Star Trails with Digital Cameras:
There are several ways to capture trails as they our earth rotates. Of course it does require long exposures, a tripod, and a fast lens.
A clear night works best away from any civilization however this does not mean going out in the country, climbing a mountain and camping out. Oh well the camping part would be fun. I do look to setup my camera and tripod looking away from any ambient lights and most likely the location be at least on the edge of city or town.
Another notation is most digital cameras do not capture extremely long exposures well. The sensor becomes physically hot producing noise or artifacts on the final photo. The method I prefer is capturing multiple exposed frames and stacking them in layers making the final photograph with software. We'll come back to this later.
To begin with for the example displayed I am using a Nikon D800 on a medium duty tripod and 16-35 F4 Nikkor lens. A faster lens will produce better results. I set my ISO for this example at 600, f-Stop wide open f-4 and make a test exposure for 30 seconds. This is the maximum time the camera's function can be set. What I'm looking for with this test is a little ambient light with speckle dots i.e. stars and a silhouette of my subject.
Looking good I next set my camera to produce "Interval Timer Shooting". The longer the 'Intervals' the longer the star trail. My example is around 90 minutes. However a trail or blurred star begins to show on a fifteen second exposure. I have set my camera set to expose a frame at 30 seconds, f-4, ISO 600, every second (camera is set to 30 seconds but the quickest response between exposures with Nikon is 1 second) for count of 250 frames. This is around 2 hours of total time my camera is set making exposures.