Using a tripod or some other way of mounting your camera to keep it absolutely still is a must for creating star trails.
Also, be sure you bring the quick release mounting plate, if your tripod requires one, with you and your tripod to the location. I left the mounting plate at home once, while the tripod was in the back of the car. Since then I carry a spare mounting plate in my camera bag.
Bring a flashlight (aka torch), cell phone, something, anything that emits a good amount of light. You will want to look around on the ground before you leave to see if you have dropped anything. A red filter on a torch comes in handy to keep your night vision intact so your eyes don’t have to readjust to the darkness.
Scout your location whenever possible during the day. This is most helpful when you are walking in unfamiliar territory trying to find a good vantage point.
Shoot RAW instead of JPEG you will get more useful information in your image files.
Some sort of cable release or remote release is really useful but there are ways around this (see below).
If you don’t have a cable release use the self-timer or use the Mirror Up feature if you have that option. “Mup” mode allows you to raise the mirror when you press the shutter release once and then it starts the exposure when you press the shutter release a second time or 30 seconds after you pressed it the first time (on my Nikon anyway) in case you forget to press it the second time like I’ve done on several occasions. Both of these techniques will work with exposures of 30 seconds or shorter (30 seconds is usually the longest time available in Manual Mode without going to bulb. A 30 second exposure is not much of a star trail I know but you can “stack” several exposures for longer trails. (If you want to use the bulb setting on your camera but do not have a cable release I have a suggestion on a pretty cool and very cheap way to work around that. I’ll describe that in another post.)
You will probably have the best results using a wide-angle lens for star trails. This allows you to have a nice wide view of the night sky.
Turn off Auto-Focus. Most of the time there is not enough to focus on anyway. This is an absolute must if you do this:
Focus on infinity and tape the focusing ring in place. You won’t have to worry that you moved the focusing ring working with the camera. I carry black masking tape that does not leave a residue with me when I’m shooting at night. With a wide-angle lens everything from about 20 feet to infinity will be in focus. You can even set your lens of choice for the evening’s festivities earlier in the day before you head out (similar to the procedure below).
Infinity is not where the lens’s focusing ring stops! There is a reason for this but I won’t go into that here. If you don’t have some way of reading focusing distances on your lens then stand about 20 feet from a tree or other object and using your light-emitting device to help you focus on said tree. Use Auto Focus if you must but tape the focusing ring and turn AF off on the camera.
Calculating exposures will be tough. If you want to increase the ISO to say 3200 to get into the ballpark be sure you translate the exposure correctly to the ISO you want to use. 3200 ISO converted to 100 ISO is a difference of 5 stops. So an exposure of 5 seconds at f5.6 at 3200 ISO = ISO 100 at f5.6 for 160 seconds. If you are calculating full stops remember to multiply or divide by 2. 3200/2=1600 (1) 1600/2=800 (2) 800/2=400 (3) 400/2=200 (4) 200/2=100 (5) 5 stops difference. 5×2=10 (1) 10×2=20 (2) 20×2=40 (3) 40×2=80 (4) 80×2=160 (5) or 5 stops difference.
Turn Vibration Reduction off. The sensor will burn up your battery fast enough without letting the VR feature on some lenses burn it up even faster. Besides a tripod is the ultimate in vibration reduction!
Talking about batteries, if you have a spare bring it with you, if not make sure to charge your battery before you head out.
For star trails that “spin” around another star, you need to face north, in the northern hemisphere at least. In the images where the stars seem to spin around another star, that start is Polaris or the North Star. Finding the north start without a compass is fairly easy. Find the Big Dipper and starting from the star at the bottom of the outside of the dipper (opposite end from the handle) trace a line that goes through the star at the top of the outside of the dipper and continues on until it hits the next bright star in that line’s path. That is the North Star. To check your star charting skills this is the last and brightest star on the handle of the little dipper. Put the North Star anywhere in your frame and the other stars in the sky will seem to spin around it!