celt_telescopes_000913 For
astronomers theres never a moments doubt that bigger is always better when it comes to telescopes.Its a matter of simple physics. The larger a
telescope lens or mirror the more light it can collect -- like putting a wide bucket outdoors to collect more rain that a narrow cup. Greater light-gathering capacity means seeing farther and fainter objects with a sharper view. The 20th century saw an explosive growth in telescope size. Telescope mirrors doubled in diameter every 35 years.
| The history of megascopes |
| Galileo and his contemporaries were amazed with the power of his firsttelescope. Since then, astronomers have been competing continuously to outdo one another. Check out our list of the greatest scopes through time |
Burning questions
As telescopes have grown, so have the burning questions beckoning astronomers. They want to look out to the edge of the
universe -- to see the first stars that were ever born, peer at the accretion disk around a monster black hole, look directly at extrasolar planets or probe the large-scale structure of the universe.We are now entering the era of "mega-telescopes," gargantuan but delicately computer-controlled architectures of steel and glass. The largest being envisioned would be the same width and half the height of Paris Eiffel Tower.
They will have 100 times the light-collecting power of todays 315- to 395-inch- (8- to 10-meter-) diameter mirrored giants. They will reveal features as much as 100 times sharper and 100 times fainter than the best of todays ground-based optical telescopes. This will invariably lead to profound new astronomical discoveries no one can begin to imagine.
A blue-ribbon panel of more than 100 U.S. astronomers recently ranked the need for a ground-based mega-telescope second in priority only to an orbiting replacement for NASAs
Hubble Space Telescope.The University of California and the California Institute of Technology (Caltech) recently teamed up to announce plans to design and build a 1,181-inch (30-meter) telescope, dubbed the California Extremely Large Telescope (CELT).
Big visions and the mega-mother
Other major observatories including the National Optical Astronomy Observatories in Tucson Arizona, the University of Texas and Lund observatory in Sweden are also envisioning 1,181- to 1,968-inch (30- to 50-meter) telescopes. They too carry the prerequisite superlatives of "Extremely Large Telescope," "Maximum Aperture Telescope" and "Giant Segmented Mirror Telescope".
The "mega-mother" of them all is the European Southern Observatorys lofty plan for a football-field sized 100-meter telescope. Called the OWL, for "Overwhelmingly Large" telescope is expected to take 17 years to build.
Though the sticker prices range from $250 million to $1 billion on these behemoths, astronomers point out that the performance gain is proportionally much greater, offering a lot more bang for the buck when it comes to raw power for harvesting light from the far corners of space and time.
Mirror costs are also coming down. "In the last 40 years optical mirror fabrication costs per square meter of aperture have dropped by a factor of 30," says Marc Postman of the Space Telescope Science Institute in Baltimore Maryland.
Similar mirror design
The CELT, and other mega-telescopes will use the same fundamental segmented mirror design that has proven so successful in the Keck Observatory, the biggest pair of telescopes presently operating, located in Mauna Kea Hawaii. At first glance,
mega-telescopes are simply a greatly scaled-up version of the Kecks.But to be affordable and workable, mega-telescopes will also require technological advances in lightweight mass-mirror fabrication and quality control, optics and computer-control systems, structural support and telescope enclosures, computer processing speed and instrument systems that match the optical quality.
The initial 10-meter Keck telescope, built in the 1980s, was the first major observatory to get away from trying to polish a huge single piece of glass for a mirror. Instead, 36 hexagonal mirrors are tiled together and arranged in a honeycomb "flys-eye" pattern.
Mirror honeycomb
By comparison, CELT will have a whopping 1,080 hexagonal mirror segments. CELTs mirrors cannot be cranked out like floor tiles. Each segment is a piece of the curve of the total mirror and so must be polished into its own unique non-symmetrical shape. Something like a giant tabletop jigsaw puzzle, this optical architecture was pioneered by Keck's designers.
Other mega-telescope schemes, such as the 2,058 segments required for OWL, must use simpler mass produced mirror "tiles," but pay the price with a less-than-perfect shape and correspondingly poorer optical quality. This must be corrected by additional mirrors downstream from the monster primary mirror.
A daunting challenge is to keep all the mirror segments aligned so that the mega-telescope yields the same exquisite image it would if it were a single huge piece of polished glass. For CELT, this calls for a staggering 3,000 actuators (tiny pistons that nudge each mirror segment to the right shape and alignment.)
Adaptive optics
As with all major ground-based telescopes -- both present and future -- CELT will need an adaptive optics system to correct for the blurring effects of the atmosphere. Adaptive optics takes the "twinkle" out of starlight by making extremely rapid and precise measurements of atmospheric distortion. A "deformable" mirror (separate from the primary mirror) then rapidly changes shape to correct the distortion of the image.
CELTs deformable mirror will need about 5,000 actuators, as opposed to 250 for Keck. Altering the positions and shapes of hundreds of corrector elements every hundredth of a second or less will require advances in raw computing power.
While there are practical physical limits as to how large any single
telescope can be scaled up from todays designs, it seems astronomers will run out of superlatives long before losing enthusiasm for building ever-bigger eyes on the sky.
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