Good going Baz... 16x20min subs is a good amount of data, there is no doubt about that...
I had a fiddle in photoshop, and I think there is a lot more in the image that can be stretched out... You posted a 42kb jpg, so compression artefacts were hard to work around, but I had a play with only levels and curves to see what I could get out of it, and I think it shows the extent of what you've captured, however using the raw files I dare say you could get much more out..
Hope you dont mind my fiddling.. Im a bit starved for data at the moment, and will process up just about anything!
hahah
Ps - I only cropped out your ASIGN Observatory label as it was messing with the histogram and it made it hard to figure out what was what..