Lincoln Laboratory supplies detectors for world's most advanced digital camera
In December of this year, when the Pan-STARRS (for Panoramic Survey Telescope and Rapid Response System) prototype telescope on Haleakala, Maui, goes operational, it will feature the world's largest and most advanced digital camera at the heart of which are silicon chips developed at Lincoln Laboratory. This telescope is one of four telescopes that will make up Pan-STARRS, a wide-field imaging facility being developed at the University of Hawaii's Institute for Astronomy (IfA) for use in sky surveys.
"This is a truly giant instrument," said John Tonry, the astronomer who led the team developing the new camera. "We get an image that is 38,000 by 38,000 pixels in size, or about 200 times larger than you get in a high-end consumer digital camera."
Pan-STARRS is distinguished from other sky survey facilities by its ability to map very large areas of sky to great sensitivity. According to Tonry, in a typical observation, the camera will be able to detect stars that are 10 million times fainter than those observed by the naked eye. Pan-STARRS photographs an extremely wide area of sky in each exposure; its field of view is 3 degrees in diameter, which is 6 times the diameter of the moon. Pan-STARRS is also unique in its ability to find moving or variable objects.
Lincoln Laboratory has been very instrumental in enabling Pan-STARRS capabilities. The Laboratory's charge-coupled device (CCD) technology is essential to the telescope's camera. In the mid-1990s, Lincoln Laboratory researchers Barry Burke and Dick Savoye of the Advanced Imaging Technology Group, in collaboration with Tonry, who was then working at MIT, developed the orthogonal-transfer charge-coupled device (OTCCD), a CCD that can shift its pixels in synchronism with random image motion. Many consumer digital cameras have mechanical means, involving motion of the lens or the chip mount, to provide camera-motion compensation and thus reduce blur, but the OTCCD performs this function electronically at the pixel level and at much higher speeds. The motivation for the OTCCD, however, was ground-based astronomy in which atmospheric effects as well as telescope shake cause image blur. During tests on small prototype OTCCDs at the MIT telescope on Kitt Peak in Arizona in 1996, the OTCCDs demonstrated significantly sharper image resolution and greater sensitivity than had been attained by other technologies.
For astronomy applications, much larger devices are necessary, but larger does not necessarily mean better for atmosphere-induced jitter compensation. As the field of view increases, the jitter in the stars begins to vary across the image, and the OTCCD with its single shift pattern for all the pixels begins to lose its effectiveness. For Pan-STARRS, with its wide field of view, large OTCCDs would have been no more effective than conventional CCDs. The answer to this problem, proposed by Tonry, was an array of small OTCCDs constructed on a single silicon chip. This architecture, called the orthogonal-transfer array (OTA), enabled independent shifts optimized for tracking the varied image motion across a wide scene. In 2004, Tonry and his group at the IfA began a collaboration with Lincoln Laboratory to develop the OTA as the answer to the challenge of simultaneously achieving high-speed, high-sensitivity, wide-field-of-view imaging.
"Not only was Lincoln the only place where the OTCCD had been demonstrated, but the added features that Pan-STARRS needed made the design much more complicated," said Burke, who has been working on the Pan-STARRS project. "It is fair to say that Lincoln was, and is, uniquely equipped in chip design, wafer processing, packaging, and testing to deliver such technology."
The primary mission of Pan-STARRS is to detect Earth-approaching asteroids and comets that could be dangerous to the planet. When the system becomes fully operational, the entire sky visible from Hawaii (about three-quarters of the total sky) will be photographed at least once a week, and all images will be entered into powerful computers at the Maui High Performance Computer Center. Scientists at the center will analyze the images for changes in the sky map that could signify a previously unknown asteroid. They will also combine data from several images to calculate the orbits of asteroids, looking for indications that an asteroid may be on a collision course with Earth.
Pan-STARRS will also be used to catalog 99% of stars in the northern hemisphere that have ever been observed by visible light, including stars from nearby galaxies. For astronomers, this three-dimensional catalog will be a resource for the study of small, distant, cool, and low-luminosity stars. In addition, the Pan-STARRS survey of the whole sky will present astronomers with the opportunity to discover, and monitor, rare explosive objects in the solar system.
Posted September 2008
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