Why Stack So Many Frames?
Many deep sky objects require an hour or more of exposure time. But many things can go wrong during a lengthy exposure. Airplanes, satellites and clouds can drift though your picture. Your mount likely will not track perfectly for that long even with computer-controlled guiding. A battery can go dead. Dew can form on a lens. Digital cameras produce much more noise in long exposures (see the Technical Photos page for more on noise).
Multiple short exposures are a better method for capturing dim objects. If a plane flies through one exposure only a minute or two is lost. Stacking many frames also drastically reduces camera noise and increases detail especially in the dim areas of your subject. Your signal to noise ratio improves by the square root of the number of frames you stack.
For the most part I shoot 2 minute unguided exposures. Even at this short exposure I throw away about 50% of my frames from bad tracking and things flying through (I live near an airport). It's tedious and time consuming, but it's cheap and trouble-free. I try to get three or four hours on a target when possible, sometimes shooting for two or three nights to accumulate enough exposure time.
M31, The Andromeda Galaxy
Photographed at 500mm f/5. Canon XTI unmodified, 1-minute exposures at ISO 1600.
Each example received identical processing. Note the obvious reduction in graininess, and the increased contrast and detail as the size of the stack increases.
