WRAL WeatherCenter Blog

Make stars sweep across sky with these tricks

Posted March 31, 2014

A meteor and resulting fireball (green streak, lower left) is captured in this image from 2011. (Credit: Ken Christison)

You’ve probably seen photographs of stars in the night sky with what appear to be "light trails." Photographers create these trails depending on which direction the camera is facing: swooping arcs when they take long-exposure photos while the camera is aimed east or west, circles when the camera is aiming north or south.

Here in the northern hemisphere, Polaris, or the North Star, creates the smallest circle. It’s position, about half a degree away from the celestial pole, creates a circle rather than just a single point. Star "trails" in the southern hemisphere lack that central bright circle as seen in the north because the closest thing to a south star, Sigma Octantis, is several orders of magnitude dimmer than Polaris.

Photographer Ken Christison ofNorthampton County has shared his images of star trails with WRAL on Facebook and submitted them as Mike Maze’s a-Maze-ing pic of the day. But unlike other forms of astrophotography that require expensive telescopes, many amateur photographers already have everything they need to begin creating their own star trails. To get started, here is what you'll need:

  • A camera capable of taking long exposures of at least 30 seconds (DSLRs are the most flexible here)
  • A steady tripod
  • Some way to trigger the shutter without touching the camera, like a remote shutter release or timer that takes multiple photos
  • Fully charged batteries in both the camera and shutter remote, as the camera will be operating for an hour or more at a time

In the days of film cameras, photographers trekked out into the desert, opened the shutter for eight hours and produced stunning star trails.  Now, light pollution in our skies will likely to turn a long exposure completely white. Also, today’s digital cameras produce digital noise in long-exposure photos that looks like colorful snow, especially on the dark backgrounds. To avoid these problems, break long exposures into 30-second increments. You can then stack the images using software to create the final image (more on that below).

To take the photo, Christison recommends the following:

  • Set the camera to manual mode
  • Use the widest aperture (lowest f-stop number) possible on the camera 
  • Set an ISO of 400 to 800
  • Pick a bright star to focus on or use the infinity mark on the lens as a guide (this setting availability varies depending on the camera and lens model)
  • Take several test shots until the desired brightness and focus is met. If the image is too bright or grainy, reduce the ISO.
  • A small hand towel secured over the camera and lens with a rubber band can help keep dew from creating problems
  • Try shooting 30-second exposures one after another for about an hour. A fast memory card in the camera helps minimize the amount of time between shots and prevents gaps in the trails.

At this point you will have a lot of uninteresting images with little streaks of light. To make them interesting you must stack them into a single long exposure. This can be a time-consuming process to do by hand but there are a number of software packages that can help. Christison suggests StarTrails for PC or StarStaX for Mac. If you are a Photoshop guru, there are also actions to help automate the process.

Once you get the hang of creating star trails, more creative possibilities open up, including positioning something of interest (buildings, trees, etc.) in the foreground.  You can keep objects in the foreground dark for a silhouette effect or manually “paint” the object with light from a flashlight while the photo is being exposed to make it appear in the final image. Paring movement and stillness in a photo can also the stunning. Just don't forget to get out there and enjoy the night sky and be sure to share your images with us.

Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and software engineer at Cisco Systems. You can follow him on twitter @rtphokie.


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