Star trails to the west, using a 20mm wide angle lens.

Introduction

Since the Earth rotates, the stars are not at fixed places in the sky but appear to move, more or less towards the west. To photograph the tracks the stars make across the sky, all you need is a camera that remains fixed to ground and have it expose for an appreciable amount of time. Such photos can be quite artistic.

The stars appear to rotate around both the north and south poles in the sky, of which you generally only see one. Stars very near the pole are almost stationary in the sky, while stars closer to the equatorial plane move quite fast. All move about 15 degrees per hour in right ascension ("horizontally"), but do not move in declination ("vertically").

Star trail photos are always eye-catching, if done correctly. The major problem doing these exposures is that they can last a long time, like several hours. If you have nothing else to do this will be boring. I make my star trail photos during every photography session that I have at night, provided the sky is clear and I stay a long time at the vantage point. You can combine star trail photography very well with astronomy in general.

What you need

All you need to photograph star trails is a camera with lens that can do time exposures (on 'B' or Bulb mode), a cable release or lockable shutter release, and a tripod. I recommend using an SLR camera.

Any star trail photo needs an exposure time of several minutes up to several hours. Make sure the camera shutter can stay open by itself without your help, and that the batteries don't drain. An old manual SLR camera is ideal.

The choice of lens depends mostly on the composition you choose. If you are at middle latitudes on the northern hemisphere, for example, and want to include both the horizon and the north pole with Polaris in the frame, you will need a wide-angle lens. Wide-angle lenses have the advantage that the star trails will all have slightly different curves, while lenses that have a longer focal length allow for a shorter exposure time to make star trails of appreciable length.

For starters, use a 28mm lens and include the pole and horizon, if that horizon is interesting; otherwise use a 35mm or 50mm lens and a tree or big rock as foreground, with the pole centered in the frame.

The tripod needs to be sturdy enough to not move by wind and such, otherwise the star trails will not be perfect.

Star trails around the north celestial pole.

Exposure times, aperture and film

You should experiment with different exposure times to find out which star trails you like best for your kind of composition: short trails where the constellations are still recognizeable, or long trails to make the photo more artistic. I generally do very long exposure times of over 3 hours using very wide-angle lenses such as 20mm or 24mm, and aim to the east or west, or a telephoto lens that points at the pole with a shorter exposure time of about 30 to 60 minutes.

When you do star trail photography, keep in mind the reciprocity error of film. Every film is subject to this, some more than others. In fact, if you are at a dark location and the glow from cities and the sky itself is very faint, the film may stop responding to the sky background light altogether, except for the stars themselves that are brighter, and thus still produce trails on the film.

You can use this to your advantage. E.g. when doing a 3-hour exposure at f/4.0 on 100 ISO slide film, the film won't be much more fogged than if it would be exposed for 30 minutes! So, unless there is a lot of background glow from the sky, like near cities, you can use low apertures such as f/4.0 or f/5.6. Do not use any lens at its maximal aperture to avoid vignetting, coma, and other abberations.

I recommend using a slow, high resolution slide film like 100 ISO. You won't need a faster film, because the amount of time of your exposure is generally long.

Things to watch out for

If you are using a fast lens and film with short exposure time, you are more prone to have satellites or airplanes, that might fly through your frame, show up on the photo. When using 100 ISO film at f/5.6, most airplanes will hardly be visible if at all, unless they fly overhead or close by.

If an airplane or satellite is going to fly through your frame, you have three choices: either let it pass and hope it doesn't show up, start a new exposure, or temporarily hold a black cardboard in front of the lens while the airplane is in the field (don't touch the lens!). However, the latter option is not so good since the star trails will be interrupted.

This exposure was started with a large lens aperture (f/2.8), a few seconds later carefully reduced to f/16 to produce trails with the stars visible at one end.

Make sure the foreground is not much brighter than the sky; a darker foreground is not a problem.

Variations

You can enhance your star trail photos with some interesting things, once you have enough of the standard star trails:

• Seek other composition: trees, tall plants and rocks do great.

• Starting an exposure, have the aperture fully open for about a minute when using standard or wide-angle lens, or half a minute if using a telephoto lens. Then very carefully close down the aperture of the lens during the exposure, taking care not to move the camera. This will create bright spots at the beginning of the trail, and the constellations are easy to make out.

• Do as above, but hold a black cardboard in front of the lens about 15 seconds after first opening the shutter, for a minute. The trails are then disconnected from the stars.

• Periodically hold a black cardboard in front of the lens for a minute or so to create dashed or dotted trails. This may require a lot of devotion and patience though!

• Bring a flashlight and "light-paint" the foreground in some color. For standard "white" flashlights, the foreground will become yellow-orange.