Charles wanted to know what things looked like before processing so here's a single frame brought straight into Photoshop with no processing, then exported as a level "8" jpeg.

And then here's that frame processed and cropped a bit to replicate the image from my last post:

 

 

What's obvious is the noise and loss of some faint detail compared to the previous post's image.  Keep in my that stacking is an AVERAGING process, not additive.    What you're doing is increasing the signal-to-noise ratio by stacking successive frames.   What that means in practice is that you can bring out a lot more detail in post processing.  Informally speaking, there is in a sense an "additive" process going on.   I haven't done it myself, but if I had stacked 100 images, I'd bring out more whispy, faint details in the disks of the galaxy -- which to me is the hallmark of an exceptional image.    There's definitely more detail to bring out in my previous post's image, but that will take time.   For example,  in M66 (the right most galaxy) there is a faint whispy extension of the spiral arm on the left most part of the galaxy.   You can barely see it in the previous post.  But it's harder to see in the above image because it's more buried in the noise.  

 

The other obvious technique is to take longer subexposures (like 10-15 minutes).    Which is always easier in theory to do than in practice.   I've found that the hit-to-miss ratio is much lower in longer exposures.  At five minute exposures, I can expect to keep 75% of the results.  With 10-15 minute exosures, I can expect a lot less.   Of course I'm assuming we're sticking with the Sirius mount.  I think that number would jump to 97 percent keepers at 5 min and 90 percent keepers at 10-15 minutes with the Astro-physics mount.

 

If you look closely at NGC 3628 (aka "the Hamburger") in my previous post, you can see a faint nebulous extension perpendicular to the disk of the galaxy pointing to the 2 o'clock position.  And to the left of the disk of 3628 (on the same side) you can see a scattering of tiny galaxies.   Those features are largely lost in the noise of the single frame.   For a reference, check out this image of 3628 from one of the masters of astrophotography, Russell Croman:

 

http://apod.nasa.gov/apod/image/0504/ngc3628_croman_full.jpg

 

It's sorta upside down compared to my image.