Short exposure astrophotography is a terrific way to learn. Not just about exposure times and iso's, but the processing skills you learn will serve you well.
LOL Not wanting to hijack your thread either, but I used to be the queen of the 20 second sub.
Here's some images I took using a Canon 20d through an ED80 scope. My mount was an alt azi iOptron Minitower.
Total exposure times on these images was 42 minutes, 26 minutes and 17 minutes. That's a hell of a lot of 20sec subs.
