Aurora and Star trails over prosperous lake, NWT - each exposure was about 10 seconds and then I combined several hundred images to create this montage of star trails and the Aurora Borealis.
Star trails appear as curved arcs of light in the night sky as the stars appear to move around the north star in the northern hemisphere. Star trails can create surrealistic looking photographs. Anyone can take these type of photographs of stars provided you can see the stars and you get to a location without too much light pollution. With film based cameras it was easy, you set up the camera on a tripod and used the B or Bulb mode setting and exposed the film for several minutes to several hours. With digital cameras using long exposures of 20 minutes to several hours (depending on the camera make and model) often results in over exposed pictures, lots of digital noise or red glow around the edges. To avoid this problem it is possible to take a series of short exposures (6 sec to several minutes) and then stack them together.
If you were to just stack multiple images one on top of the other the resulting image would become very dark. However, if we stack the images and blend them together using a "Lighten blend mode" the parts of the image that are black or dark become transparent and the lighter pixels are added or combined with the previous images creating the appearance of long star trails. The length of the star trails depends on the total exposure time of the series of photos.
Star Trails and Aurora over Pontoon Lake, NWT. This photo was taken with a 24 mm F1.4 Canon lens wide open at ISO 800, each exposure was about 10 seconds and the total time I photographed the series was about 15 minutes.
BASIC STEPS To Make STAR TRAIL PHOTOGRAPHS
1. Set up your camera on a sturdy tripod in a location where you can see the stars.
2. Set the ISO speed between 200-1600, higher speeds makes the star trails brighter but also adds more noise, you may have to do some test photos to determine the optimum exposure for a single frame. You can use Auto White balance, but may see colour shifts between frames and some photographers prefer to set white balance manually to tungsten - makes warm lights and the night sky look more blue.
3. For convenience and the best results use an intervalometer to automatically trigger one photograph after the other, alternatively you can set your camera to fire continuously and lock it in this mode using an electronic cable release. You want each exposure to come right after the other to avoid gaps in the trails. Exposure times can vary between 30 seconds to several minutes. Set you camera to record JPG files (you can shoot RAW, but you will have to convert them to JPG in order to use the recommended Stacking software).
4. Stacking your images involves importing the images into an image editing program and then combining them into a single image. The easiest solution I have found is a shareware program called Star Stax created by Markus Enzweiler. You need to import your series of JPG files (does not work with RAW), select the blend mode (choose Lighten) and then press the button (top 4th from the left) to start processing. The images will be stacked and presented in the middle window. The time it takes to process the images on my computer varied between 10-30 seconds to process up to 500 images (I have a fast computer). Once the image is stacked you can save the stacked image File>Save as. The software is remarkably easy to use.
You can experiment with different blend modes and the software also has the ability to subtract dark a frame from the photos to reduce digital noise. A dark frame is just a picture you take of the night sky with the lens cap on and the camera records any "hot" pixels. These hot pixels and\or digital noise can then be subtracted from the images. Using a dark frame is common in astrophotography. I did not use dark frames in any of my photographs.
Screen shot of Star Stax software with the files loaded on the left and the resulting stacked image in the middle.
Star trails and Aurora over cabin at Peterson's Point Lake Lodge in the Northwest Territories - total exposure about 1 hr.
Star trails and Aurora over Point Lake, NWT - in this series I clicked the shutter manually over a period of about an hour. Because the time between exposures varied some of the star trails have gaps.
Star Trails and Aurora over Prelude Lake, NWT Total duration was about 90 minutes.
To create time lapse movies where the star trails appear to grow is easy using Star Stax software and Adobe Photoshop CS4\CS5 extended. With Star Stax you select the cumulative save feature and select the folder where you want the series of photographs to be saved to (see below).
To create a time lapse movie of star trails so that they appear to grow I open the cumulative images files in Photoshop CS4\CS5 extended as a series in animation mode (described in detail in my time-lapse photography article) and then render the video as a Quicktime movie. I import the Quicktime movie into Adobe Premiere Pro CS4, add titles and then render the movie to a streaming flash (.FLV) format for presentation on the web (see below).
1 min move showing the Aurora Borealis and Star trails expanding in time - photographed near Yellowknife, NWT
For the photographs in this article I was primarily interested in making time lapse movies of the Aurora, but since I had the individual frames I decided to try and stack the images to create star trail photos. I attended an astrophotography workshop by Alan Dyer recently and I recall him saying he wished there was an easy method by which to create time lapse movies where the star trails would grow - well here is how to do it. Below I have listed some links to other photographers web sites that include some beautiful star trail photos and movies to inspire your imagination. RB