Clear skies are not enough for imaging. Turbulent atmosphere can make your star move all over the place making it impossible to get a good images. The atmosphere is in continual motion with changing temperatures, air currents, weather fronts and dust particles. These factors cause the star images to twinkle. If the stars are twinkling considerably we have "poor" seeing conditions and when the star images are steady we have "good" seeing conditions.
Two older seeing scales are well known to
visual observers
1.
The Antoniadi scale 2.
Pickering Scale A Modern Scale of Astronomical Seeing for
Imagers gives you an indication of when not to image
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here To get good images two conditions are necessary; Clear sky and Good to Excellent Seeing.
cheers
