A project of the Dark Energy Survey collaboration


Catching Old Light: DECam Style


A photon is born, its birthplace the heart of a star, billions of light-years away and eons ago. Its brothers and sisters headed off in all directions, but this one—this one found its way in one particular direction, toward a civilization that had just learned to command the elements enough to peek beyond the terrestrial veil. After a long, long journey this photon—this old, old light—encountered a mountain top in the Chilean desert, where it found a new home.

At the Cerro Tololo Inter-American Observatory (CTIO), an old workhorse has learned some new tricks and received some serious cyber upgrades. The Victor M. Blanco telescope (left image) was commissioned in the mid-1970s, along with its near-twin sister telescope, the Mayall, which is located in Kitt Peak, Arizona. The Blanco is a reflector-type telescope with an equatorial mount. Situated on one end of the Blanco is large mirror (left image: just right of center), which collects and redirects light toward lenses made of specially crafted glass. These lenses focus the light on a detector composed of a material that converts light into electrical signals, which are then transformed by software into images visible to the human eye.  The back of the detector is shown in left image (just left of center) and the front of the detector is in the right image.

Blanco’s primary mirror is four meters in diameter, long enough for two tall people to lay across, end to end. From the primary mirror, near the base, to the detectors at the top, one could stack about 15 tall people head to toe across the 28-meter span; this is also roughly the length of a basketball court.

Every time the telescope moves to look at a new patch of sky, the motors have to shift  300 tons of glass and metal. This is the equivalent of 4300 people, 150 cars, 10 Humpback whales or 5 Brachiosaurus dinosaurs. It would take about seven Blanco telescopes to equal the weight of one space shuttle at the time of liftoff.

The telescope’s detectors are known formally as charge-coupled devices (CCDs), which are similar to photographic film, in that they are made of materials that absorb and react to light. They are also the very same kind of detector that is found in digital cameras like in your point-and-shoot, or in your mobile phone.

Over the decades of its life, the Blanco telescope has evolved, and most recently, the Blanco was retro-fitted with many new pieces of instrumentation, including new optical elements, a new shutter and other components. Critical to the mission of the Dark Energy Survey is also a new set of detectors that were developed at the Fermi National Accelerator Laboratory in Batavia, Illinois. These 62 new detectors are state-of-the-art CCDs that make up the 570 Megapixel Dark Energy Camera (DECam), which is shown in the image at the right.

DECam is not just over 50 times larger than your average point-and-shoot camera: it has unprecedented sensitivity. This camera is so sensitive it could detect light from a 100-watt light bulb as far away as the moon.

Thanks to the upgrades at the Blanco, old, travel-weary light from a billion trillion miles away, which happened to makes its way toward Earth long ago, will be welcomed with open eyes and ready minds.

Written by: Det. B. Nord, PhD [Fermi National Accelerator Laboratory]
Image Credit: Reider Hahn [Fermi National Accelerator Laboratory (Visual Media Services)]

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