Photography and Imaging
The eye can only see what is in front of it at any given time; no matter how long you stare at a dim object, it won’t seem to get any brighter. This is not the case with a camera, which is why photography is such a powerful tool for astronomers. When you use a camera to take a normal photograph the shutter (be it mechanical or electronic) is usually open for a tiny fraction of a second, to ‘stop’ the motion of moving objects. Typically this ranges from 1/100 s to 1/2,000 s for a modern camera, hand-held. As you might expect, ten times more light arrives on the CCD (charge-coupled device) chip (that replaces the film in a modern digital camera) if the shutter is open for 1/100 s than it does if it is open only for 1/1,000 s. Obviously, this is not the same way that the eye works.