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  • Moving Sky Espied by Digital Eye

Photonics Spectra
Oct 1999
Daniel C. McCarthy

Scientists estimate that there are between 1000 and 2000 asteroids capable of colliding with Earth and causing a mass extinction similar to the one that brought down the curtain on the dinosaurs. Finding these killer rocks is the focus of the Near Earth Asteroid Tracking program run by NASA's Jet Propulsion Laboratory.

The asteroid Kytheria appears in the center of the upper left, middle center and lower right 25 x 25-pixel panels. Digital imaging integrated into the Oschin telescope helped identify and track the movement of the potentially Earth-threatening asteroid against a static background. Notice that fixed stars, if any, remain in the same relative position in each horizontal row. Courtesy of Jet Propulsion Laboratory.

Until recently, program efforts were limited to six nights a month at a single telescope in Hawaii. Now the lab intends to add two telescopes and scale up observation at all three instruments to 18 nights per month. One of the additional observatories, the 1.2-m Oschin telescope atop Palomar Mountain near San Diego, is being equipped with a high-end charge-coupled device (CCD) from Lockheed Martin Fairchild Systems to facilitate its mapping ability.

Since 1949, the Oschin telescope's wide field of view has surveyed the sky using photographic plates or film, said Steven Pravdo, the project manager for the lab's program. "But for [the telescope] to work in more modern applications, we needed to change the guts, which is not to say the optics or the light-collection components, but the electronics and its interface with the outside world. That included how images were captured. Photographic films were meant for the nonmoving sky, but detecting asteroids requires real-time imaging to allow astronomers to follow up the next night. We're now looking for the moving sky."

Another of film's disadvantages involved sensitivity. "It's a couple magnitudes less sensitive than digital imaging," said Pravdo. "That means we'd miss objects that appear six times dimmer."

He selected Lockheed's 4000 x 4000-pixel CCD largely for its comparatively low $10,000 to $18,000 price range. A high-end backside-illuminated chip used in similar telescopes captures images much faster, observed Pravdo, but those CCDs cost $500,000. "Some applications need the quick readout of that chip," he said. "Ours takes 20 seconds to read out. For finding asteroids, that's not as much of an issue."

More at issue is a forward-looking plan to expand Oschin's capabilities. Lockheed's chip covers only about 2.5 in. of the telescope's generous focal plane, which measures 14 in. on a side. Over the next two to three years, Pravdo plans to develop a next-generation camera integrating several more of the CCDs to utilize the entire focal plane. With the additional chips, the telescope could cover 10 times more of the night sky.

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