Are short subs a barrier to extracting faint detail?

[sternenben]

What is the best way to measure sky noise and read noise, so I can calculate whether it makes sense to push for longer exposures?  

I have been shooting 5 minute exposures at f/6 under what I estimate to be a bortle 4 skies.   I want to be able to draw out faint outer spiral arms in galaxies, and I'm trying to decide whether to try for 10 minute exposures or to just shoot a lot more 5-minute exposures.  

Does ISO have any bearing on this at all? 

[russellhq]

You'll find that with a high ISO you will saturate the pixels faster which will make I'd difficult to get long exposures. That's my experience.