Secrets of Deep-Sky Observing
Okay, you're pretty sure you've finally got your telescope aimed at the object of your desire. The crosshairs of your finderscope are on its exact location, according to the star charts you're using. Now what can you hope to see?
Probably a lot less than you were expecting but more than the disappointing first impression leads you to believe.
If your target is a bright star, it will be obvious and beautiful but contain no detail. Seen in a telescope, a star is a tiny blaze of brilliant light that looks pretty much as it does to the naked eye, only brighter. Much more interesting but generally more difficult are deep-sky objects: nebulae, star clusters, and galaxies. Hundreds of these ghostly glows and subtle spatterings are within reach of a modest telescope.
Let's say you've targeted Messier 87 (M87), an enormous elliptical galaxy in the springtime evening sky 55 million light-years away. At the eyepiece you'll see a small, shapeless, very dim gray smudge floating among a few pinpoint stars. While finding it should bring a thrill of accomplishment, many novices are let down by the sight. "Is that all there is . . . to galaxies? It's nothing like the pictures in the books!"
You've just come up against the fact that the human eye does not work well in the dark. It cannot perform anywhere near as well as a camera does at very low light levels. We are daytime animals who evolved under a blazing sun; our eyes are not built for observing the distant night universe. Your eye's view of a galaxy will never match the spectacular photos in books and magazines. But here lies the challenge. Many deep-sky objects do show a surprising wealth of detail when studied long and well even with the eyes that nature stuck you with for other purposes.
A telescope serves a different function on deep-sky objects than it does on the Moon, planets, or scenes on Earth. In those cases, its main purpose is to magnify distant detail. With deep-sky objects, on the other hand, a telescope's main function is to collect a lot of light for your less-than-sensitive eye. The main obstacle to seeing deep-sky objects is not that they're too small to see without optical aid. It's that they're too dim.
Accordingly, deep-sky observing involves its own techniques. All are aimed at helping the eye to see in near-total darkness. Here are some pointers every observer should know.