Clear Seeing

August 18, 2008

The sky is blue… the sun is setting, and it looks like it will be a perfect night for stargazing. Or will it? Blue sky doesn’t necessarily lead to a great night for observing. Here’s how to read your sky and get the most out of your stargazing session.



The Basics

• Aside from clouds, the atmosphere degrades your view in two ways: by reducing transparency and by decreasing the sharpness of images in your telescope.

• “Transparency” describes the clarity of the atmosphere. If the air holds dust particles, water droplets, or smoke, then starlight is blocked from reaching your telescope and faint stars and deep-sky objects are much harder to see.

• “Seeing” describes the image sharpness as affected by the atmosphere. In a turbulent atmosphere, masses of moving air of different temperatures swirl and mix. Because warm and cold air bend light a little differently, this turbulence slightly scrambles the light along your line of sight and makes stars twinkle or appear like unseemly blobs instead of tack-sharp points in your eyepiece.

• Seeing and transparency are often independent from each other. So transparent sky might be unsteady, and rock-steady sky might be murky and not particularly clear. Of course you want both, but this is not a perfect world and steady and clear skies are rare in many places.

A Deeper Look

• Good seeing (and therefore steady sky) is great for observing fine lunar and planetary detail and resolving double stars, even if transparency is poor.

• Transparent sky is great for extended objects such as nebulae and galaxies, even if the seeing is not particularly good.

• How do you know if you have good seeing? If the stars are twinkling like crazy, that’s a bad sign. And if you look through your telescope and you see stars that look like angry boiling blobs, that’s a very bad sign. Skip planets and double stars on such a night, but maybe try galaxies if sky has good transparency.

• And transparency? After a cold front passes through, the sky is often highly transparent but not very steady. A transparent sky is often a dark, rich cobalt blue before sunset. A whitish-blue sky and high humidity during the day are sure signs of poor transparency.

Good To Know

To minimize the effects of the atmosphere, try observing a celestial object near culmination… the time when it’s highest in the sky. Technically, culmination is the time each day when an object crosses the imaginary great circle that passes from the pole through the zenith. Just remember… look higher, see better.

Personal View

Great nights when the sky is clear and steady are rare where I live. While I might get 100 clear nights a year, only 5 or 10 have good seeing and good transparency. And I try not to waste them watching TV!

Posted in Telescopes and Observing