Sky Lights

Subaru Telescope/NAOJ

By Bob Berman|Saturday, July 01, 2000

On desolate mountaintops in Hawaii, Arizona, and Chile, a group of billion-dollar super-telescopes is about to expand the boundaries of what we can see in space. The language used to express these accomplishments may seem like hieroglyphics. How many people will understand the technical meanings of "magnitude" and "arc second"? But what really matters is that we'll be able to spot fainter stars and galaxies, and they'll appear sharper than ever before.

Despite the boasts of "500-power" used by manufacturers of cheap toy-store telescopes, the truest measure of an instrument's worth is the diameter of its main lens or mirror, not its magnifying power. The Hale Telescope on Mount Palomar, which was the world's largest for half a century, has a mirror 200 inches, or 5 meters, across. The just-completed Japanese Subaru Telescope is 8.2 meters across, the twin Kecks are 10 meters each, the two Gemini 'scopes are 8.1 meters, the upcoming Large Binocular Telescope sports dual 8.4-meter mirrors, and the European Very Large Telescope will be an amazing 16.4 meters when completed in 2003. The amount of light a telescope collects depends on the surface area of its main lens or mirror. Each Keck mirror, twice as wide as the one on Mount Palomar, can detect objects four times as faint.

Astronomers quantify the brightness of celestial objects with a confusing term: magnitude. Higher numbers indicate fainter things; each magnitude is about 2.5 times dimmer than the one before, so an increase of five magnitudes denotes a hundredfold brightness drop. White Vega and orange Arcturus, the bright stars now high in the sky, are zero magnitude. The human eye can barely pick out sixth-magnitude stars, some 250 times fainter. Amateurs' telescopes collect enough light to make out stars of magnitude 14. Keck dips down to the 29th magnitude. Hubble's smallish mirror - 2.4 meters across - can detect 30th-magnitude objects because it orbits above Earth's blurry atmosphere, so it can concentrate starlight more precisely. Thirtieth-magnitude objects shed as much light as the glow of a cigarette in Hawaii as seen from Boston. The Very Large Telescope will do even better, despite being earthbound.

Bigger mirrors also produce sharper images. Here the standard scientific measure is the arc second, an angle 1/3,600 of a degree. The sun and moon appear just under 2,000 arc seconds across. A good backyard telescope reveals Jupiter's moons as little disks, a 1-arc-second resolution. Hubble can resolve features as small as 1/20 arc second. That is equivalent to standing in Brooklyn and distinguishing the two headlamps of a taxicab in Paris.Although the new telescopic giants must peer through layers of turbulent air, novel de-smudging techniques should allow them to surpass even Hubble in sharpness. Declassified adaptive-optics technology will allow them to compensate for atmospheric distortion using deformable mirrors attached to motorized pistons.

The Kecks and the Very Large Telescope will eventually combine light from several telescopes, improving their resolution to once-unimaginable levels. They may get down below 1/1,000 arc second, good enough to pick out those headlights on a taxi driving across the moon, 235,000 miles away - a feat as astonishing as what the cab's meter would be reading.

All of the major new telescopes are on the Web:
Hubble (, Keck (, Very Large Telescope (, Subaru (, Gemini (, and the Large Binocular Telescope (

Sky & Telescope's Web site has great tutorials on how to pick and use a telescope of your own ( and how to make sense of magnitude and other astronomical terms (
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