Estimating the seeing

Astronomical seeing refers to the stability and clarity of telescopic images at high magnification, not to be confused with transparency. Seeing degradation is mainly caused by layers of air having different temperatures. Wind shear produces turbulence that mixes these layers. The seeing can be estimated in many ways; with instruments and specialized software, or by eye through a telescope. William H. Pickering (1858-1938) developed the Pickering scale using a 5-inch refractor, and Damian Peach has made a very nice set of animations illustrating the appearance of a star at each step of the 10-step scale. The size of the telescope aperture and its secondary obstruction will have an effect on the star image; the larger the aperture the better it will show atmospheric turbulence; i.e. a seeing rated 6 through a 5 inch aperture will probably look more like 3 to 4 through 10 inches of aperture. Thus, Pickering seeing estimates between different telescope sizes are not really comparable, unless the observer stops down the aperture to 5 inches, or compensates by guessing what the seeing rating would have been through a 5 inch aperture. Today many amateur astronomers use apertures larger than 5 inches. I made a "new" Pickering scale for a 10 inch telescope, with some Saturn images and a few absolute seeing measurements (in arc-seconds), made with the software DIMM Seeing monitor during its 30-day trial period. I hope in the future to able to fill some of the gaps with more seeing measurements. The star images where simulated with Aberrator. 

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In 2002 and 2003 I made many seeing estimates. Seeing better than 6 is nearly unheard of where I live, but at nearby hilltops the seeing does once in a while get as good as 7-8. Catching good seeing is a matter of being in the right place at the right time.