Star Trails, Digital Style; Exposure And Stacking Techniques

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The earth rotates tilted on its axis approximately 23Þ. This fact renders most forms of astrophotography maddeningly complex. There is one form of night sky photography that is given to us in relative simplicity by our home planet's relentless spin: star trails. All we need to create gorgeous star trail shots is a camera, a tripod, and a remote or cable release.

Before I owned a D-SLR I took star trail shots with film cameras made in the 1970s. These classics don't need batteries to keep the shutter open. Today, battery life is an issue, but digital cameras keep improving, and some of them are amazingly robust. It doesn't even require a D-SLR anymore, as some digicams have become so noise-free as to allow considerable opportunity for sky imaging.

Camper At Pyramid Lake
Use moonlight to your advantage. This stack of three images at 3 minutes each, taken at f/7.1 and 18mm, ISO 400, presents a moonlit dreamscape. When I saw the camper on the shore near the rocks, I knew I had an interesting composition. Nine to 10 minutes under fairly intense moonlight made for a bright photograph.
All Photos © 2007, Art Rosch, All Rights Reserved

One effective way to approach star trail imaging with a digital camera is to use a technique called "stacking." This allows numerous exposures of shorter duration to be combined into a single photograph. A star trail shot can be built over a period of hours, slowly accumulating exposure time. It's not arcane or difficult. Mostly, it requires patience and a thermos of coffee or hot chocolate. And it sure saves on battery life.

Location, Location
By sheer cosmic coincidence, the star Polaris, known as the North Star, is located less than half a degree off of the celestial North Pole. If you stand at the North Pole, you will see Polaris directly overhead. As you move south, the star sinks farther and farther in the sky until, at the equator, it vanishes over the horizon. At the equator, something strange happens to photographic star trails. They become two arcs, one bending north, one bending south.

Southern Hemisphere photographers have no such touchstone as Polaris on which to train their cameras. In the Northern Hemisphere, Polaris is easily found. As a child, I was told to look for the two end stars in the Big Dipper's bowl. Draw a line through them, and, like an arrow, that line points to a lonely star of middling brightness: Polaris. Around this star, because of its location, the entire panoply of the heavens' wheels.

Layer One
This initial exposure, and the rest of the exposures, was taken with a Tokina 12-24mm lens at 14mm at f/4.5, ISO 200 for 61/2 minutes. Camera was a Canon EOS 20D and the Canon timer switch was used to precisely time each exposure.

It's a good starting point to take a wide angle lens; locate Polaris, then set up your tripod and begin taking pictures. A simple formula describes how long it takes for stars to move from discrete points to blurs, and then into arcs. Divide the 35mm equivalent of your lens' focal length into the number 600. If you're using a 20mm lens, you have 30 seconds before the earth's movement starts to bend the star's image. In just a few minutes, a significant amount of arc is revealed to film or sensor.

Stay Focused
Focusing on a star is like focusing on a blank wall. Where is it? Twisting your lens barrel to the end isn't the same as focusing at infinity. I like to pre-focus, on the moon, or use a bright planet or a distant light that my camera's autofocus can lock on. Then I turn off the autofocus and take care not to disturb the focus ring. A bit of experimenting will show you where the focus point is for any particular lens. It would be a shame to spend hours trying to make an image only to discover that you have bloated trails and a fuzzy foreground.

Because a stack of star trails is going to accumulate light over a period of time, stopping down just a little, to between f/4 and f/5.6, will control a tendency for the final image to be overly bright. Middling ISO settings, 200-400, seem to work best.

Layer Three
As you can see, this exposure is considerably brighter than the others. A neighbor turned on his porch light for a few minutes. I used the Opacity slider and a little Levels adjustment to blend it into the stack. See how the trails are accumulating.

One of the points of stacking images is to avoid noise. Ten 6-minute exposures will be far less noisy than one 60-minute exposure. A good result will need minimum or no noise reduction software in post-processing. Digital noise increases with ambient heat, so cool nights can be better than warm nights for this work.

Other shooting tips are a matter of common sense. If you live half a block from Yankee Stadium on game night, don't expect to see good star trails. Light pollution or an overly bright moon can limit the number of stars visible to your sensor. A lens hood can help eliminate distracting light from other sources. If a lens hood isn't available, fashion a tube of black construction paper; just make sure it doesn't get into your field of view.

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