Many thanks guys. Some interesting info there. Craig made quite a few points I hadn't considered. I also came across this which made for an interesting read:
https://www.galactic-hunter.com/post...c-experiment-3 I noted the differences between the 60s exposures v the 15 minute exposures were not as dramatic as I had imagined. I also had not considered the % of the histogram as a measure - cheers JA. My camera (Pentax K50 DSLR) will most likely perform a little differently depending on temperature. It does it's own dark frame subtraction - if I take a 5 minute shot, I have to wait 3 min 50s extra for the noise reduction to do it's thing but it does seem quite effective. Certainly WAY better than the original Pentax 1stD I had several years ago. At least now I've got some autoguiding rigged up on the AZEQ6, I will be able to experiment with some longer exposures and see what results I get out of the Pentax. (which will eventually get replaced with a ZWO294MC or similar - my knees are gonna thank me for that!!)