Thanks! So you focus on the mountain/infinity then take the shot?

I'm using a nikon d5300 with a sigma 18-250mm f/3.5-6.3 lens. Though up till recently I've mostly shot on a nikon kit 18-55 lens I've noticed the body doesn't handle noise well on higher ISOs. I've gotten some decent aurora shots but it's hit or miss as to whether I get the whole aurora in focus (as opposed to a "blob"). How important is iso when shooting auroras? Also I'm guessing you were able to get good illumination bc of both high iso and a fast lens right?