Friday, September 30, 2011

Starry Night

The stars are the jewels of the night and perchance surpass anything which day has to show. - Henry David Thoreau

Spellbound. That's how I spent most of my summer nights as a boy, lying on my back in our backyard staring up at the sky. It was pitch black, at least for a city kid, and on a clear evening the view was spectacular -- billions and billions of bright, twinkling stars! I'd be entertained for hours, my mind wandering with the freedom and peace I long for today. But it would all end too soon, my Mom's voice shattering the silence, calling me back inside our house. Recently I discovered a way to recapture that feeling and make a nice picture to boot -- Star Trail Photography.

As the Earth rotates around its axis every 24 hours, the star field we see in our night sky rotates around the polar axis in a complete circle -- this phenomenon provides the basis for star trail photography. With a digital camera, there are two techniques you can employ to shoot the stars: one long, single exposure, or a series of short exposures that are "stacked" together using special software; we will focus on a single exposure image. But regardless of the method you choose, star trail photos require the following equipment: a camera capable of "Bulb" mode and long exposure noise reduction; extra camera batteries; a locking cable release; and a sturdy tripod.

The next decision you'll need to make is the type of star trail photo you want to take -- a polar-aligned shot, which results in a circular star trail pattern around the North Star, Polaris; or a non-polar-aligned image which offers nice arcs but not complete circles. For the polar-aligned shot (top photo), you simply locate Polaris by aiming your camera north; pointing the camera in any other direction will yield only a pattern of arcs. Other important factors to consider are the weather forecast and the moon phase -- you are looking for a totally clear night and a last quarter or new moon phase. Excessive moonlight (and other light pollution) will overexpose the image and render it unusable. The final consideration is finding an interesting foreground object for the stars to frame; in my two examples, I used tents at camping trips and they were lit by waving a flashlight around inside of them for 30 seconds at the beginning of the exposure.

Once all the preparations are finished and the photo is composed, determining the exposure completes the process. Through experimentation I have found that an aperture reading of f/4 to f/5.6 at an ISO of 200 with a 30 minute exposure works well. Just set your camera to "Bulb", focus to infinity, turn the auto-focus and vibration reduction settings off, turn the long exposure noise reduction on, trip the shutter and enjoy the view for the next half-hour!

In my two examples, the top photo was a polar-aligned image taken during a new moon phase with an exposure of 30 minutes at f/5.6 at 200 ISO. The bottom photo was my first-ever attempt at star trail photography and was taken during the 1st quarter moon phase (note the abundance of light in the canyon and in the sky -- that's moonlight, the photo was taken at 10 p.m.!) with an exposure of 30 minutes at f/4 at 200 ISO.

For those that like to stay up late and commune with the night, star trail photography may be just what you've been looking for and the real beauty is you won't have to worry about your Mom, she'll be fast asleep before you even begin!

Looking at the stars always makes me dream, as simply as I dream over the black dots representing town and villages on a map. Why, I ask myself, shouldn't the shining dots of the sky be as accessible as the black dots on the map of France? - Vincent Van Gogh