How Much Color Can I See?
The vivid red, yellow, blue, and green images of nebulas and galaxies in magazines and textbooks look very appealing, as do the dramatic colors of the magazines' planetary photos. Unfortunately, the human eye is not very sensitive to color when the light is dim. The old expression "In the dark, all cats are gray" could easily be changed to say that astronomically "In the dark, all nebulas are gray." A camera stores up the light of faint deep space objects during long exposures, slowly building an image one photon at a time. Your eye doesn't have that luxury. It has to show you an image right now
. If there's not enough light to trigger the color receptors in your eye, you will see only silvery-grey nebulas and galaxies. You might see faint nebula color with 10" and larger telescopes from dark sky observing sites – but for most people the bright colors of nebulas and galaxies will be visible only in long exposure photos.
Deep space objects seen through the average 8" telescope are usually closer in appearance to less-detailed versions of the black and white photos in magazines and textbooks than they are to the long-exposure color photos.
Stars, on the other hand, show colors quite well, even without a telescope, because their light is concentrated enough to trigger the color receptors in your eyes. The subtle blue, yellow, green, and red colors of individual stars – and the often sharply-contrasting colors of binary pairs – make the sky a veritable jewel box.
The planets show colors, even through a small refractor, but the colors are subtle and muted. Magazine photos are usually computer-enhanced, and often taken from spacecraft only a few thousand miles above the planet. Through a telescope, you're seeing a planet from across millions of miles of space instead of a few thousand and through hundreds of miles of Earth's dirty and turbulent atmosphere. This makes for difficulty in seeing a wealth of planetary detail unless you are willing to spend hours of time waiting for those rare moments of perfect seeing. You can see planetary detail with most telescopes, but it's rarely as contrasty and detailed as the computer-enhanced magazine photos.