Beating the Seeing
NASA spent $2.1 billion to escape from poor atmospheric seeing; that's what it cost to put the Hubble Space Telescope in orbit. However, backyard observers on smaller budgets need not despair of improving their fuzzy, shimmering views. You can avoid the worst effects of atmospheric turbulence by understanding its nature and learning a few tricks.
Viewed at high power from the bottom of our ocean of air, a star is a living thing. It jumps, quivers, and ripples tirelessly, or swells into a ball of steady fuzz. Rare is the night (at most sites) when any telescope, no matter how large its aperture or perfect its optics, can resolve details finer than 1 arcsecond. More typical at ordinary locations is 2- or 3-arcsecond seeing, or worse. Planets appear fuzzy at high magnification and and won't quite focus. Heat waves seem to shimmer across the Moon. Close double stars that your telescope ought to resolve look single.
In an ideal world the air would affect every part of a light wave equally. But if the refractive power of the air down one part of the telescope tube differs from the rest by more than just one part in 1,600, the ¼-wave tolerance will be breached. Such a change results from a temperature difference of just 0.2° Celsius.
Add the miles of air that the light wave traverses before it even gets to the telescope, and it's a wonder that we can see any detail at all on objects above our atmosphere.
The air's light-bending power, or refractive index, depends on its density and therefore its temperature. Wherever air masses with different temperatures meet, the boundary layer between them breaks up into swirling ripples and eddies that act as weak, irregular lenses. You can see this where hot air from a fire or a sunbaked road mixes with cooler air above; those ordinary heat waves are astronomers' poor seeing writ large. Our windy, weather-ridden atmosphere is almost always full of slight temperature irregularities, and when you look through a telescope you see their effect magnified.
Much of the "seeing" problem, however, arises surprisingly close to the telescope, where you can take steps to reduce it.
Inside the Scope
Seeing problems often are at their worst a fraction of an inch from your telescope's objective lens or mirror. If the objective is not at air temperature, it will surround itself with a wavy, irregular, slowly shifting envelope of air slightly warmer or cooler than the ambient night. So will every other telescope part. Therefore, one of the most important ways to "beat the seeing" is to give your telescope time to come to equilibrium with its surroundings. Amateurs soon learn that the view sharpens within about a half hour after bringing a telescope outdoors. The full cool-down time for a large, heavy instrument may be much longer. It pays to set up early.
Usually the telescope is too warm, especially if it is stored indoors. But sometimes the opposite happens. Whenever a telescope begins to collect dew or frost, you know that it has grown colder than the air, thanks to radiational cooling. In this case gentle heat not only prevents dew but also keeps the scope closer to the air temperature thus sharpening its resolution.
"Tube currents" of warm and cool air in a telescope are real performance killers. Reflectors are notorious for tube currents, but closed-tube Schmidt-Cassegrains and refractors can get them too. Amateurs today agree that any open-ended tube should be ventilated as well as possible. This means designing lots of open space around a reflector's mirror cell, keeping cell itself light and airy, and keeping the tube walls at least an inch away from the optical path.
For decades, the conventional wisdom in the amateur world was that reflectors cannot be as optically excellent as refractors. Many books say an 8-inch reflector equals a 5-inch refractor, at least for image sharpness. The discovery of the wonders of fans in reflectors has gone a long way to ending this disparity!
Do tube currents trouble your images? It's easy to check. Turn a very bright star far out of focus until it's a big, uniform disk of light. Tube currents will show themselves as thin lines of light and shadow slowly looping and curling across the bright disk. Turn on a fan; the lines quickly swirl, break up, and almost disappear.