If you’re a space geek like me, you know that this has been a great season. Not only did we get to witness the extraordinary transit of Venus across the sun in June, but we also enjoyed a spectacular annular solar eclipse in May. And more recently, NASA announced that one of the Voyager probes has finally left the solar system for interstellar space.
Such events turn my attention to astrophotography. In this post, I’ll focus on shooting star trails by taking lots of relatively short exposures and then combining the results.
[Photo by Flickr user Moyan_Brenn]
Astrophotography for beginners
You have a lot of ways to capture the beauty of the night sky with a camera, but shooting star trails is among the easiest, mainly because you can do it with almost any camera. There’s something magical about these kinds of photos, because they reveal the mathematical precision of the cosmos generally hidden from the naked eye – it’s easy to see that the earth spins under a blanket of stationary stars.
To take a star-trail photo like the one above, all you need is a camera with a manual exposure mode; such a camera will let you dial in the shutter speed and aperture setting independently. To make things easier, you’ll want either a remote shutter release that you can lock (so that the camera takes photo after photo with no intervention from you) or an intervalometer mode, in which you can schedule the camera to take a set of photos automatically. Check your camera’s menu or user guide to see whether it has such a mode.
Taking the shots
Ready for some astro action? Get started while there’s still a little light left in the sky. Set your camera on a tripod, and compose the shot so that it includes something interesting aside from the stars. You might want to include some buildings, for example, or an unusual arrangement of trees or mountains. Including even a simple group of trees, as in the photo here, is better than just pointing the camera straight up into the empty sky.
Check your camera’s settings, too. Since you’re going to combine a large number of shots into a single photo, make each shot a brief exposure to minimise digital noise. Set the ISO to 400 or 800, and open the aperture all the way, to f/4 or the smallest f-number your camera supports. Finally, set the shutter speed to 30 seconds.
If you have an intervalometer mode, tell your camera to take one photo every 30 seconds for several hours. (You might need to make that every 31 or 32 seconds, since some cameras ‘miss’ every other shot if the shutter speed is exactly the same as the shooting interval.)
If you don’t have an intervalometer, attach a remote shutter release to your camera and be ready to lock it down so that every time one exposure is completed, it will automatically start the next, until you choose to unlock it.
That’s everything you need for the setup. I like to start shooting before the sky is completely black – you’ll find that if even a few of your photos capture a deep-blue twilight sky, it will add a rich colour to your completed photo. Now just start shooting, and leave the camera alone for several hours. How long is long enough? You can get a nice photo with 75 or 100 photos, but the longer you can shoot the sky, the longer the trails will be. Try for 300 photos.
Stacking the exposures
Thankfully, taking the photos is the hard work; stacking them together is not too complicated, though you’ll need a specialised star-trail stacking program. For Mac-owners, I recommend StarStax, which is easy to use and free.
The StarStax interface
Simply download and open the program. Then select File -> Open Images, and choose all of the photos from your star shoot. Click Edit -> Start Processing and you’ll end up with a completed photo that you can save to your Mac.