The Dark Energy Survey

The Dark Energy Survey

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A prototype of the Dark Energy Survey camera, DECam (Credit: Fermilab)
Unlike the human eye, photographic film and digital cameras can stare at the sky for a long time and store more and more light. By replacing the human eye with cameras, astronomers can detect fainter and more distant objects.

Cameras used for optical astronomy are usually composed of an array of digital chips called charge-coupled devices (CCDs). CCDs convert light into electrons. Each chip is divided into millions of pixels. The electrons generated by the light that hits each pixel are converted to a digital value that a computer can store or display.

In concept, these are the same devices that make up the heart of any home digital camera. However, unlike home cameras that are used to record images of things that are very bright, astronomical CCDs must be souped up in order to detect the tiny amount of light that reaches us from faint and/or distant objects. Much of the light from extremely distant galaxies and supernovae has been redshifted into long-wavelength red and infrared light, which conventional CCDs do not detect very well.

The science requirements of the Dark Energy Survey drive the construction of a brand new camera. Named DECam, this large, 570 Megapixel camera will hold 74 CCDs constructed specifically to be sensitive to the redshifted light from distant galaxies and stars. DECam will have the widest field of view in the NOAO ground-based optical/infrared system of imagers. Its 2.2 degree field of view is so large that a single image will record data from an area of the sky 20 times the size of the moon as seen from earth. This wide field of view requires that DECam use a system of five lenses, each one uniquely shaped to correct a variety of optical aberrations, with the biggest of these lenses being almost 1 meter in diameter.