Great gratings
Light is tricky stuff. We may see it as one color, but in fact most beams of light are composed of many individual wavelengths, or frequencies. When light is divided up into a spectrum -- a trick first done by Isaac Newton -- its individual frequencies can be analyzed. The different "lines" in a spectrum show the presence of specific elements. In other words, a line at, say 666 nanometers, might indicate the presence of whyfilium, an element so new it's not yet on the periodic table.

(A spectrograph can also detect the motion of stars and galaxies. A Doppler shift causes objects moving away from us to shift toward a longer wavelength -- to be red-shifted.)

It turns out to matter how you create the spectrum. The standard "diffraction gratings" used to separate light are awkward and fragile to handle, because they are imprinted onto the surface of glass. Worse, they reflect at most 60 percent of the light striking them, says Bershady, so almost half of the precious, long-traveled photons end up in Cincinnati or Kalamazoo rather than in the spectrograph where they belong.

In a newer technique that goes by the awesomely awful moniker "volume-phase holographic" (VPH) gratings, diffraction occurs inside a gelatin sheet encased between glass plates. Much like photographic film, the gelatin is sensitive to light; scientists use a laser or two to change its index of refraction, thus changing how the grating will diffract light.

By varying the laser treatments, it's possible to make gratings that diffract specific wavelengths, Bershady says. And because the grating is inside glass, it's easier to coat and clean than standard, surface gratings. Best of all, VPH diffracts 90 percent or more of incoming light. That may make things dark in Cincinnati, but it should light up an astronomer's face.

Normally, you'd expect to find a new technology like this in a new telescope, but as the upgrades to the Hubble Space Telescope showed, you can boost the accuracy of an existing scope by adding new instruments. The emerging VPH technology is starting to be used on telescopes built years ago. Bershady, for example, is preparing new gratings for the WIYN telescope on Kitt Peak in Arizona. "You can take an existing spectroscope and redesign it for a moderate cost to get improved performance," he says. By boosting the efficiency of this one component, "you can easily get a factor of two improvement in throughput."

To view light in the near-ultraviolet, the SALT telescope will, appropriately enough, use lenses made of salt. Sodium chloride transmits this form of electromagnetic radiation, while glass blocks it. But salt is tougher to handle: Glass doesn't mind getting wet... Courtesy Eric Burgh/SALT team

No tilt!
Before a telescope is even built, it runs up against a key constraint in gathering photons: cost. (Telescopes are expensive -- We've even heard of a proposal to build a 100-meter monster. Over 15 years of construction, it's expected to cost around $1 billion.) Even giant mirrors in the 10-meter range, used to gather faint light on an increasing number of 'scopes, are expensive. And their great weight entails massive expense for steel to hold the mirror steady as it tracks stars across the sky.

If the earth did not rotate, a stationary telescope would work just dandy. But that rotation, which causes the "motion" of the sun and other stars, requires telescopes to have elaborate -- and expensive -- star-tracking systems.

A few new telescopes, including SALT, which builds on the Hobby Eberly design, have a mirror that does tilt to track stars (the mirror will rotate around the horizon on air bearings. Think air-hockey).

Seven of the 91 segmented mirrors on the new SALT (South African Large Telescope) are installed in this photo. It's vastly cheaper to build small, hexagonal mirrors than traditional, one-piece monster mirrors. Speaking of monsters, this mirror will be 10 meters by 11 meters -- a record for an optical telescope in the Southern Hemisphere. Courtesy Matthew Bershady, SALT project.

Instead of tracking stars, this scope will gawk at whatever happens to be overhead. It will, however, be able to follow stars for an hour or so by moving the scientific instrument -- initially a big spectrograph -- to catch the moving reflections from the mirror.

From its location in the Southern Hemisphere, SALT will be able to gather data on skies that have eluded astronomers in the Northern Hemisphere.

And while it might be better to track stars in the conventional manner, the design does slash costs, says Bershady. The scope, due to be commissioned in fall, 2004, will cost 20 percent of what a conventional, tracking scope would cost, he says.

Do astronomers have other tricks up their sleeves?

There are 1 2 3 pages in this feature.
Bibliography | Credits | Feedback | Search

©2003, University of Wisconsin, Board of Regents.