Many eyes: Each component of the orthogonal transfer CCD array consists of a five-centimeter device made up of 64 CCD chips. The large eight-by-eight array only contains 60 devices because the corner elements would be too far from the center of the focal plane to collect useful data.
Such a design will likely be the way of the future for very large focal-plane cameras, says Donald Figer, an astronomer and the director of the Rochester Imaging Detector Laboratory (RIDL), in New York.
Tiling the camera’s focal plane into numerous CCDs and using the orthogonal transfer technology allows it to avoid a problem that often affects larger CCD chips, Figer says. This issue, called blooming, occurs because of contrasts in the intensities of light coming from a field of stars. A very bright star can create a large electrical charge in a particular row and column of a CCD chip, because its intensity overwhelms the part of the sky imaged on the chip. CCDs deliver their data along the rows and columns of the semiconductor circuits, so a strong light signal can overwhelm the other pixels in the same row and column. But by using many chips, the effect can be localized, and by moving the image using orthogonal transfer, the peak intensity can be corrected.
“The orthogonal transfer capability allows it to shuffle charge along the segments,” Figer says. “It allows you to effectively get a clearer image. Other cameras do something like that, but they do it by deforming the mirror.”
Pan-STARRS’s approach is different from that used in large telescopes in other observatories, such as the Keck Observatory’s two 10-meter telescopes on Mauna Kea, in Hawaii. Large telescopes typically use adaptive optics to correct for atmospheric turbulence by taking advantage of a bright object, known as a natural guide star, near the target. By adjusting the telescope’s image to correct for aberrations detected in the guide-star image, a much clearer picture–corrected for atmospheric turbulence–results. However, in 99 percent of viewing cases, a natural guide star is not available, so Keck 1 and Keck 2 use a laser guide star, which is created by sending a sodium-wavelength laser beam into the upper atmosphere to excite a thin layer of sodium atoms there. This creates a reference point near the target of observation, similar to a natural guide star.
A ground-based telescope equipped with adaptive optics can produce images with a resolution comparable to that of the Hubble telescope. However, the approach is too expensive for smaller telescopes, such as the 1.8-meter Pan-STARRS scopes. At lower cost, however, the image correction performed by the OTCCDs results in a picture of similar, if not quite as good, quality.