A portion of the Hercules Globular Cluster showing the effects of seeing on the resolution of astrophotographs
On dry land we are really at the bottom of a pool looking up. Have you ever looked up from the bottom of a swimming pool? The water distorts the image of the world above. If the water is moving, there is more distortion; if the water is calm, there is less distortion. Standing on dry ground the same holds true for our atmosphere, the crispness of the stars depends on the atmosphere. Astronomers call this phenomenon “seeing.” You can judge the seeing by looking for the twinkle of the stars. More twinkling equals worse seeing. Less twinkling equals better seeing and better astrophotographs. Professional astronomers can afford to locate their equipment on the tops of high mountains where the atmosphere is thin and seeing is likely to be better.
Here in eastern North Carolina I’m at the bottom of the atmosphere near sea level. More atmosphere usually means worse seeing. What comes to the rescue is the fact that I’m near the ocean living on the flat coastal plain. The air currents have had thousands of miles to “smooth” out into what’s called laminar flow as it travels across the Atlantic toward North Carolina. These smooth currents of air do not affect seeing as badly as the turbulent currents that can occur around mountains and other topography inland. In fact, most professional observatories are located near coasts at as high of an altitude as possible to take advantage of laminar flow.
To illustrate the effects of seeing on astrophotographs take a look at two versions of the Hercules Globular cluster (M13). These were taken just days apart with the same equipment. You can clearly see much better resolution of the stars when the seeing is good.