From my experience shooting auroras past years. Currently i mostly only use Sigma 14mm F/1.8 true performer on my eos 5div. Irix 15mm f/2.4 is also really good even its bit darker F wise. Also the new version of the samyan/rokinnon 14mm f/2.4 xp is told to be quite sharp. Sigma 20mm art 1.4 is also really good, bit easily gets frosted/foggy but little heat on it does the trick.

This is taken with sigma 14mm.
October lights by Kasaari, on Flickr

Since i mostly shoot here in sw part of finland i rarely get auroras like that one on the picture. I usually use something like 1-4s and f/1.8 iso varying from 2000-5000, yes i use rather different isos than the native ones =)