Barn-door Tracker for Astrophotography

The finished barn-door tracker. These are also called “Scotch Mounts.”

Explanation & Objective

Astrophotography often requires long-exposure imaging. The shutter may be open for many seconds, to several minutes. However, while celestial objects do not move much over this amount of time, the earth does. The earth rotates at the sidereal rate of 0.25 degrees/minute.

Imaging without compensating for this rotation results in stars leaving light trails, rather than individual points of light. The barn-door tracker cancels the earth’s rotation out by rotating in an opposite direction, about an axis parallel to the earth’s axis of rotation.

The theory is identical to the one used in my equatorial platform, but since DSLR’s are much lighter than telescopes, building a barn-door tracker is usually best. Additionally, a Scotch Mount can be used at different latitudes, as the inclination is set with the angle that the hinge axis makes with level ground (and again, equal to the latitude where the imaging is taking place).

A time lapse taken with the barn door tracker.

Construction

There are a few different styles of tracker, with most differences occurring in the way the camera is driven about the axis. I chose to use a curved threaded rod as a leadscrew, because there is zero tracking error with this method. A straight threaded rod, for example, would incur trigonometric error at the limits of travel, and would have a nonzero angular acceleration.

I traced a curve on a large piece of cardboard with the radius needed for the threaded rod.
Then, I bent the rod carefully, until it approximated that curve. I began with a much longer length than needed, and cut out only the section of threaded rod that appeared to have the best curve fit.
Testing the circuitry to drive the stepper motor. An Arduino Nano was used, along with an inexpensive stepper-driver breakout board. A lithium polymer battery powered the system.

Decorative Stain & Drive System

A 3/8″ threaded stud protrudes from the top. This thread is commonly used for ball-mounts, and it’s where the camera will attach. I located it near the hinge to reduce the amount of torque needed to rotate the leadscrew nut.
I sourced these two nylon gears from an old printer, and repurposed them for this project. The gear-reduction ratio worked very well. The leadscrew nut is a standard 1/4-20 nut, attached to the gear on the underside with epoxy putty. The slight curve of the rod did not interfere with rotation, due to the large radius and existing loose tolerances in the thread.

Motorizing the Mount

Quick calculations using the thread pitch, sidereal rate, leadscrew radius, and gear reduction provided the step-rate needed.

I added a small set screw on the pinion gear to affix it to the stepper motor shaft.
The stepper motor was attached in a way that allowed some adjustment of gear tooth engagement.

The Finished Barn-Door Tracker

Aside from occasional electrical issues with the stepper driver, the mechanical-side of the tracker performed flawlessly. To use the tracker, the hinge axis is pointed toward Polaris (the North Star). This places the hinge axis parallel to the earth’s rotational axis, because Polaris is nearly coincident with the earth’s axis of rotation, and at an effectively infinite distance from the earth.

Another 1/4″-20 female thread is on the bottom of the tracker, so that it can attach to tripod mounts.
Gear engagement was sufficient for the light loads found on this tracker.
The tracker worked well enough, and rotated precisely enough, to utilize longer focal lengths. This lens is a 60mm-300mm lens.
Stained red oak came out well. I added a radius on some edges and corners with a router for aesthetic purposes.
The aluminum disc underneath the upper ball-mount allows the locking lever to rotate fully.