The Dark Energy Survey

The Dark Energy Survey

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Additional Information about DECam

The Dark Energy Survey camera, DECam (Credit: Fermilab)
The science requirements of the Dark Energy Survey (DES) drive the construction of the Dark Energy Camera, DECam. DES will look deeply into the universe, observing galaxies at great distances to record their motion and conditions in the distant past. To do this, DES needs a camera which will view relatively large areas of the sky at once while being sensitive to the redshifted light from these distant objects. To accomplish this task, DES has designed a camera with five major systems: Just as DES is an international collaboration, DECam is an international camera with components built in five different countries.

The 570 megapixel CCD Camera

A prototype of the Dark Energy Survey camera, DECam. The front ring holds the detecting CCDs and is 45cm in diameter. (Credit: Fermilab)
In order to complete its science goals, DES will use CCDs that engineers at Lawrence Berkeley National Laboratory (LBNL) specifically designed to observe red light from distant galaxies. For example, the chance of detecting long-wavelength light is increased when it travels through more silicon, so DES CCDs are about 10 times thicker than conventional CCDs.

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 10-6 Torr.

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.

Hexapod Adjustors

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.