A deep image will typically consist of many 20-minute exposures. You can't have the exposure be too short because the CCD read-out process introduces noise, so

A deep image will typically consist of many 20-minute exposures. You can't have the exposure be too short because the CCD read-out process introduces noise, so you want as few read-outs as possible. But if the image is exposed for too long parts of it may saturate and start to ruin the rest of the image. (Plus you may have the whole thing ruined by a cosmic ray or a satellite or something.) Something around twenty minutes is generally the sweet spot. Throwing out an image or two that had a satellite or cosmic ray pass through out of a few dozen isn't so bad. But if every single one of the images has a satellite trail you may be out of luck.