Sky Survey Project Could Open Up The Universe `By Magnitudes'

PEOPLE who search the night sky - either professionally or for pleasure - could soon have at their fingertips a map of the heavens incredibly more detailed than anything available before.

Like much in astronomy, ``soon'' has to be taken in relative terms. The scheduled completion date for the Sloan Digital Sky Survey is 2001. But the project, undertaken by the Astrophysical Research Consortium, which includes Princeton University, is already well under way.

The initial steps have been planning, designing, and building, with the ``construction phase'' now begun. Three major items have to be built: a new 100-inch telescope for the consortium's Apache Point, N.M., observatory, the camera that will use an array of large silicon chips called CCDs (charge-coupled devices) rather than photographic plates, and the special double spectrograph that will record the light spectra coming from celestial objects and thus enable astronomers to estimate their distances.

When the project is complete, the astronomers will start gathering data about the sky at a pace and depth that will be ``magnitudes better'' than previous sky surveys, says Jeremiah Ostriker, head of Princeton's department of astrophysical sciences.

The information coming off the new telescope will be measured digitally in ``terabytes'' (a million million), explains Princeton astrophysicist Gillian Knapp. When all that is put in a form accessible to someone at a computer terminal, such as on a CD-ROM, researchers will be able to do with a ``click'' what before could have taken months, even years - such as find all the objects of comparable brightness to a star or galaxy under study.

``In astronomy,'' Mr. Ostriker says, ``surveys have produced a huge amount of what we know. If we can design something to see more of the universe, important things will be discovered. It happens every time.''

As an example, he points out what happened when radio astronomy was developed after World War II. ``A completely new universe'' opened up as the radiation, not just the visible light, coming from distant parts of the universe was detected and analyzed.

Ostriker says the Sloan survey - named after the Alfred P. Sloan Foundation, which has contributed $8 million of the $18 million capital cost of the survey - could have a similar impact.

The most revolutionary facet of the project's technology is the use of silicon chips in the camera. Thirty large chips built into the camera will make it possible to take five simultaneous pictures of a targeted object in five different colors, each of which will reveal different characteristics of the object. In a typical night's observation, lasting six hours, scientists will collect about 11,000 images, says Michael Richmond, another Princeton astronomer involved in the survey. @bodytextdrop =

The Sloan survey is not taking on the whole universe, just the quarter of the night sky visible from its New Mexico observatory.

The last such survey, done from California's Mt. Palomar observatory in the 1950s, has been the state of the art for 40 years. Though that survey used the same size telescope, it was limited to pictures in only two colors, and the photographic plates used then captured only a fraction of 1 percent of the light coming from stars and galaxies.

The electronic equipment used in the new survey will get 60 percent of that light, Ostriker says. That means objects many times fainter than those visible in the previous survey will show up in this one. And the survey's spectrography will allow a mega-jump in the numbers of galaxies and quasars (star-like objects that emit large quantities of light and radio waves) whose distances can be measured.

Ostriker explains that when the ``red shift'' phenomenon, which allows scientists to determine the distances of celestial objects by analyzing their spectra, was discovered by astronomer Edwin Hubble in the 1930s, it took many nights to get the spectrum of a single galaxy. The Sloan survey spectrograph will use fiber optics to get the spectra of 600 objects at a time. When the survey is completed, it will include the distances of up to 1 million galaxies and 100,000 quasars, as contrasted to the relative handful of such distances known now.

This kind of detailed and accurate data should throw fresh light on such basic problems of astronomy as the origin and structure of the universe, Ms. Knapp says.

The multidimensional map supplied by the survey won't simply be the province of professional astronomers. The project's sponsors - including the National Science Foundation - plan to disseminate the information to the general public through CD-ROMs available in libraries and schools.

In addition to the consortium members - Princeton University in Princeton, N.J., the University of Chicago, The Johns Hopkins University in Baltimore, the University of Washington in Seattle, and the Institute for Advanced Studies in Princeton, N.J. - other collaborators in the Sloan survey are the Fermi National Accelerator Laboratory and the US Naval Observatory.