New Video: Astronomical Seeing

[theskyhound]

Interesting as a very basic guide to seeing but my experiences are that telescope aperture also has an effect on seeing as you are potentially looking through more air cells and that impacts the view as well.

Hi Owen,

I've heard that repeated all my life, but I never thought it made sense, if for no other reason than these simplified "cells" likely vary in size throughout the atmospheric column. Yet people do report that a larger aperture telescope appears to be more affected by seeing, including in side-by-side tests.

An alternative explanation that has also been around for a long time is more subtle: the resolution limit of a smaller aperture is less than that of a larger aperture, so when viewing stars in telescopes of identical design, but different apertures, there is a tendency for the larger aperture to be employing a higher effective magnification. As a result, the turbulence is more apparent in the larger aperture.

But there is another factor, and I hazily recall this being explored in an old article in Sky & Telescope: when we compare large and small apertures we are seldom comparing telescopes of the same optical design. Rather, we are typically comparing a small refactor to a larger reflecting telescope. In this scenario, it is internal thermal currents in the reflector that may account for the apparent difference in seeing. There is also the added time it takes for to reach thermal equilibrium, including the thick mirror. This difference is something that may also show up when comparing large and small reflectors. 

All that being said, ultimately astronomical seeing is subjective. For visual observing we aren't usually putting that fine of a point on it. An experienced observer can usually estimate the relative quality of the seeing by looking at at bright stars via the naked eye. For a more precise estimate, one would use the Pickering scale on a star nearly overhead.

The same applies to imaging, except that the FWHM can often be measured for a star near the zenith.

In the end, when we make our own estimate it doesn't really matter if the person next to us may have a somewhat different experience.