angeles鈥� weather that tends to keep pollution trapped in the city below 鈥� the rising temperature with altitude, or inversion layer 鈥� also limits the turbulence at higher altitudes, keeping the air around the summit of Mount Wilson calm. Telescopes sited there had already made amazing observations of nebulae, star clusters, and sunspots.
Then, in 1906, hardware magnate John D. Hooker from Los Angeles presented the observatory with $45 000 to build a mirror with a diameter of 100 inches for what was to become the biggest telescope in the world. 鈥淭here is reason to hope that a 100-inch would add 100 000 000 still fainter stars, many of them lying beyond the boundary of the Universe as at present known,鈥� wrote George Ellery Hale, the solar astronomer and founder of Mount Wilson, in 1915.
The telescope took more than ten years to build. The 100-inch mirror, which weighs 4.5 tonnes, was made out of wine bottle glass by the Saint Gobain Plate Glass Company in Paris 鈥� the only glass-maker that would agree to undertake such an enormous task. The Fore River Shipyard in Quincy, Massachusetts, which built some of the world鈥檚 largest battleships, was commissioned to build the telescope鈥檚 huge mounting. When the parts were ready, they were transported to the mountaintop on trucks and mules along a metre-wide track that had to be widened twice to make room for the enormous telescope parts.
On 1 November 1917, an expectant group, which included Hale, Walter Adams, the director of Mount Wilson, and the English poet Alfred Noyes gathered for the telescope鈥檚 first test. They were expecting a pristine image of the planet Jupiter, but were horrified to see six or seven overlapping, fuzzy orbs. The surface of the mirror had become distorted when workers left the dome open in the midday sun. When Hale and Adams retested the telescope at 3 am, focusing on a star this time because Jupiter had long since left the night sky, the mirror had cooled, and the star appeared bright and clear. For the next thirty years, the 100-inch Hooker Telescope reigned supreme.
Hubble was the most famous astronomer to use the telescope. Before it was built, astronomers believed that the entire Universe was contained within our own Galaxy. But with the Hooker Telescope, Hubble saw that what were previously thought to be faint, fuzzy blobs in our Galaxy were actually galaxies themselves. And then, in the astronomical discovery of the century, Hubble realised that the galaxies were all speeding away from Earth at millions of kilometres an hour: in other words, the Universe was expanding.
Allan Sandage, who played a key role in devising methods for determining the ages of stars, was another prolific user of the telescope. He鈥檚 an astronomer at the Carnegie Observatories in Pasadena, the organisation that owned the telescope before its premature retirement. Sandage went to Mount Wilson in 1948 as a postgraduate, and spent thirty years there, first as Hubble鈥檚 observing assistant, then as an astronomer in his own right. 鈥淚t was a remarkable time when the ideas of stellar evolution were just nascent,鈥� he recalls. 鈥淭he electricity was indescribable.鈥�
But gradually another kind of electricity took over: the glow of Los Angeles began to taint the observatory鈥檚 night skies. And although objects in the Galaxy were bright enough to outshine the city lights, new crops of young astronomers at the Carnegie Institution in Washington DC were no longer interested in the telescope. They wanted to study cosmology, the history of the Universe 鈥� which meant looking at objects billions of light years away. Already strapped for cash, the Institution decided to shut down the Hooker Telescope in 1985 and turn to cosmology research with its other telescope in Chile, where the skies are darker.
Single-minded mission
But astronomer Arthur Vaughan, who spent nearly twenty years at Mount Wilson, refused to watch the site slip into obscurity. He founded the Mount Wilson Institute in 1986, and persuaded the Carnegie Institution to turn over the operation of the observatory鈥檚 telescopes to it. In 1991, Vaughan, who was by then devoting his time to working out what was wrong with the Hubble Space Telescope, handed his duties over to Robert Jastrow, a member of the Mount Wilson Institute鈥檚 board. Jastrow, who had directed NASA鈥檚 Goddard Institute for Space Studies for nearly twenty years, had a single-minded mission: to rescue the Hooker Telescope, refurbish it with modern controls, and return it to the astronomical world.
At the same time, the US military declassified some of its research on a new way to remove atmospheric distortion from images, which they had hoped would help them to identify Soviet satellites from the ground. Atmospheric distortion, the chaotic fluctuations in air density that cause stars to twinkle charmingly, is devastating to astronomy. Light from stars and galaxies travels unimpeded billions upon billions of miles through space. But when the light encounters the Earth鈥檚 turbulent atmosphere, what was an image of a double star system or a dense cluster of stars might arrive at the telescope as a featureless blob.
Adaptive optics is an ingenious way of measuring the distortion, and corrects for it thousands of times a second (see box, 鈥淗ow stars lose their twinkle鈥�). With it, telescopes can achieve the clarity only possible previously with the more expensive option of sending telescopes up above the atmosphere into space. Adaptive optics systems typically cost a few million dollars, compared with $1 billion for the Hubble Space Telescope.
There is a problem, though. Every image taken using adaptive optics needs a bright 鈥済uide star鈥�, which might be a star close to the observed object or light from the object itself, to provide the basis for the correction. The system works out how much the light from the guide star has been distorted and feeds appropriate corrections back into a deformable mirror, whose shape can then be changed to cancel out the distortions in the image of interest. But not many stars are bright enough or near enough to objects of interest to be used as guide stars, so only a small fraction of the sky can be observed. Even so, there are still between 10 million and 12 million potential guide stars that the Hooker Telescope can use.
In 1992, Jastrow hired Chris Shelton, an expert on adaptive optics at the aerospace corporation TRW, to design and build an adaptive optics system for the Hooker Telescope, and the US Air Force donated a deformable mirror worth $450 000. The final system cost less than a million dollars, a sum that was met by Californian philanthropic organisations, including the Ahmanson and Parsons foundations.
Sallie Baliunas, the deputy director at Mount Wilson, has already received dozens of replies to her announcement last August that the telescope would soon be back in business. Because the telescope is privately funded, the Institute can choose how to allocate time. That and the cheap nightly fee of around $700 makes possible projects that require large amounts of time, something astronomers couldn鈥檛 dream of getting at the other large observatories around the world.
Hal McAlister, director of the Center for High-Angular Resolution Astronomy at Georgia State University and a longstanding user of the old telescope, is a good example. Astronomers are increasingly aware that most stars in the Universe appear to be organised in pairs, known as binary star systems, or in multiple groupings. With a grant from the National Science Foundation, McAlister will combine the power of the adaptive optics with an instrument called a 鈥渟peckle鈥� camera that snaps hundreds of short exposures. By comparing the exposures with each other, a computer can calculate which portion of the image remains constant and which is the result of distortion. Together, these instruments should be able to resolve very faint binary systems that are currently impossible to see.
Binaries are not only important because of their ubiquity: from the motions of the two stars orbiting each other, astronomers can determine their masses. 鈥淚t鈥檚 important because all of our theories of evolution stars show that stellar mass is the single most important parameter,鈥� McAlister says. Astronomers are particularly keen to pin down stellar evolution theories in the light of the Hubble Space Telescope鈥檚 recent discovery that the Universe appears to be several billion years younger than some of its stars (New 杏吧原创, Science, 29 October 1994).
Michael Shara, who is head of the Science Program Selection Office at the Space Telescope Science Institute in Baltimore, Maryland, wants to use the Hooker Telescope to study 鈥渃ataclysmic binaries鈥�, star systems consisting of a white dwarf and a star similar to the Sun. The white dwarf, a collapsed star with an intense gravitational field, sucks hydrogen-rich material from its companion. The material collects on the white dwarf like stellar snow until it is about a kilometre thick. Then this layer explodes, producing a brilliant fireball known as a nova. Unlike a supernova, in which an entire star detonates, only the surface material blows up, and the white dwarf resumes its parasitic behaviour afterwards.
Novae occur over periods of days, and if Shara can arrange simultaneous telescope time in Australia, South Africa, Chile and California, he can observe them constantly, as they flare up, cool down and settle back into relative quietude. The adaptive optics will make Shara鈥檚 job easier because at least two of his cataclysmic binaries are located inside a globular cluster 鈥� a dense group of bright stars. 鈥淲e need good seeing to separate the stars I鈥檓 interested in,鈥� Shara says.
Funding permitting, Jordi Gutierrez, an astronomer at the University of Barcelona, says that the Hooker Telescope could help in his theoretical studies of supernovae. Some scientists believe that supernovae come from young, massive stars, which exist in the arms of spiral galaxies along with patches of ionised hydrogen. With its increased resolving power, the telescope might be able to spot these hydrogen patches in galaxies where supernovae occurred, thereby establishing whether this model is correct.
Many astronomers are happy just to have another large telescope to relieve the pressure on the relatively short list of instruments. People like Don Kurtz, from the University of Cape Town in South Africa, have problems getting the time they need on big telescopes because allocation committees give priority to astronomers who want to use the telescopes鈥� superior light-gathering power to look at very faint objects. He and his colleagues study 鈥渞apidly oscillating peculiar A stars鈥�, a type of star that pulsates with periods of about 6 to 15 minutes. This jiggling can be seen as tiny fluctuations in brightness, and can provide clues to a star鈥檚 mass, age and atmospheric structure. But the atmospheric distortion that makes stars twinkle can mask the oscillations, which are only between 0.1 and 0.2 per cent of the star鈥檚 brightness. Larger telescopes, which average light over a larger area, reduce twinkling, so the Hooker Telescope could solve Kurtz鈥檚 problems. After concentrating on the southern hemisphere, he would like to find some of these oscillating stars in the northern sky.
Perhaps one of the most intriguing questions in astronomy is whether other planets exist around stars like the Sun. Such a discovery would not only tell astronomers a great deal about the structure and formation of solar systems, but could indicate a possible site for extraterrestrial life. Baliunas and her colleagues are hoping to use the Hooker in this long-term search.
Outshone by their parent star, planets are nearly impossible to spot directly. Instead, astronomers look for tell-tale wobbles in a star鈥檚 orbit, which could be caused by the gravitational tug of planets. These wobbles can be detected as a minute shifting of the lines in the visible spectrum of the star as it lists back and forth. But picking this up requires very concentrated light. 鈥淚n the old days, even though the seeing was good, the star would be a large blob, so you got only a little slice of light,鈥� Baliunas says. Adaptive optics will bring a punch of concentrated light to the very narrow slits of the Hooker鈥檚 high resolution spectrograph. The increased sensitivity may make it possible to spot larger planets, those of the size of Jupiter for instance, around the stars in the Milky Way.
Within the next couple of years, the Hooker Telescope may get a new adaptive optics system that will eliminate the need for a guide star. Laird Thompson of the University of Illinois, Urbana-Champaign, is currently designing a system that creates an artificial guide star by shooting a laser into the sky, a technology pioneered by Robert Fugate, director of New Mexico鈥檚 Starfire Optical Range run by the USAF Phillips Laboratory.
In the meantime, with 90 nights already booked for observing, the creators of the new Hooker Telescope can bask in the glow of their new star. Mount Wilson鈥檚 exceptional position, the paucity of large telescopes, and the great amount of work still left to be done in observing the Galaxy should ensure that the telescope has a long and fruitful second life.
How stars lose their twinkle
SINCE the first telescopes were built, astronomers have been hindered by the smearing and fuzzing effects of the Earth鈥檚 atmosphere in their attempts to view the heavens. The distortion is so profound that NASA spent $1 billion to send the Hubble Space Telescope into orbit.
Adaptive optics offers a far cheaper solution to much of the problem. The heart of adaptive optics is a deformable mirror that assumes the opposite shape of the image distortion, effectively cancelling it out and producing a clear, crisp picture.
The idea was first proposed in 1953 by Horace Babcock, a former director of the Mount Wilson and Palomar Observatories. He said that distortions could be cancelled out by using an electron beam to change the thickness of a liquid film on a mirror. But the technology of the 1950s wasn鈥檛 up to the idea, and it was never pursued.
In the 1970s, the US military resurrected adaptive optics as a possible tool for spotting Soviet satellites in space. Meanwhile, in the 1980s, astronomers in France and at the European Southern Observatory (ESO) in Germany, began to develop their own adaptive optics system. Led by Fritz Merkle, a former astronomer with the ESO, they built an infrared adaptive optics system known as COME-ON (CGE Observatoire de Haute-Provence ESO ONERA), which was first tested at the 1.52-metre telescope of the Observatoire de Haute-Provence in the late 1980s and was then installed at the 3.6-metre telescope at La Silla, Chile.
When the US Air Force declassified their research at the end of the Cold War, military and civilian astronomers began to collaborate and adaptive optics research blossomed.
In a typical adaptive optics system, a telescope collects an image from a 鈥済uide star鈥�, which might be a star close to the observed object or light from the object itself, to act as a reference point. Light from the guide star falls on an array of small lenses. These lenses divide the distorted light into a set of spots, and focus each spot onto a detector. If the spherical wavefronts emanating from the guide star were undistorted, the lenses would produce an array of spots that fell nearly in the centres of the detectors; because the wavefronts are distorted, the spots deviate from those centres. Measurements of the deviations are then fed into a processor, which translates the information into voltages, and sends them to the deformable mirror.
The mirror is deformed from the back by an array of piezoelectric crystal cylinders called actuators, which expand and contract in response to the voltage. The movement necessary to correct the distortion is usually only a few micrometres. Because the atmosphere can change thousands of times a second, adjustments need to be made very quickly.
The number of actuators on the few systems already in use varies greatly, from less than 20 to more than 200. The Hooker system has 241 actuators for observing in visible wavelengths. Adaptive optics systems for infrared observations need far fewer actuators because infrared鈥檚 longer wavelengths suffer less distortion. Most systems also use a 鈥渢ip tilt鈥� mirror to correct for any overall 鈥渢ilting鈥� of the image, caused by the atmosphere or telescope vibrations.
As the technology is in its infancy, there are only a few working systems. The Starfire Optical Range in New Mexico, run by the US Air Force鈥檚 Phillips Laboratory, where much of the adaptive optics research took place, has a system on its 1.5-metre telescope, and is now building a system on its 3.5-metre telescope with 500 actuators. An upgraded version of ESO鈥檚 COME-ON system, called COME-ON Plus, has 52 actuators. Mount Wilson鈥檚 60-inch telescope, one of four other telescopes at the observatory besides the Hooker, has been using a 69-actuator system donated by the military, called ACE (Atmospheric Compensation Experiment), for over a year. Astronomers there are also using an experimental 12-activator system on the 2.2-metre telescope at the Beijing Observatory in China. More than 14 new systems are either being designed or constructed, including systems for the 10-metre Keck Telescope in Hawaii, and the 200-inch Hale Telescope on Palomar Mountain in California.
Many of the systems now being built are for telescopes that use infrared wavelengths, including the 3.8-metre UKIRT (UK Infrared Telescope) on Mauna Kea. But a major stumbling block with adaptive optics is that only a fraction of stars are bright enough to be used as guide stars. This seriously limits the number of objects that can be studied. The guide star problem can be dealt with by using a laser beam as an artificial 鈥渟tar鈥�, a technology first proposed by French astronomers Renaud Foy and Antoine Labeyrie in 1985 and later developed at the Starfire Optical Range.
The laser can be focused anywhere in the sky, and its light, backscattered from the atmosphere, is collected by the adaptive optics and used to correct distortions. A laser guide star system is being constructed at Mount Wilson under the guidance of Laird Thompson, of the University of Illinois in Urbana-Champaign. Laser guide star adaptive optics systems are also planned for many other telescopes, including the Hale and Keck telescopes and the 3-metre telescope at the Lick Observatory in California.