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OK, here is the image i did before just cropped to 1080 high in original resolution from the camera. Lets me do better quality jpg so less noise added and removes the bent stars to the right of the image.
I use the 450D, and I recommend doing 4 or 5 by 5min exposures. On fainter targets if there are no bright stars in shot then go for 10minutes (possibly longer? I have not yet tried with 450D???). It does make a big difference. On objects with great contrast then do both 5 min and 10 min and do HDR or create a gaussian blurred mask of the brighter image and apply the dim image over the bright image using this to bring out over exposed cores (M42 comes to mind).
Doing 2min exposures on a faint target is not real good as the edges of the object are about as bright as the noise of the camera. Once stretched I'm seeing a lot of red/blue blotchy pixelation between the stars, we're stretching out the noise of the camera. Doing lots of exposures is great if you have high initial brightness of the target being captured. Exposures are not added together as they are stacked. What the software is doing is comparing them to remove the noise hence increasing the contrast of the brighter target and letting you stretch the image more without showing up more noise and grain.
Don't waste too much time at first doing 20 shots. Try for 5 or so longer shots. More is better, but not if you have limited time. Spend more time polar aligning and setting up. If your stars are not round at 2min, then they will be worse at 5min.
Allow for dark, bias and flat capture time. I noted you mentioned nothing about bias shots. They are important to remove readout noise from the darks and flats and images. Again, just run 5 of these at first. You can use darks and bias from previous nights work if you forget to take them. I run them with the cap on the camera body whilst I pack up the scope so they are taken at the same temperature as the main images.
Your image without flats caused the center to go bright. Without a lightbox this can be removed pretty well with software and is important to do, especially with this round target near the center. When you push to 10min exposures on faint objects you will bring out the dust donuts on the camera sensor, so a light box is better.
Hope this helps,
Mark
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