Near the Scope

Some seeing problems arise just a few feet in front of the telescope. Obviously, you should try to keep your breath and body heat out of the light path. This is one reason to put a cloth shroud around an open-framework tube.

A telescope's immediate surroundings should have low heat capacity so they don't store up the warmth of the day. Grass and shrubbery are better than pavement. The flatter and more uniform the greenery the better. Heated buildings are disasters of poor seeing, especially if you find yourself looking over a chimney.

If you build an observatory, use thin materials that cool quickly: plywood or sheet metal, not masonry. Paint it white or a very light color to reflect solar heat. (Special heat-reflective paints are available; they reflect infrared as well as visible light.) Ventilate the building very well. A thick rug belongs on the floor.

A roll-off roof that opens the whole room to the sky provides quicker cooling and better seeing than a dome with a chimneylike slit. If you insist on a dome, install a large fan in one wall to suck air down through the slit past the telescope, just as professional observatories do. It's widely considered a poor idea to put an observatory on a heated house unless you resign yourself to low-power work. If you must do so, try to put it on the upwind side — and make sure the floor of your attic is well insulated and the attic is well ventilated.

Much poor seeing hugs the ground, so an elevated observing platform is a good idea if you can manage it. A telescope is likely to show the stars and planets more sharply if you can get it up just a few feet closer to them.

High-Altitude Seeing

Now we come to the unavoidable heart of the problem. There's not much you can do about the air thousands of feet up. But you may be able to predict when and where it will be smoothest.

Telescope users recognize two types of seeing: "slow" and "fast." Slow seeing makes stars and planets wiggle and wobble; fast seeing turns them into hazy balls that hardly move. You can look right through slow seeing to see sharp details as they dance around, because the eye does a wonderful job of following a slowly moving object. But fast seeing outraces the eye's response time.

An old piece of amateur folklore is that you can judge the seeing with the naked eye by checking how much stars twinkle. This often really does work. Most of the turbulence responsible for twinkling originates fairly near the ground, as does much poor seeing. But rapid, high-altitude seeing escapes this test. If the star is scintillating faster than your eye can follow (the eye's response time is about 1/10 of a second), the star will appear to shine steadily even if a telescope shows it as a hazy fuzzball.

Astronomers often talk of seeing cells — air-eddy lenses, millimeters to meters across, that swarm through the sky. These eddies originate wherever air masses rub past each other — either horizontally in winds, vertically by convection, or both. Sometimes, when watching an extended object like the Moon or a planet, you can focus the telescope on a horizontal layer of "shear turbulence" a few thousand feet high. The ripples sharpen up when you turn the focuser slightly to the outside of infinity focus (moving the eyepiece farther from the objective). This is the signature of an inversion layer, in which a mass of warm air flows across cooler air below. The actual temperature difference may be very slight.

Large or slow-moving eddies cause slow seeing, but they don't stay large forever. No matter what size the eddies are when they originate, they break up into smaller and smaller ones. When these finally become small enough to measure in millimeters, they die out and dissipate their energy as heat via the air's fluid friction (viscosity).

This complex situation belies an often-repeated piece of astronomer's lore: that seeing cells are 10 centimeters (4 inches) in size. In fact they come in all sizes. But cells in this middle range do have an important property: they affect a large telescope more seriously than a small one. If you have a 4-inch scope, cells 4 inches and larger passing through its line of sight will make an image move around while staying relatively intact. The same cells passing in front of a 12-inch aperture will superpose multiple images at once.

This fact has led to another piece of folklore: that when the seeing is bad, a large telescope shows less detail than a small one. Therefore, supposedly, you can improve the view in poor seeing by stopping down a large aperture with a cardboard mask.

Technically there is a bit of truth in this, but in practice the improvement is nonexistent. I have never seen any improvement by stopping down a telescope when the problem was poor seeing. The most that can usually be said is that on a really rotten night, large- and small-aperture views will be equally poor. Even then, if you constrict the aperture you miss the chance for the momentary high-resolution views that the full aperture will provide if the air briefly steadies.

There are reasons why you may indeed see more sharply through a stopped-down telescope. Most of them are bad — and have nothing to do with the atmosphere. Maybe your eye was dazzled by a too-bright planet; in that case an eyepiece filter would solve the problem better than a reduced aperture. Maybe the aperture stop is masking off the optical errors of a flawed objective. Maybe it's just allowing a mediocre eyepiece to perform better by increasing the telescope's f/ratio. Poor collimation of the optical parts is also less damaging when the f/ratio is increased.

On a reflector or Schmidt-Cassegrain, a small, off-axis mask can give you the advantage of a clear aperture. A clear aperture, mathematical analyses have shown, is slightly less affected by atmospheric turbulence than an obstructed one. But in this case the loss of aperture is huge.