Astronomy News
Get a Grip on Sharper Seeing The very air we breathe conspires to ruin our view through a telescope. Here are clues to detecting the culprits and how to take action. by John Shibley You probably think that image quality in a telescope begins when light bends through a lens or bounces off a mirror. Well, think again. Most of the time telescopes don't work at their theoretical limits because of what's called "bad seeing," air turbulence somewhere along a path starlight takes shortly before reaching your eye. This turbulence may lie several thousand feet up in a jet stream or within your telescope as heat currents. Tiny clues will betray which is the case; knowing what causes turbulence lets you avoid it and the bad seeing it causes. Lowly Turbulence, Lofty Effects Jupiter's in the eyepiece. The image is doing this slow, undulating crawl around a significant portion of the field of view. Welcome to "slow seeing," caused by turbulence from nearby sources that radiate heat into the cool night air. Because you can isolate and usually eliminate these sources of thermal pollution, slow seeing is easy to combat. Start by directing attention to the telescope. Let it cool and equilibrate to ambient temperature for at least one hour before observing. Some observers with open tubes, like reflectors, hook up battery-powered fans to speed up the process. Telescopes with closed tubes, like refractors, offer no such option and must wait out the cooling process. This may not be as bad as it sounds; a closed tube won't belch out currents of warm air. Check for this type of slow seeing by slightly defocusing a star's image into its diffraction rings. Look for slow-moving blobs that seem to ooze through zones of the ring pattern. Those are currents of warm air escaping the telescope tube. Another source of slow seeing may lie in the immediate vicinity of your scope. Make sure nobody's standing near the scope's line of sight. And if you're set up inside an observatory, use a fan to draw the day's heat from the building. Even observatory design comes into play here: A roll-off roof surrenders a building's heat to the open sky more efficiently than a chimney-like slit. Buildings made from wood or sheet metal also have less thermal inertia than those made of concrete block or masonry. Another remedy for slow seeing might rest in a change of venue. Avoid sighting over things that soak in the Sun all day, like a neighbor's roof or a parking lot. Don't set up inside a depression. Cold air sinks into such pockets during the course of an evening, so it's more likely you'll be looking through a turbulent boundary where cold air meets warmer air. Also, stay away from the edge of a depression; air racing past you on its way to lower ground leads to unsteady seeing. Finally, don't observe too close to the horizon. Many more sources of rising air lie in that direction than, say, 45° up. Lofty Turbulence, Lowly Effects High-altitude turbulence arises from eddies only a few inches across that form layer between warm and colder air. All it takes to create a turbulent layer are temperature differences as small as a few hundredths of a degree. These boundaries lie near areas where different types of air meet and blend in our atmosphere: jet streams, cold fronts, and around the edges of clouds. Such mixing betrays itself in images that quickly churn and boil, but stay put in one area of the field of view. These effects happen very fast, rippling across an image in a fraction of a second, hence the popular moniker "fast seeing." And although fast seeing is easy enough to detect, there's little to do but anticipate it. Pay careful attention to where jet streams snake across weather maps. The middle latitudes suffer especially during the winter months, when the polar jet dips south and carries along winter storms. If a jet stream passes over your area, you'll see the effects of high-altitude turbulence. Find out where these streams are by watching the Weather Channel or the weather segment of your local news. A quick look out a window will also tip you off to fast seeing. Count on plenty of blurry viewing immediately after a cold front passes. While skies may look transparent after a front, they are not that steady; air is still in a state of thermal flux. For steadiest views, wait until the high-pressure system that follows a cold front settles into place. Also, keep an eye out for approaching warm fronts, marked by high "mare's tail" cirrus clouds. One to three days before a warm front's arrival, southerly winds carry air that is warmer than the surrounding ground. This puts a lid on rising air to yield steadier views. Finally, avoid observing near the edge of middle-level clouds. Clouds are nothing more than billowing columns of moisture-laden air that punch into a cooler layer of air. There's no shortage of turbulence in the immediate area where this happens. The least likely environment for fast seeing is stagnant, homogeneous air. Believe it or not, some of the steadiest seeing comes on hot, hazy summer evenings. Bigger Isn't Always Better . . . Even with this good advice, it's rare you'll avoid bad seeing entirely. Larger scopes are especially sensitive to unsteady nights. Why? Ironically enough, it's because they suffer from better resolving power. Atmospheric turbulence spreads light over a circle appropriately called a "circle of confusion," measured in seconds of arc. Bigger eddies, the result of a lot of turbulence, create larger circles of confusion that affect a wider range of telescope apertures. As turbulence dies down, the eddies responsible for smearing light get smaller, as do the resulting circles of confusion. While observers with smaller scopes might perceive an improvement in seeing, those using larger apertures see no change at all. Larger scopes continue to resolve the blurring effects of still-lingering eddies. Consequently, larger scopes are more susceptible to some finer forms of bad seeing. In a specific example, assume that air turbulence on a certain night creates a 1" circle of confusion. A 10-inch scope capable of resolving detail down to 0.5" sees this turbulence as a rippling, "fast" form of bad seeing. On the other hand, a 60mm refractor, which can only resolve down to 2" reveals a relatively sharp image that occasionally jumps around the field of view. For the most part, smaller aperture scopes only detect coarser, avoidable forms of poor seeing, such as "slow seeing," created by nearby thermal sources. Knowing this, some observers "beat the seeing" by stopping down a large-aperture telescope with an aperture mask. Although this technique might keep an image from rippling, it won't keep it from jumping around. Besides, most seeing changes from second to second. When it does improve, you'll want a full aperture to glean as much detail as possible, however fleeting. This underscores a point worth remembering. Patience is often the most overlooked factor when dealing with bad seeing. Your eye can grab a myriad of detail in just a fraction of a second when seeing clears. Rather than waiting for a night of perfect seeing, keep an eye out for reasonably steady skies and observe as often as possible. This alone will guarantee you many spectacular views for years to come.
