go to www.dxomark.com/index.php/Came…
Find your camera and see what iso settings you can push your camera to and still retain descent image quality, this is especially important when shooting auroras, a high iso sensitivity means you can push the shutterspeed under the magical 8 seconds to retain some of the movement of the aurora in the image.
(I personally recommend 6 or less)Preventing STAR TRAILS
Use the rule of 600 to keep stars in the picture from trailing/streaking.
Divide 600 by the true focal lenght of your lens, true is measured by 35mm full frame, so a 30 mm full frame lens would yield this simple equation: 600/30 = 20 second (or less) exposure and stars will look nice.
For cropped sensors you have to time your focal length with 1.5 for Nikon and 1.6 for Canon.
So 16 mm wide angle lens on a Canon cropped sensor would equal 25.6mm true focal lengt (16*1.6) giving this equation: 600/25.6 = 23.43 seconds or less to prevent startrails.
(ps: superwide and ultrawide lenses will show startrails faster on the edges, so divide 450-500 instead of 600 for best results)A Tripod and remote shutter
In low light situations handholding goes out the window and you need someting to keep the camera still, and everytime you click that shutter and remove your hand you induce a little bit of vibration in the camera, so for optimal sharpness I recommend a remote shutter.
Tripod gets a bump in price as soon as you get over to carbon fiber, but you can get great aluminum and even wooden tripods for much less, and a fast look on ebay will give you 3rd party remote shutter as low as a 10th of the price of branded ones.
But you can workaround both if you are on a low budget.
A towel to lay your camera on, a book to elevate the lens and a surface like for instance a rock and you have your custom "tripod", and if you use the self timer on the camera your hand will be away long before the shutter goes off.Focus and test shots
Getting focus at night can be very easy, find the brightest star turn the focus ring until it becomes sharp, go over until it blurs just a bit, then back again until it is sharp.
This technique will make sure you get the sky in focus, be careful though as you will be at the further end of the scale and if you have something in the foreground close it might get quite soft.
Another way is to use the infinity mark on the lens, but keep in mind that those are often guidelines and not set in stone, for instance on my 14-24mm F2.8 lens, having it on the infinity mark gives slightly soft images, I have to keep it just a touch under.
When you think you have gotten the focus right do a highly overexposed image so the ground is clearly visible, check the shot and zoom in to see if it is sharp, if it is you are all set, if not repeat the focus process.
If you want to have foreground sharp and background acceptably sharp you have to resort to hyperfocal distance ( from wikipedia: The hyperfocal distance is the closest distance at which a lens can be focused while keeping objects at infinity acceptably sharp).
It is some math behind this but for simplicity there are numerous apps that does that for you, all you enter in is your F-stop and Focal range and it calculates for you where you should move your focus.(ex: "DOF Calculator", "Hyperfocal Distance" a search will lead to more)Preserve your night vision
It takes time for your eyes to adjust to darkness and nothing kills that faster than a big ass white torch of a flashlight, and for the previous step being able to see the stars through the viewfinder a good astronomy flashlight will go a long way.
Red light will hurt your nightvision the least, and flashlights like the Rigel Starlite Red Flashlight and the Orion 5756 DualBeam LED Astronomy Flashlight will help you preserve nightvision better, both have adjustable brightness settings the orion also has 2 white LEDs for when you need to pick stuff of the ground and such.
And also your photography friends will thank you for not blinding them when out shooting at night.