Deep Sky Blog: Astrophotography: Barn Door Tracker

October 12, 2007

Astrophotography: Barn Door Tracker

Ever pictured what the sky would look like with about ten times the amount of stars we have now in it? It sure would be beatiful. It actually does look like that, but your human eyes can only collect so much light, so little in fact, that we can't see nearly as many stars as other things.

Your camera is one of those other things. Your aperture is the setting that determines how much light gets let in. A wide open aperture (usually f/2.8) is a great start for astrophotography and will bring bright, high contrast stars into view with exposures of 15 seconds and even less.

But what about those super starry pictures you've seen? Well those must be on exposures much more than 15 seconds; more like 15 minutes! After your first fifteen minute exposure, you will learn a valuable lesson. The Earth rotates. You will have what is called "tracking errors" or blurs and in some cases, streaks. Star paths make good photos, however, to get a decent starry sky picture, you will have to compensate for the curvature and rotation of the Earth.

Ok, break out your graphic calculator and dust off your ancient calculus skills; this a math problem.

Just joking... thankfully one wonderous astronomer has already figured out all the trig, angles, and square roots of the problem and provided us with a simple solution for under $10: the barn door tracker (also called a Scotch mount).

With precise measurements listed in the construction details, you track the rotation of the Earth through a camera near perfectly up to fifteen minutes. With an exposure this long with precise compensation for the rotation of the Earth, you will get astonishing photographs! Check out this photo of the Andromeda Galaxy with just a simple camera.