Welcome to the Forums!!! So, here in Arizona we are too far south to be able to enjoy the Northern Lights - but it was 92F today. That said, we do shoot a lot of the Milky Way here so, here would be my suggestions.... I shoot with a K5IIs
- RAW format
- (M)anual or (B)ulb mode - maybe even Av if you are shooting under 30 seconds
- ISO - 1600 to 3200 - if it appears too dark, I would even try higher. Dial in what appeals to you in terms of best results.
- Shutter Speed - 13 seconds to 30 seconds, but I would also try the (B)ulb setting and leave it open for up to a couple of minutes to see what it captures. I say about 13 seconds, because that's about as long as you can shoot and keep the stars from trailing with an 18mm lens.
- Aperture - This is dependent on your lens, but wide open.
- 2 second delay to let the vibrations of tripping the shutter dampen out.
You are going to need to focus, so I would go out during the day and focus on something far away - a mile or two. Then take some tape and tape down the focus ring on the lens, and switch the body to manual focus.
A tripod helps quite a bit, along with an external shutter release. On the external shutter release, the K5 takes the same release as a Canon body.
What it really comes down to is to go out and experiment. The film is free. I would shoot perhaps 30 minutes, come in load the images up on the PC and take a quick look at them. See what works and what didn't. Then go back out and shoot some more, doing more of what worked and less of what didn't. If you shoot for more than say 30 seconds, also try dropping the ISO down to perhaps 800 - as you will get less noise that way.