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The charge-coupled device, or CCD

         When I was grinding my 24" mirror for my Dobsonian telescope, holding the mirror in my hands I could cause telephone poles to soon gout flame at the point where the 24" image of the Sun came to focus on the pole. What fun! This is the major function of all telescopes: to intensify the images falling on them by focusing that image with a collecting lens or mirror. By virtue of its enhanced brightness, the focused image can then be magnified many times. Although in principle it can be magnified as many times as it has been brightened, telescope images are usually brightened millions to billions of times but magnified only a few hundred times. This is due to the limiting effect of atmospheric turbulence, and because the need to be make the object brighter far exceeds the need to make it larger. This subsequent magnification can be carried out by a variety of eyepieces or objective lenses placed in the various focal planes (places where the light comes to focus) of the telescope.

         In practice, however, the world's largest telescopes rarely form images using a magnifying eyepiece. Instead, their light comes to a focus on a silicon wafer called a CCD ("charge-coupled device") whose surface is composed of millions of microscopic transistor gates. Like film, CCDs can gather images over time. But whereas film records only 1% of the photons that fall on it, a CCD is far more sensitive, recording 60% - 70% of the photons that strike it. They are thus far more sensitive than both the human eye (gathering an image in 1/14th of a second through an eyepiece) and time-exposed film.

         A 36-inch telescope equipped with a CCD camera can record fainter images than even the 200-inch Mt. Palomar telescope using photographic plates. And most famous images taken by the world's largest telescopes are formed from many hours of exposure on CCD plates. For example, the Hubble Ultra Deep Field (HUDF) image, released March 9, 2004, was formed from 278 hours of exposure on a CCD during 400 orbits of the Hubble Space Telescope.

         The HUDF image was acquired using Hubble's Advanced Camera for Surveys (ACS) and the Near Infrared Camera and Multi-object Spectrometer (NICMOS). Even though the ACS (installed in 2002) has twice the field of view and a higher sensitivity than its predecessor, the NICMOS sees even further than the ACS.  The NICMOS reveals the farthest galaxies ever seen because it utilizes the infrared portion of the spectrum, and the expanding universe has stretched the light of the most distant celestial objects into that part of the spectrum. The size of a telephone booth, the Hubble's ACS camera captured photons of light that began traversing the universe before Earth existed!  These most ancient of photons from the very faintest objects in the universe arrive at a mere one photon per minute, compared to light from nearer galaxies which arrives at millions of photons per minute.

 

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Carl Woebcke, The charge-coupled device, or CCD, 1991-2016. All rights reserved.