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Figure 1.10. A schematic of a CCD array.

price to pay for quantum efficiencies of 30-40% at blue wavelengths and 80-90% at 6,000-9,000 A.

The largest optical CCDs currently manufactured consist of arrays of 4,096 x 4,096 pixels, with individual pixels 12-15 ^m in size; 2,048-square optical devices with 12-25 ^m pixels are also in use on many telescopes, while the largest infrared arrays have 1,024 pixels to a side. None of the individual chips is much larger than ~ 5.5 x 5.5 cm, so a single-CCD camera cannot provide the same areal coverage as a single photographic plate. However, the solid angle surveyed can be increased by constructing arrays of many CCDs, as has been done at the Japanese Kiso Schmidt [S1], and in the main camera used in the Sloan Digital Sky Survey [G1]. The latter camera includes 30 2,048-square CCDs, which sparsely sample the sky in a 6 x 5 grid on the focal plane, scanning a strip of sky ~ 2.5 wide. With improvements of nearly a factor of ten in quantum efficiency over photographic plate material, these arrays can detect objects substantially fainter than the limiting magnitude of the large-scale photographic sky surveys.

The response of optical CCDs declines significantly beyond 9,000 A, but specialised infrared-array detectors can be used to cover the 1-5 ^m region of the spectrum. These devices are fabricated from semiconductors such as InSb and HgCdTd (mercadtelluride), and the technology is less advanced than is the case for the visual-wavelength detectors. However, increasing numbers of 1,024-square arrays are becoming available at observatories throughout the world. PbS and SiAs detectors are used at wavelengths beyond 5 ^m, where low atmospheric transmission and the high background due to thermal emission make observations extremely difficult. For highest efficiency, the detectors have to be cooled to substantially lower temperatures than optical devices, usually requiring liquid helium as a coolant.

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