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Week of September 14, 1998

For many things in life, what you see is what you get (on the 'net, that term is usually abbreviated WYSIWYG, and is pronounced "whizzy wig"). You'd think then that what you don't see you don't get, but in astronomy, sometimes what you don't see you can get anyway!

Long before the telescope was invented, humans still used a phenomenally complicated, intricate, and sensitive detector to observe the heavens: the eye. Our eye is amazingly sensitive to light, and has a huge range of light it can detect: the brightest astronomical object on the sky we can safely see is the full Moon, which is about 40 million times brighter than the faintest star you can see!

Yet with all that, the eye is a peculiar thing. The retina is the light sensitive tissue in the back of the eye. When light is detected, an impulse is sent to the optic nerve, which then carries that signal to the brain. The problem is, the human eye has a concentration of color sensitive receptors (called cones) near the center of the eye, and these receptors are not very good in low light conditions. To see something dim, you have to move your eye a bit to get the image back on the more light-sensitive receptors (called rods).

The trouble with this less-sensitive spot becomes clear when you look at a faint object through a telescope. Since the spot is located almost dead center in the retina, when you look straight into a telescope, a faint object in the center of the field of view will disappear! It's a very odd and disorienting effect, but luckily you can cure it by just looking a little to the side. By doing that, the light from the object will fall on a sensitive part of the retina again, and you can see it. This technique is called "averted vision", and every experienced amateur astronomer knows how to use it. It takes getting used to, but once you get it you'll use it without even thinking about it.

Incidentally, sometimes objects are so faint that astronomers are not sure whether they are actually seeing it or not. In that case, we say they are using "averted imagination". ;-)

©2008 Phil Plait. All Rights Reserved.

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