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.

It's not hard to understand why. The usual definition of an optically "good" telescope is one that keeps all parts of a light wave entering it nicely squared up to within quarter-wavelength accuracy by the time the wave comes to focus. But that same light wave, in traversing just three feet of air inside a telescope tube, is retarded by about 400 wavelengths compared to where it would be if the telescope contained a vacuum. Clearly the air is an important optical element.

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.