redshifted light from these distant objects. To accomplish this task, DES has designed a camera with five major systems:
The DECam focal plane will consist of a science array of sixty-two 2048 X 4096 CCDs. Additionally there are four 2048 X 2048 guider CCDs and eight 2048 X 2048 focus and alignment CCDs. The quantum efficiency of these LBNL-designed CCDs with their anti-reflective coating is red optimized to be > 90% at 900nm and over 60% over the range of 400-1000nm. The DES CCDs are fabricated by Dalsa with further processing done by LBNL. They are packaged and tested by Fermilab.
DECam will operate at -100oC in order to minimize noise and dark current with the cooling provided by liquid nitrogen. To prevent condensation on the surface of the CCDs, DECam will operate at that the extremely low vacuum pressure of
The Low Noise Readout System
The electronics will allow an entire digital image to be read out and recorded in 17 seconds, a very short period for an image of this size. This will allow the camera to be read out in the time it takes the telescope to move to its next viewing position. The noise of the readout system will be so low that while an image may produce a full-well specification of 130,000 electrons there will be less than 25 electrons of noise in each pixel. The electronics will record the data in the form of a mulit-extension fits (MEF) file
and will also provide real-time instrument health and quality checks.
The Wide Field Optical Corrector
The optical corrector system is a Wynne-style five lens, two asphere design. It provides a 2.2 degree field of view image at 0.27"/pixel while contributing less than 0.3" FWHM to the image quality. The lenses have been cast by Corning in New York and are being polished by SESO in France. The biggest of these lenses will be 98 cm in diameter and will weigh 380 pounds.
The Combination Shutter-Filter System
The cartridge-style filter changer holds up to eight filters, which, at 62cm in diameter, are the largest produced. DES will use five filters, which astronomers have designated g, r, i, z, and Y. Each will let through a relatively broad spectrum of colors, such as red light, green light, or blue light. By comparing the relative amount of light detected through each filter for each object in an image, astronomers can make a very good estimate of the redshift of each object. (For more information on the meaning of a galaxy's redshift and how DES will estimate it for each galaxy, click here.)
These filters are being manufactured by Asahi Spectra, headquartered in Tokyo Japan. The shutter, manufactured at Bonn University in Germany, is the largest shutter of its kind. The filter changer was manufactured at the University of Michigan.
The hexapods provide a real-time focus and alignment system to maintain high image quality. They are being manufactured by ADS International in Leco, Italy.