Capturing star trails is easy and, for me, very exciting. The best, and safest way for you to get these images is to stack many 30 second exposures into a single frame, rather than using the bulb mode of your camera.
Shooting in blub mode can cause sensor damage, especially in the duration required to capture these photos. By taking many exposures, you are allowing the sensor a little "break" between the shots, and keeping things from getting too abused...
Things you will need...
1. Digital Camera (SLR)
Must be capable of taking timed exposures. My camera is a Nikon D3000, with the 18-55 zoom lens that came with it.
Needed to hold the camera still. You'll want one with a fully articulating head, so that you can point it at the stars. You'll also want good functioning legs, so that you can level the shot(s).
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It's 100% free and totally awesome, because that's how German photographer/programmers roll.
4. Rubber Band
5. A Post-It Note
Step 1: Make a button holder
You'll set your camera release mode for continuous shooting, so holding the button down will keep taking pictures. So, we will need a way to hold the button down for quite a while...
Enter the rubber band and the post-it note...
Fold your post it in thirds like so.
Then half again, like so.
Then half again, but this time, like this kind of so.
Now, get your roll on, and roll that little guy up.
And check for fit.
Now, rubber band that little fake finger in place.
Check and test your finger substitute with a faster shutter speed, so you don't have to wait 30 seconds before you know whether or not it's actually working. You should hear it clicking off photo after satisfying photo.