The $87-million Keck Telescope being built in Hawaii (upper left) has been described as “beyond the state of the art.” Now experts involved in the project concede that building the world’s largest telescope has proved far more difficult than expected. : Dimmer Future for Keck
There is growing concern among numerous experts that the Keck Telescope, destined to become the world’s largest when it goes into operation in a couple of years, will not perform as well as had been hoped.
The 10-meter instrument will be the most complex telescope on the planet, requiring revolutionary and unproven construction techniques and 36 separate mirrors that must be aligned 120 times a minute to function as a single mirror.
The $87-million telescope is being built as a joint venture by two of the nation’s foremost institutions of higher learning, Caltech and the University of California, and some of the best minds in the field are involved in the program. But several experts intimately familiar with the Keck say the project has proved far more difficult than had been expected.
“We’re trying to build something that is beyond the state of the art,” said one source, who asked that his name not be used.
Even the project manager, Gerald M. Smith, conceded that all has not gone as well as hoped during a recent tour of the facility here where the mirrors are being ground.
Even the project manager, Gerald M. Smith, conceded during a recent tour of the facility here at which the mirrors are being ground, that all has not gone as well as had been hoped. “What we are doing is hard, and it may not work as well as we would want,” he said.
But even if it performs below expectations, the Keck telescope should play a major role in astronomy in the years ahead because it is designed to be four times as powerful as the largest U.S. telescope in operation today--the 200-inch (5-meter) Hale telescope at Palomar Observatory in northern San Diego County.
Despite the rash of problems that have plagued the project, the Keck should still rank as the world’s most powerful telescope when it is completed, although other instruments now in advanced planning stages could eclipse it by the end of the century. The Keck, if it works even close to the standards set by its designers, should reveal much about the dimmest objects in the universe, the distant galaxies and quasars on the fringes of the cosmos that are almost as old as time itself.
“I’m sure its going to be an exceedingly good telescope and a major advance in astronomy,” Smith insisted.
The most critical problems lie in the fundamental design of the telescope, which will use 36 hexagonal mirrors that will be computer-controlled to act as a single mirror 10 meters in diameter. It could still function even with three-fourths of those mirrors missing, but at a considerable sacrifice in power.
The design grew out of a desire to build a telescope that would be much larger than scientists thought would be possible to build just a few years ago. It was generally believed that mirrors larger than the 5-meter giant at Palomar would be too costly to grind, too difficult to support in a movable structure that could follow celestial targets, and subject to deformation from the force of gravity.
A few years ago, UC Berkeley astronomer Jerry Nelson suggested that it might be possible to get around that limitation by building a telescope that would use many smaller mirrors that would be lighter and cheaper to grind, and link them together through an “active control system” that would keep the mirrors perfectly aligned.
The idea was immediately controversial because many astronomers thought that such a telescope would be far too difficult to operate and maintain, and the technology available at the time was not adequate to meet the exacting requirements for each segment of the giant mirror.
“It was obvious that what was being proposed was just not feasible,” said one source who was involved at the time.
But in time, other experts began to warm to the idea, believing that it might make it possible to build the kind of instrument they had long dreamed of.
The University of California initially proposed building the telescope, but the state-run institution could not come up with adequate financing. Then, Caltech entered the arena with a $70-million grant from the W. M. Keck Foundation.
The two universities joined forces, and the dome that will someday house the Keck telescope has been built atop Hawaii’s Mauna Kea volcano, a mountain so tall (14,000 feet) that oxygen will be pumped into the rooms where the scientists will work.
Here, outside Boston, in the closely guarded workrooms of one of the world’s leading optical firms, Itek Optical Systems--which is more accustomed to grinding mirrors for spy satellites than research telescopes--workers are struggling to overcome a series of problems. No one is following their progress more closely than Nelson, the man who proposed the multimirror concept.
“There’s a lot of money riding on this project and a lot of people are anxious to get it up and working,” said Nelson, now the project scientist. “I’m one of them.”
Engineers claim that they have made substantial progress in recent weeks, making it possible for Nelson to breathe a little easier.
“We have had unusual problems, so we have come up with innovative solutions,” he said.
But if the Keck telescope is to perform anywhere near the desired level, workers here must continue to break new ground as they polish 42 mirrors, including six spares, to near perfection, using a technique that pushes technology to new limits.
Normally, grinding a telescope mirror is a laborious, exacting and time-consuming, but routine, process. A telescope mirror has a symmetrical, concave surface that reflects light from all areas to a single point. It resembles a shallow dish, nearly flat close to the center and sloping more steeply toward the edge.
Such a mirror can be ground routinely by systematically removing less and less glass as the grinding tool gets closer to the edge.
However, that process will not work for a mirror that is composed of 36 segments, because the segments will no longer be symmetrical, just as the surface of the pieces of a broken dish would vary according to their distance from the center. But if the pieces could be glued back together, the overall effect of the dish would once again be symmetrical, even though the individual pieces would not.
The concept behind the Keck mirror is to grind 36 asymmetrical mirrors that, joined together, will form the symmetry required for a telescope mirror.
Traditional grinding procedures cannot be used for asymmetrical mirrors, just as a round bit cannot be used to drill square holes.
So scientists at UC Berkeley decided to fine-tune an innovative process, called “stress polishing.” The concept had been tried unsuccessfully before, but backers of the Keck design were convinced that with state-of-the-art computers they could make it work.
The idea behind stress polishing is to place the mirror blank under stress by literally warping it through forces applied to handles bonded to the underside. The blank is then ground and polished in a slightly warped configuration.
The grinding produces a concave surface, and then “we let go” of the handles, Smith said. With the stress removed, the blank reverts to its normal shape, slightly distorting its new concave surface. If the stresses had been exactly right, and the individual piece of glass correctly understood, the mirror should achieve “the shape we want” when the stress is removed, Smith said.
“But many problems have been more difficult than we expected,” Smith said. The most critical problem is that the first mirrors did not assume quite the shape that is required and included some minor flaws. Other problems have forced abandonment of plans to fine tune the mirrors with a computerized polishing system.
The project is now about a year behind schedule, and engineers now believe that it will be necessary to add additional stresses to the mirrors after they are installed on Mauna Kea, rewarping each segment slightly to achieved the desired shape.
Smith said he expects that the first of the mirrors will be installed at the observatory by next spring. Nine mirrors will be enough to test the telescope, and if they work well it would be the equivalent of a 5-meter scope, making it one of the most important telescopes in the world.
If all goes well, by sometime in 1991, all 36 mirrors should be in place and the Keck telescope should become the most powerful on the globe.
But some experts believe that is when the problems could get really interesting.
As the giant telescope scans the heavens, an “active control” system will be required to align the segments twice each second because of wind, plus expansion and contraction of the telescope due to changing temperatures. Each mirror will be supported by a lattice structure called a “whiffletree,” each of which sits atop three “actuators,” which align each segment.
The 108 actuators must function precisely 120 times a minute, keeping the array aligned to within one-millionth of an inch, about the distance spanned by 10 atoms.
The Keck telescope’s “active control system” is far more sophisticated--and potentially more troublesome--than other telescopes.
“My expectation is that there will be lots of bumps on that road,” Nelson said. “But there will be success.” However, many astronomers doubt that the telescope will ever function quite as well as had been hoped.
One expert noted that the Keck, and especially its segmented design, pushes technology so far that the project is a major test for the scientists, engineers and institutions trying to build it.
“Nobody knows how to do this,” he said. “But these are good people. There was a time when nobody knew how to fly, either.”