Seeing is the term astronomers use to describe the stability of the atmosphere for observing. Seeing significantly affects both the appearance of stars and of extended objects like planets, galaxies and nebulas. Seeing affects all astronomical and terrestrial, day and nighttime, visual and photographic observations.
Air can bend light just like a lens and it can distort rays of light from distant objects. It does this because of different air temperatures and densities in the path of the light through the atmosphere. Because air is constantly in motion, the amount of blurring, or smearing changes from moment to moment.
When there is little change or distortion, this is called good seeing. Fine detail is visible on the brighter planets like Jupiter and Mars, and stars are pinpoint images. When there it a lot of change and distortion, it’s called bad or poor seeing. Images are blurred and star images are diffuse. Seeing conditions can be numerically rated on a five point scale, where one is worst and five is best.
A particular type of seeing can be correlated to where the turbulent air is and how large the disturbed part – or parcel – of the atmosphere is compared to your scope’s aperture. Some seeing conditions can be controlled or avoided.
Type 1 seeing is caused by air currents either close to or actually inside your scope. The moon appears to shimmer and stars appear double. Tube currents are cured by allowing your scope to reach thermal equilibrium (cool off or warm up to outside air temperature) before you use it to observe. Fans in Dobsonians or Newtonians can help by drawing ambient air through the tube. Also be sure no one stands directly under or in front of the tube and to not place heat-generating equipment near the tube.
Type 2 seeing creates slower moving blurred images. Fine detail is lost and contrast is low for extended objects. Stars appear spread out and lack sharpness. These conditions are created by lower atmosphere heating from the ground or buildings. The solution is to select a good observing site like a grassy field or hilltop. Avoid parking lots, heated air over plowed fields and trapped air in valleys. If you can’t change location, then wait until the early morning hours and uniformly cool air, stable conditions of atmospheric inversions, etc.
Type 3 seeing comes from upper atmospheric turbulence. Fast ripples create shimmer in the visual field, affecting otherwise sharp images. For extended objects, fine details are visible, but images shift around the field. Stars are crisp points, but they rapidly move small distances within the field of view. Since you have no control over it, you can only wait it out until moments of good seeing or simply put off observing until another night.
Astrophotography is limited by seeing, as even with fully thermalized scopes, a great site and excellent weather conditions, the atmosphere still blurs stars to sizes of about 2 arc-seconds during a typical exposure. This is much less than the theoretical resolution of even small telescopes as the chip or film records every part of the blurring during the exposure. Only the world’s best sites, special techniques and equipment can improve upon this figure.
Terrestrial uses such as bird-watching, harbor and ocean patrols, surveillance and target shooting are heavily impacted by seeing. These activities almost always occur in daytime and are usually done with a nearly horizontal line of sight. So effects of surface heating and turbulence are at a maximum and can significantly degrade both viewing and photography.