December will bring us the first in a set of four asteroid-tracking telescopes. With the help of the world’s largest digital camera this piece of modern technology will scan large portions of the sky in search of asteroids and other near-Earth objects.Each telescope is equipped with a 1.4 billion pixel digital camera capable of taking highly detailed photos of the night sky. The Panoramic Survey Telescope and Rapid Response System or Pan-STARRS project aims to scan the sky from high on top of Mount Haleakala in Maui Island, Hawaii. Three times a month they will be searching for asteroids and other NEOs ( near-Earth objects ) starting from sizes as small as 300 meters in diameter. Pan-STARRS is going to have at a minimum three times the collecting power of today’s NEO telescopes.
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Each of the cameras in the project consists of charge-coupled devices or CCDs arranged in a 40 centimeter square array.

The maker of the cameras, Barry Burke, who is also a senior staff member at MIT’s Lincoln Laboratory said that the most innovative aspect of the CCD is it’s ability to electronically shift an image to counteract atmospheric blur and deliver clearer astrophotography.

“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, 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.” he said.

The orthogonal transfer CCD or OTCCD is a technology that electronically adjusts an image instead of doing it mechanically by tilting a camera’s lens or mirror. The same technique is used in consumer cameras with image stabilization. Distributing the technology to each cell of the CCD array allows for a more granular adjustment to localized atmospheric turbulence, resulting in a sharper image.

The CCD array structure leads to a more reliable and cheaper system. Burke says “The chip could not possibly be made to that size, so we are forced to break the camera down into tiles,”. A camera is made of an eight by eight array of devices, each containing an eight by eight array of CCD cells. Every cell is about six millimeters on a side, a carefully determined size. If the chips were much larger the cost of making them would be too great because the number of defects on them would rise, and if they were much smaller it would be harder to organize them into the camera’s focal plane.

This kind of design that results in tiling the camera’s focal plane into many OTCCD cells will likely be the way of the future. Blooming is a problem that often affects CCD chips that can easily be avoided by using this kind of technology. Blooming occurs when the light from a very bright star creates a large electrical charge in a particular row and column of a CCD chip, overwhelming other pixels in the same row and column because of the data transmission occurring between the chips along the rows and columns of the device. By using a lot of chips the effect of the very bright light can be localized and by using orthogonal transfer technology the peak intensity can be corrected.

Classical large telescopes usually use adaptive optics to correct for atmospheric turbulence. Adaptive optics takes advantage of the presence of a bright object called a natural guide star near the telescopes target. Focusing on the guide star, the telescopes image is adjusted to compensate for aberrations detected while observing it, resulting in a much clearer picture corrected for atmospheric turbulences. However, in almost all of the viewing cases a natural guide star is not available, so telescopes such as Keck 1 and Keck 2 at the Keck Observatory on Mauna Kea, Hawaii, use a laser guide star. A laser guide star consists of a sodium-wavelength laser beam sent into the upper atmosphere to excite a thin layer of sodium atoms there, creating a reference point near the target of observation.

Even though adaptive optics can produce images similar to that of the Hubble telescope when quality is concerned, the approach is too expensive for smaller telescopes. The OTCDD technology results in a lower cost and a comparable or even quite as good image quality when it comes to the 1.8 meter Pan-STARRS scopes.



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