Star scanner is cataloging the universe

by Peter Kauffner

Cataloging the stars in the sky is not a job for the easily frustrated. Not only are there millions and millions of stars to be mapped, but the equipment breaks down, too.
“Basically you have to smile at (the Automated Plate Scanner) very nicely and do an incantation,” said Roberta Humphreys, a professor of astronomy and the principal investigator for the University’s Automated Plate Scanner Lab. “It works very well for months at a time, and then all of a sudden it goes broke.”
The scanner is used to create digital images from photographic plates taken by the Palomar Observatory in California. The lab has cataloged more than 100 million stars and galaxies during the past six years to create the largest star catalog in existence. Most of the data has already been made available on the World Wide Web.
The plates are made of glass that is 14 inches wide and one-sixteenth of an inch thick.
“You have to be careful when you handle them,” said Chris Cornuelle, an astronomy research associate. “I think in all these years of scanning plates, we have only broken one or two.”
Such plates are no longer manufactured, and astronomers have shifted to digital photography.
“Kodak is no longer making these plates, because one of the ingredients is a chemical from water buffalo hoofs, which for some reason is hard to find,” Cornuelle said.
The plates Humphreys’ team is working with are part of the Palomar Observatory Sky Survey, which was conducted in the 1950s. The survey consists of 1,000 sets of plates and covers two-thirds of the sky.
The Automated Plate Scanner was manufactured by Control Data Corporation for astronomy Professor Willem Luyten in 1970. The machine was modernized and refurbished in the 1980s.
There are only three other machines that can compete with the University’s scanner, Humphreys said. “One is located at the Royal Observatory in Edinburgh, (Scotland); another is located at the Paris Observatory, and another one is located at the Naval Observatory in Flagstaff, Arizona.”
The machine scans two plates of the same area of sky simultaneously, one taken with a red filter and the other with a blue filter. The catalog is therefore able to give the color of every object listed, a feature unique among major star catalogs.
“The photographic plate is basically transparent, with images of stars and galaxies on it,” Humphreys said. The scanner uses a laser beam, focused by a rapidly rotating eight-sided prism, to scan across the plates. Humphreys said it takes about five hours to scan the plates.
The project is funded by grants from the National Science Foundation and the National Aeronautics and Space Administration. It has received about $4 million since 1980.
Star catalogs help astronomers plan surveys and determine what is already known about the objects they are studying.
“If you have a radio telescope, for instance, and you have a strong radio source somewhere, you can go to a catalog and see if it corresponds to a known galaxy,” said Cornuelle.
The University’s catalog was also used to help determine the shape of the Milky Way galaxy and the sun’s position in it.
The project is currently producing a second catalog that measures proper motions, or the movement of deep-sky objects during a period of about 15 years.
This is done using a second sky survey begun by Luyten in the 1960s. “We do it by scanning our (Palomar) plates from the 1950s at the same time that we scan one of his plates from about 1970.”
The new catalog will be completed in a few months, Humphreys said.