Composite image of star trails around the North celestial pole. 6 eight-minute exposures were stacked together to give the equivalent of a single 48 minute exposure.

Because exposures with DSLR cameras are limited in length to 10 to 15 minutes at most due to thermal signal, it would seem impossible to shoot star-trail photos longer than this. This is not so!

Layers blended via Lighten

Through the magic of digital imaging, we can composite as many short sub-exposures together as we would like to equal a longer exposure. We even get the added advantage of being able to shoot star trails at light-polluted locations because the sky will not fog in these shorter exposures.

Shoot a series of individual exposures, each several minutes long, and paste them all as layers on top of each other in Photoshop. The trick is simple. Change the blending mode for each layer to "lighten". Then the sky will be contributed from only the background layer, and only the stars will be added from each of the other layers.

Set up your camera on a fixed tripod and make sure it does not move in between exposures. Use all of the same techniques that we discussed here earlier.

You can use freeware programs like Startrails, by Achim Schaller, or StarMax, by Gilbert Grillot and Sylvain Weiller, to combine your exposures. These programs were specifically designed to create star trail images.