Thanks for the info but probably not the best option for aurora shots.
Good point. Tracking only helps with stars and deep sky objects.
I have no experience with Aurora. I thought that they are bright enough so that aperture doesn't matter as much. But they also move faster, so exposure time becomes a greater issue.
I just bought a Fornax Lightrack recently to get better night skies and eventually start doing deep sky stuff when I've got polar alignment figured out reliably and accurately. Seeing how uncertain the future of cameras is right now, I thought it made more sense for me to by a lower priced lens for the gear I use (16mm 2.0 for the 80D) now and a more expensive bit of Equipment that will work with anything I might use in the future, regardless of whether that will be APS-C or FF, DSLR or Mirror less, Canon or something else.
So I thought I'd mention the option.
But just in terms of quality, there's not much you can do wrong by going with an Art lens, especially for something that requires manual focus anyway, like anything Astro does. My Samyang 16mm 2.0 is quite sharp wide open, but it looks pretty poor when comparing it directly to my Sigma 35mm 1.4 Art. Looking at TDPs tool, the story doesn't seem to change much when comparing it to the 14mm 2.4 full frame lens:
View the image quality delivered by the Sigma 14mm f/1.8 DG HSM Art Lens using ISO 12233 Resolution Chart lab test results. Compare the image quality of this lens with other lenses.
www.the-digital-picture.com
It's too bad that throwing in affordable into the requirements usually means making some compromise. Astro is such a niche and unfortunately Canon hasn't done much to serve it in the past.
For Canon DSLR, all options I'm aware of have been listed here. There are more for mirror less and other systsms, but that doesn't help for your 7DII.