To photograph the stars, you need a gadget that can track the revolving night sky in a perfectly timed arc. Otherwise all you’ll see is streaks and blurs.

You can buy fancy motorized “equatorial mounts” for telescopes and cameras, but it’s way cheaper and more satisfying to build your own simple “barn door” tracking mount using a long bolt or threaded rod as a drive screw. You mount your camera on the “door,” then aim the hinge straight at the North Star, Polaris. The motor opens the door very slowly to match the sky’s rotation, for blur-free exposures of minutes or even hours. You can set the speed using a microcontroller or a simple circuit.

But there’s a catch: A straight drive screw turned at a constant rate won’t produce a constant angular motion. It’s called the “tangent error” — and here’s how some of our favorite DIY barn door trackers solved it.

Analog Trackers

Sky & Telescope contributing editor Gary Seronik of Victoria, British Columbia, built a lightweight, portable tracker that drives a simple 4RPM DC motor with an adjustable voltage regulator to dial the rotation rate, and a curved bolt to reduce tangent error. He’s shared his design and schematics in this great tutorial. “It’s hard to beat a DC motor and simple regulator circuit for simplicity and performance,” he says.

Seronik went on to create an even more compact Hinge Sky Tracker using an 8″ strap hinge in place of the plywood doors. A straight bolt introduces tangent error, but he solves that by taking shorter exposures and “stacking” them in freeware called DeepSky Stacker.

» For the best of both worlds, build Seronik’s new motorized Hinge Sky Tracker, with a curved bolt, DC motor, and regulator circuit. Find the project here.

Digital Trackers

Chris Peterson in Guffey, Colorado, used a straight bolt in pivoting mounts and cleverly programmed a Freescale/Motorola 68HC705C8 microcontroller to drive a 1.8° stepper motor at a variable rate to produce constant angular motion. He’s taking 20-minute exposures with a 300mm lens, and has shared his schematics and code.

University student David Hash (now an aerospace engineer) updated Peterson’s build with an Arduino Pro microcontroller, 1.8° stepper, and Pololu microstepping driver board to give 3,200 microsteps per rotation (Figure B). He’s shared his build and code on Reddit. He gets great photos by stacking multiple 90-second exposures; check out his Andromeda Galaxy pictured above, and more on Imgur.

Finally, Alex Kuzmuk in the Ukraine designed custom laser-cut doors and acrylic gears for his curved-bolt tracker, and drove it with a $2 stepper, Arduino Uno, and LCD Keypad Shield for easy speed adjustment. Check out his build and code here.