Just to remind you, here is a shot of 9 stacked 30sec exposures at ISO200 using the 350D, Sigma 70-300mm APO DG lens piggybacked on a fork mount:
farm3.static.flickr.com/2278/2231649780_b5ec8c0327_b.jpg
Very little nebulosity is visible in this, and even less in individual frames.
just so i understand this correctly, if you had 100 30 second images, and stacked them , could you theoretically come out with an image equal to 50 mins total exposure of a system capiable of 50 min exposures?
Not quite. If in the 30secs of exposure you managed to get a few photons of the object under scrutiny then those photons will stack and become more apparent in the end result. However stacking can only improve whats already there. A single image of 50 minutes exposure will have lots more detail (including noise). Stacking is a way of emphasising more detail and reducing noise, but if the data isn't there because the exposure wasn't long enough to start with then .................
  