Just because the skies are clear doesn't mean that it will be a good night for astrophotography. Sometimes the skies are turbulent because of winds or temperature changes. On those nights the stars will twinkle ferociously and looking at the sky through a telescope will be like looking at the bottom of a clear lake on a windy day.
Astronomers call that "bad seeing" and it causes the light from distant stars to flicker and jump. Needless to say this distorts astronomical images, leaving them blurry and wavy.
The image on the left is a shot of the galaxy M101 under good seeing conditions. The one on the right shows the same galaxy, taken with the same equipment, on a night of poor seeing.