Giant Telescope to Map Vast Expanse of Sky

Jennifer L. Morey

Astronomers have been able -- even with the greatest of telescopes -- to see only a small portion of the sky at a time. Professor Jim Gunn, a Princeton University astronomer, hopes to overcome this obstacle by building a telescope that images the sky piece by piece and reconstructing those images into a vast map of the sky. He said the project will allow astronomers to gather quantitative data on the distribution of mass in the universe.

A front view of the aspheric surface shows the device's color filters. Reflection is from a fluorescent ceiling light.

The Sloan Digital Sky Survey -- the brainchild of Gunn and his University of Chicago colleagues Rich Kron and Don York -- is being constructed in the mountains of southern New Mexico at Apache Point Observatory. The telescope will spend an estimated five years imaging the sky piece by piece and assembling the data into a map with a distance sample 100 times larger than the sample for any survey conducted to this point. The map will also be much more quantitative and sensitive, recording much fainter and more distant objects than those found on previous maps.

The telescope will be equipped with a charge-coupled device (CCD) camera containing 2048 × 2048-pixel detectors made by Site of Beaverton, Ore., and will perform time delay and integration mode scanning. The camera will image 100 million celestial objects and then return for a closer look at 1 million galaxies, taking measurements to determine their distance from Earth. The camera will capture the images in stripes. For each stripe, six columns of detectors will take two passes to cover the camera's 2.5° field of view.

The survey has evolved into a project involving five universities, the US Naval Observatory, a national laboratory, a group of Japanese scientists and a full-time manager. Such a project couldn't have been started any earlier, Gunn said, because five years ago the necessary detectors weren't available.

Test run

Gunn said his team recently passed a milestone by engineering the device's first light run. Because it operated under a full moon with stray light, the data gathered represented little of scientific interest. However, Gunn said, the test proved that the telescope's engineering works. He admitted that the device needs a fair amount of tweaking but said the team hopes to begin gathering real data by this fall.