3-9-12 11:39 pm f4.5 ISO1600 197 seconds (3.3 minutes)
To do a star trail image, I set up my camera on a tripod (ESSENTIAL), set the aperture to a small number, and set the camera setting to Bulb or B mode. The last essential tool is a shutter release cable. When you are ready to start recording an image, you push the shutter button on the cable in and keep holding it :) I have a lock on mine so I can walk away and not worry about it. For the image above, I had the ISO up so high because I wasn't sure what I was doing :) The resulting image was very blown out, but I was able to recover most of it. I also was shooting in RAW and the 3 minute exposure time equaled at least 3 minutes of processing time on the card.
This was my second attempt. I left my setup outside after pointing it north and stayed inside.
For this shot, I pointed the camera up in the sky to be sure to capture the earth's movement and left it outside for nearly a half hour. Processing time was at least 30 minutes, but that's fine.
I should have lowered the ISO for this one, but I wasn't thinking. :) I do like how dreamy looking it is though!
4-23-12 11:18pm f4 ISO400 243 seconds (4 minutes)
For this last shot, it was still too light outside, but I was impatient. I had a car drive by right as I was finishing up. I wasn't sure if it would ruin the image or if the visual interest would be worth it. I think it's alright.
Those are the only star trail photos I have taken so far, but I find them fascinating and will be doing it again.