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Leon I've done a LOT of research on this matter with great input from members here. The base 500 rule works reasonably but depending how far from the central true south (I can't remember the fancy name, my heads very messed up at the moment-oh, the pole) the value will drop because the further fringes will rotate more or faster (I can't even remember the terms for this condition, sorry) than the central point. A lot of Astro people tend to more use the 400 rule for wide-field from what I found, same as the 500 rule but slightly shorter exposures and less chance of drift. I myself used a more complex mathematical equation I found and came up with 16 seconds using my 14mm f2.8 Samyong (or whatever it's called). I can do 30 second exposures if close to the southern pole but when I get further out along the fringes of the MW my shots are so much crisper at 16 seconds.
I believe JA is extremely well versed and experienced in this particular subject and a very smart, helpful gentleman.
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