Brian Carter
BFA Industrial Design
The University of Kansas (2003)
Bachelor of Fine Arts - KU

Barn Door Tracker

Brian Carter

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. Using a barn door tracker and image processing techniques like stacking can reveal unseen details in the night sky. A barn door moves the camera to follow the apparent motion of stars and sky during long exposures.

Given the above, this personal project started as a hypothesis. Could a lightweight barn door tracker be built primarily from LEGO parts? Could I mount the tracker directly on the pan head quick release of a small tripod? Could this be done without modifying any LEGO parts?

Connecting LEGO parts and non-LEGO structures

I restated the hypothesis as goals and explored how to connect LEGO parts and non-LEGO structures in non-destructive ways. I traveled to hardware and hobby stores in search of material for physical ideation. I didn’t know what the final structure would be when I began.

The selected aluminum tubing is aesthetically harmonious, is easy to cut with a low-cost hobby saw, is easy to file, is available in a variety of nestable inside and outside diameters, and most importantly fits inside LEGO beam holes and around LEGO axles. The aluminum tube parts seem strong enough to load with a few pounds. I have not done destructive load testing aside from regular use without issue. Your mileage may vary.

Link: Step-by-step Flickr photos

 

Motorized crank

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.

Link: Video 01: Barn door tracker with motorized-crank
Link: Video 02: The motorized-crank only
Link: EV3 program

 

360 degree panoramas

The camera mount from the barn door tracker can be placed on the motorized crank for 360 degree panoramas. The motorized crank uses an EV3 Intelligent Brick, one EV3 Medium Servo Motor, two worm gears, two round gears, and a turntable with 56 teeth. 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 will move the turntable for 1 minute, 1.5 minutes, and 2 minutes of rotation, all with a 10 second delay to match my camera. In the above photo you can see me simultaneously pressing both buttons.

Link: Video 03: Modified to take 360 degree panoramas
Link: 360 panorama sample via video processing software
Link: EV3 program

 

Bluetooth

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. How this was done: I picked a night with a new moon, arrived before sunset, and afterwards aligned the tracker with Polaris by looking through the two sights. The camera’sbuilt-in delay was used to take three two-minute exposures at 400 ISO. Using the side of my finger, I moved the barn door handle to match the second hand position of a clock. On the last exposure, my hand covered the lens to take a dark frame. The exposure was adjusted as necessary and the process repeated. Once home, the DNG images and dark frames were combined to remove noise via Deep Sky Stacker. The resulting 32-bit images were further processed with Photoshop CS6 or CC.

My phone and camera display were covered with red cellophane. Using only red light allows our eyes to adjust and see more stars, and avoids disturbing any nearby stargazing. I also disabled the interior lights in my car. If there are other astrophotographers nearby, be aware that they may be capturing a long exposure and any light from you or your car may disturb them. Be courteous.

The Materials and Software link further below includes a list of astronomy, astrophotography, and processing resources I used to plan, capture, and process images.

Notes

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.

Link: Hand-crank barn door tracker building instructions w/ parts list
Link: Motorized-crank building instructions w/ parts list
Link: Barn Door Tracker Materials and Software
Link: Step-by-step Flickr photos

 

References

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.

About the author

I am a maker, hacker, hobbyist, artist, designer, and builder living in the SF Bay Area. Posted 8/24/2014. Updated 9/7/2014.