Use manual exposure, shoot wide open at f/4 as a starting point, and record RAW. A good baseline with your 17-40mm on a 60D is about 5–10 seconds at ISO 1600 for a moderate, fairly static aurora. If the display is faint, you may need 20–30 seconds and a lower ISO such as around 640–800, but longer exposures risk both star trailing and smearing the aurora’s motion. If the aurora is bright and moving quickly, shorten the shutter as much as possible and raise ISO—ISO 1600 to 3200 may be necessary.

In practice, aurora settings depend heavily on brightness and speed, so adjust ISO to get the shortest shutter that still gives good exposure. For a mostly static green arc, longer exposures can work; for active, waving lights, 30 seconds is usually too long. High activity is difficult with an f/4 lens, and very short exposures are easier with a faster lens. A tripod is essential. Long-exposure noise reduction and high-ISO noise reduction on the 60D may help, but shooting RAW gives you the most flexibility.