I recently went up to the Griffith Park Observatory on a moonless evening to see if 1) I could capture the launch of a Delta II rocket from Vandenberg AFB, 2) to see if I could get star trails in a light polluted area, and 3) just to have fun taking pictures. The results are here.
Below are a few images of some experiments doing star trails shots with a digital camera. To see the results, put your mouse over the picture. The pictures were taken on the grounds of Quaker Meadow Christian Camp, located at 7200' in the Sequoia National Monument. All night time images were taken with a Nikon D200, with the 17-55 DX lens. All images were at 17mm. The D200 had a battery grip attached with two batteries.
This first image is of "Slate Mountain". The large rock structure to the right is called "Indian Head". On the night picture, you can see the trees illuminated by the lights of the pool. The camera is facing southward, lens at f/2.8. (I accidentally had the camera set for ISO200, I wanted 100.)
This second picture is of the lake in front of the Dining and Recreation Hall of Quaker Meadow Christian Camp. The camera is facing NW, lens at f/5.6 (because of the light from the dining hall), ISO100.
All images are taken with a tripod. (Important!!) These star trail images were done by taking a single shot while there was a little bit of light still left in the sky, then starting the camera taking 20 second exposures every 21 seconds. (Pretty much back-to-back.) Because the D200 is somewhat a battery hog, I had the external battery grip with two batteries. About an hour into the process, I swapped out the expended battery for a freshly charge one (being VERY careful to do any action in the 1 second window between exposures). All images were brought into Adobe After Effects and layered. The evening (light) shot was on the bottom. All pictures except for the bottom one was set to "Lighten" for the transfer mode. I exported the result into Photoshop to resolve any stray issues, then reduced in size and quality for the web. The evening picture was also modified in Photoshop to remove some stars from the evening shot that were visible at that time, as well as to address a "dashed" line effect due to the way the images were taken.
BTW - I chose After Effects to do the layering because of the ease of importing hundreds of photos and stacking them. This can also be done in Photoshop, but After Effects is a LOT more efficient in doing this operation.
I also made two movies that should explain how the process works on the lake picture. The first movie is simply all of the images of the lake sequence played sequentially. See the movie here.
The second movie shows the cleanup work on the dining hall/lake area. Each frame of the movie is done the same way as the final image - rendered each image in lighten transfer mode, one new added image every frame. This one took a while to render on After Effects. I did it this way because I wanted to see if there were any variations as it was being built. (None were seen.) See that movie here.
Questions/comments, you can reach me here.