Stellar Trails

One of the simplest, easiest, and most rewarding astrophotos you can take are star trails photos. All that is required is a camera capable of relatively long exposures, a tripod, cable release, a free computer program, and a moderately dark site.

So what are trails and star trails photos exactly?

Well, as I mentioned in the Camera and Tripod Astrophotography post, trails in any other picture than star trails pictures are a problem for long exposure astrophotography. Trails are caused by the slow apparent movement of stars (or other objects in the sky) due to the Earth's rotation. The reason trails appear in longer exposures of around 20-30 seconds (depending on the focal length lens you're using) is the same reason the sun rises, appears to move across the sky during the day, and then sets. As the Earth rotates, the stars slowly shift to different positions in the sky, and as they move they make lines or trails in your picture.

But, who said trails are not nice looking? Indeed, a star trails picture aims at showing the trails themselves!   

To take a star trails photo, first set up your camera and tripod at your chosen site, and focus your camera on the sky as described in the Camera and Tripod Astrophotography post. Next, aim your camera at the portion of the sky you want to photograph. Aiming for a star trails photo shouldn't be too difficult, as you can catch trails practically anywhere in the sky, as long as you can see stars there. It is nice to try to include a few bright stars in your picture or some terrestrial scenery.

Next, take a trial shot of around 10-30 seconds at the ISO of your choice. Again, a lower ISO might make the image too dim, but a very high one might introduce noise into the image. If you can control the f-ratio, set it a few stops above the minimum f-ratio available for your lens. A higher f-ratio will make the image look dimmer, but also makes the star trails look sharper. If you don't see any stars in your test photo and you can't increase the exposure time, try a lower f-ratio.

As long as you see stars in your test photo, you're good to go! The trick to making a star trails photo is to take several consecutive images while your camera is fixed on the sky, allowing the stars to move across the field you're photographing. Here, it's important to make sure the time between consecutive images is very small. Most DSLR's have a continuous shooting mode that triggers the camera to take consecutive pictures of a set exposure time. If your camera doesn't have this feature, then you may have to trigger the camera manually as quickly as possible between exposures, preferably using a cable release.

Once you've taken several consecutive exposures, you can download the pictures to your computer and combine them into one image with a program such as startrails.exe. And voila! Now you've got a star trails photo!

Here's one I took from my house's east facing balcony. It is a combination of 120 photos, 30 seconds each:

You may be wondering why you should take consecutive exposures, and not just one long exposure to capture trails. Well, the main reason is to avoid something called sky fog. Sky fog is basically background brightness caused by light pollution close to urban or suburban areas. This "fog" washes out your image, and the longer the exposure, the more sky fog accumulates. This may not be a problem from a very dark site, but many sites do suffer from sky fog to a certain degree, especially close to cities. Still, you can take a single long exposure if you want to.

Be creative with your star trails images, in terms of where in the sky you photograph and what terrestrial scenery you include. If you aim your camera at the north or south celestial pole, you'll see that the stars make circles as they move around the pole. Photographing the celestial equator will show relatively straight trails, which move away from the equator on opposite sides. The length of the trails of course depends on the total exposure time, but stars also move faster further away from the celestial pole, so the time required to get long trails is less further away from the pole. For terrestrial scenery, you can use a technique called "painting" to brighten objects in the foreground. Painting consists of shining a light such as a flashlight onto a foreground object and moving it over that object as the camera is taking the picture.

Finally, because short focal length lenses have wide fields of view, it is better to use shorter focal length lenses for star trails photos rather than longer ones, as this will allow you to capture more of the sky, and thus more trails. This also helps you fit in any terrestrial scenery in your photo.

Here's a link to another star trails picture on Flickr: http://www.flickr.com/photos/mikeisaak/6107708695/in/faves-astroz1/. You can also explore this star trails group on Flickr for more examples.

Thanks for reading :)

and clear skies!