Under an ocean of air

Sirius under high magnification and poor seeing, 2004
Bowen Celestial Observatory, 8" reflector (obsolescent)
Cuesta College, San Luis Obispo, CA
http://www.youtube.com/watch?v=yV1RUsZORig

Astronomy 10 learning goal M1.4

Even if the sky was perfectly clear at night, at a location as distant as possible from urban light pollution, there is always turbulence to contend with--the cause of poor seeing, and the twinkling of stars.

To help students understand this, draw an analogy with being at the bottom of a swimming pool. No matter how clear the water is, unless it is perfectly still, you will not get a steady view of what is above the water. Literally at sea level we are at the bottom of an ocean of air! This unsteadiness, while making the stars twinkle, degrades images taken through telescopes.

So what are some ways to alleviate the effects of atmospheric turbulence?

• Go to higher altitudes--the less air you look through, the less of an effect it has.


• Wait for lucky seeing--moments when the air in your line of sight just happens to be perfectly still. This can be done in a systematic way by recording a digital movie, throwing out the frames where the image is unacceptably distorted, and "stacking" the frames where the image is acceptably steady and clear. In a related method, speckle interferometry is where the frames of this digital movie are much faster than the unsteadiness caused by atmospheric seeing, thus "freezing" out its effects, and all frames are stacked after compensating for slight shifts in position.


• The effects of atmospheric turbulence can be "undistorted" by adaptive optics, where shape of a flexible objective mirror of a reflector is changed on the fly by a computer, in order to bring a guidestar from a twinkling, wobbling mess back to a point image. Thus the rest of the field of view would also presumably be undistorted as well. This has allowed very large aperture telescopes to be built, thus maximizing light-gathering power and improving resolving power.


• Or go to outer space--the most costly method by far, and precludes large apertures as a trade-off. But extremely-long exposures are possible, as you would not even have to worry about day or night (along with the weather and cloud cover), thus resulting in effectively powerful light-gathering.