Barn Door Tracker
Built with LEGO parts
The recent advent of hack kits for small digital cameras reduces the cost and weight required for entry-level astrophotography. A tracker moves the camera to follow the apparent motion of stars and sky during long exposures. When used with processing techniques like stacking, unseen details in the night sky can be viewed. Given the above, this personal project started as a hypothesis. Can a lightweight tracker be built primarily from LEGO parts?
Connecting non-LEGO parts
How to attach non-LEGO parts? The selected aluminum tubing is aesthetically harmonious, easy to cut, is available in a variety of nestable diameters, and most importantly fits inside LEGO part holes and around LEGO axles. I have not done destructive load testing aside from regular use without issue. Your mileage may vary. Link: Step-by-step Flickr photos
The motorized crank is optional and attaches to the bottom of the barn door tracker. The design may also retrofit a variety of existing barn doors. The combination of the isosceles geometry and my goal to not modify any LEGO part pushed me to a unique design solution. The LEGO turntable fits around the hand crank, allowing the barn door crank to travel through the middle of the turntable, while an arm on the outside of the turntable turns the hand crank. In addition, the motorized-crank assembly can disengage for an easy reset of the barn door tracker, and a small offset in the design allows for tension adjustment between the gears.
360 degree panoramas
The camera mount from the barn door tracker can be placed on the motorized crank for 360 degree panoramas. The gears have a 1:1 ratio. One complete turn of the motor moves the turntable by exactly one tooth, so the motor must do 56 full revolutions to move the turntable 360 degrees. I built three EV3 programs that move the turntable for various rotation times, all with a 10 second delay to match my camera. In the above photo you can see me simultaneously pressing both buttons.
Using an API available on CodePlex, I’ve previously authored an app to control an EV3 TRACK3R via Bluetooth smartphone. I plan to build a similar app to control the motorized crank. I will post details here when complete. In the meanwhile, I use a rubber band to secure the EV3 Intelligent Brick to the tripod. Pressing the shutter and EV3 buttons simultaneously is easy to do with this configuration.
Capture and process images
The above image of the Milky Way was captured using this barn door tracker and a low-cost camera hacked with CHDK. The camera’s built-in delay was used to take several two-minute exposures at 400 ISO. Using the side of my finger, I moved the barn door handle to match the hand of a clock. On the last exposure, my hand covered the lens to take a dark frame.
Using the tripod pan head to align the barn door tracker with Polaris while looking through the two sights is manually easy, but it requires visual patience. The sight beam automatically aligns with the hinge as designed. Tracking seems accurate enough for multiple wide field images taken over a period of several hours. Hand crank operation requires watching a timer. The motorized crank gives more time to look at the night sky, but with the addition of cost, motor noise, and batteries. I highly recommend taking dark frames at regular intervals, writing notes on exposure settings and durations, and occasionally checking Polaris alignment through the sights.
Building instructions. Materials. Software.
All LEGO parts I’ve used are available in the MINDSTORMS EV3 Home Edition kit, with the exception of the "4624645 Turntable 4.85" available on BrickLink for $1.25. If you just want to build the hand-crank barn door, you don’t need the entire EV3 kit. Try ordering individual pieces from LEGO Bricks & Pieces or BrickLink instead.
The center of the 10-32 stainless steel straight rod is located 7.14 inches from the center of the hinge. Reference: A Tracking Platform for Astrophotography, Gary Seronik, http://www.garyseronik.com/?q=node/52 (last visited Aug. 24, 2014).
While building, I realized an isosceles mount would be easy to do and nearly as accurate given the materials and process used here. Reference: Barn door tracker, http://en.wikipedia.org/w/index.php?title=Barn_door_tracker (last visited Aug. 24, 2014).
The hand crank moves the 10-32 rod one revolution per minute. Multiple references suggest thinking of the handle like the second hand on a clock. The handle position should reflect how many seconds have passed during each minute of exposure.