Seeing

"Seeing" is a term used to denote the quality of an optical image. Bad seeing is caused by atmospheric turbulence, which causes the star images to appear to jump around, as in this simulation. The eye sees this effect as "twinkling". When a long exposure is taken, with photographic or digital detectors, the "twinkles" are all recorded, and the image smears out. Thus bad seeing results in a blurred image. The effects of seeing can be seen in this comparison between a Hubble Space Telescope image and one from the ground.