Tracking vs. fixed camera
As the Earth rotates once every 24 hours, heavenly objects appear to move in circles across the sky. In long nighttime exposures they will appear blurred unless a special motorized mount is used that moves the camera to track them across the sky. The mount exactly compensates for the Earth's rotation.
Tracking with a motorized mount is a specialty and takes some knowledge of astronomy. At the low end of complexity is a driven mount which simply holds a camera. But most people doing this sort of photography use a camera mounted on a telescope. This is more elaborate. One needs a good telescope and a beefy motor drive, for starters. Even the most precice motor drive drifts slightly off course in time, and so the astrophotographer also needs a telescope attachment (called an "off-axis guider") which permits monitoring of the drive and mid-course corrections as needed. Since the objects being photographed are invariably faint (unless it's the moon), specialized, souped-up fast films are often used. This can be a joyous way to spend a starry evening and the results can be stunning, but it's not something you jump into lightly.
A simpler way to do astrophotography is to use a camera mounted rigidly on a tripod, and live with the motion of the Earth. Not only live with it, but use it. You'll need a camera with a "bulb" shutter setting -- this lets you keep the shutter open for as long as you want. You'll also need a locking cable release to trip the shutter and keep it open.
Stars appear to rotate around the north celestial pole, which is very near Polaris, the Pole Star. You can use any film to take star trails. Since light from the stars falls onto the film in very small spots which scroll across the film as the Earth moves, exposure time has absolutely nothing to do with how bright the star trails appears on the film. A trail's brightness is controlled entirely by the f-stop of the lens and the film speed. Exposure time only determines the length of the trails. When I'm shooting my beloved, but slow, Velvia film, I just open up the lens to its widest aperture. If you are shooting faster film, you may be able to stop down the lens -- a little experimentation might be in order for you to establish your optimum shooting aperture with the film you use. But I doubt you'll be disappointed if you just open the lens up wide, no matter what film you are using. For focus, set the lens to infinity. Then trip the shutter open with your locking cable release and leave it open as long as you like.
Since the Earth rotates 360° every 24 hours, the sky appears to move at a rate of 15° per hour. Stars close to the pole will make circles around the pole, and 15° an hour doesn't move them very far; stars on the celestial equator will move the fastest and leave the straightest trails. This helps you figure out what lens to use and how long an exposure to make to get the effect you desire. For example, a 200 mm lens with a field of view of 12° will take 48 minutes for a star on the celestial equator to move from one corner of the frame to the diagonal corner, leaving a trail of light across the entire frame. As another example, a 24 mm lens and a one hour exposure will give equatorial trails that are about 8 mm long on the film (remember that a frame of 35 mm film measures 24 mm by 36 mm). Here is a table that tells how long an exposure it takes for a star trail to fill the frame from one corner to the diagonally opposite corner (for stars near the celestial equator):
I treat every star trail photo attempt as an experiment, a game. I never count on any attempt working, and I get a delightful surprise on the light table when it does. I try short exposures, long exposures, whatever. This is a version of bracketing, trying several things hoping at least one will turn out well. Sometimes I'll leave my camera with the shutter open taking a star trail picture when I turn into my tent at night, and stop the exposure whenever I happen to get up in the middle of the night. Of course, this unattended camera approach works best when backpacking or in a deserted campground. For one thing, you don't have to worry about theft; for another, you don't have to worry about wayward headlights illuminating foreground objects or shining into the lens and causing flare.
Aesthetically it is almost always best to have a foreground object in the frame along with the star trails. If it is a dark night the object will appear as a black silhouette. If there is a moon out or you use very long exposures, the foreground object may have visible detail. Both are interesting effects. You can also step in and play an active role by using a flash to illuminate selected foreground objects. You can achieve a similar effect by "painting" the foreground object with a flashlight.
A couple of caveats are in order when taking star trail pictures. Keeping the shutter open for long periods of time, like hours, can quickly drain the batteries of some cameras. Mechanical cameras are better for this sort of photography. The other thing to know is that light from a viewfinder display, if it doesn't turn off after a few seconds, can leak back inside and record on the film. I learned this the hard way with my Nikon FM-10: the red viewfinder exposure meter stays on as long as the shutter button is depressed, and I had red splotches on almost all of one evening's work of star trails. Only the last image I attempted that night was clean; I'm guessing that the cold of the night and the continual use of the batteries had run them down so the red display had flickered out by that time.
Here are two examples of star trail pictures, showing some of the range of effects you can get. One has long trails, one has short. One has an illuminated foreground, one does not. One has long, curved trails, one has short, straight trails.
Extended objects (e.g. comets)
Bright comets are rare, but a chance to photograph one is not to be missed. One can also do successful fixed-camera exposures of a few astronomical nebula with a long lens. The exposure for an extended celestial object, such as a comet or a nebula, follows the usual rules for exposure, that is, both the exposure time and the lens aperture affect the final exposure. Since these objects are traveling across the sky just like the stars, exposure times with a fixed camera must be kept relatively short to keep the object from blurring excessively. Just how short depends on the focal length of the lens you are using -- the object will appear larger with long lenses, so blur is more apparent -- and how close to the celestial pole the object is -- objects closer to the pole move across the sky more slowly. You'll need fast film, probably one of the excellent 800 speed color negative films available today. So choose a lens to get the composition you want, open it up as wide as it will go, and trip the shutter for as long as you can get away with before blurring becomes objectionable. Be sure to bracket! I have had good success with comet pictures using exposures up to 2 minutes with a 24 mm lens. Compositionally it is often pleasing to have a foreground object in the frame along with the celestial object.
Timothy Edberg / 6511 Homestake Dr. South / Bowie, MD
(301) 809-5857 / 1-877-471-6414 (toll-free)