I’ll just add my 2c worth to what young Ken has described. A single 300 second exposure would tend to have a lot of signal (good stuff) and a relatively small amount of noise (bad stuff).
Signal is what you want – lots of it. These are the photons of the galaxy that you are trying to image. With a good mount and auto guiding, these photons will arrive consistently and linearly at the same place on the CCD chip, building up the signal strength.
Noise is usually more random and does not build up consistently and as quickly as the galaxy photons being captured at the same pixel locations. Noise comes from thermal effects in the CCD, read out processes, electrical circuit effects, etc.
However, 300 seconds demands a good mount and good auto guiding to avoid trailed stars. In some cases, an airplane flying across the 300 sec frame can ruin the frame.
Now if we take say 10 x 30 second frames, we have the following:
- Tracking and auto guiding errors are slightly less critical.
- The airplane will only ruin a single 30 sec frame – you can do a median combine with the other (good) 9 frames and the airplane trail will be rejected.
- However, with only 30 sec exposures, the signal is so much weaker and thus the noise is not so much “drowned out” by it. But, the signal will build up consistently from 30 sec frame to 30 sec frame so when you stack these, the signal re-enforces itself across all the frames. The noise in each frame is mostly random and so when to stack the frames, the noise is not consistently amplified by the stacking process, so it is averaged out, and thus less dominant than in a single 30 sec frame.
The single 300 sec frame will still have a better Signal to Noise ratio than the 10x30 sec frames stacked, and I have forgotten the formula that describes this.
Cheers
Dennis