Monday, November 24, 2008

Giant Camera Tracks Asteroids

The camera will offer sharper, broader views of the sky.

By Robert Lemos

Keeping watch: The prototype Pan-STARRS telescope, PS1, focused on Comet Holmes during trials in 2008. The detail is about one-half of what is expected when the telescope goes online in December. Credit: Institute for Astronomy, University of Hawaii

The first of four new asteroid-tracking telescopes will come online next month in Hawaii, promising to quickly scan large swaths of the sky--thanks to the world's largest digital camera.

The project, known as the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), aims to scan the entire sky visible from the summit of Mount Haleakala in Maui Island, Hawaii, three times a month, searching for asteroids and near-Earth objects (NEOs) as small as 300 meters in diameter. At the heart of each telescope is a 1.4-billion-pixel digital camera that can photograph broad swaths of the night sky in sharp detail.

The first prototype telescope using the camera will go online in December. This telescope will scan the night sky, searching for asteroids and comets that could pose a threat to Earth. Pan-STARRS is designed to have at least three times the collecting power of current NEO telescopes.

The Pan-STARRS's cameras, each consisting of a 40-centimeter-square array of charge-coupled devices (CCDs), bring new technology to the optics used in astronomy. Perhaps the most innovative aspect is the ability of each CCD cell to electronically shift an image to counteract atmospheric blur and deliver clearer astrophotography, says Barry Burke, a senior staff member at MIT's Lincoln Laboratory, which makes the cameras.

"The atmosphere is the limit to the quality of the image, but there is a special feature of these chips that allows them to remove some of the blur due to atmospheric effects," Burke says. "It allows the image to be shifted in any direction in the chip in a way that matches the motion of the stars and that takes out a significant part of the blur."

Known as orthogonal transfer CCD (OTCCD), the technology uses electronics to adjust the image rather than mechanically tilting a camera's lens or mirror, a more common technique used in consumer cameras that have optical image stabilization. Because the process is electronic, the technology can be distributed to each cell of the CCD array, allowing for much more granular adjustments to localized atmospheric turbulence. The result is an image that is sharper than what a ground-based observatory could produce.

The mosaic structure of the CCD camera also leads to a more reliable system and less expensive manufacturing costs, Burke says. "The chip could not possibly be made to that size, so we are forced to break the camera down into tiles," he says.

Each Pan-STARRS camera consists of an eight-by-eight array of devices, each containing an eight-by-eight array of CCD cells. The size of each cell--about six millimeters on a side--is determined by a sweet spot: if the chips where much larger, the number of defects on them--and thus the overall cost of making them--would be too great; if they were much smaller, it would become much more difficult to organize them into the camera's focal plane.