The Drones’ Hackers

A sidereal (L. “of the stars”) tracker is a camera mount that turns at exactly the speed of rotation of the earth but in the other direction, so the stars appear stationary. This allows long exposures of the stars — up to 3 hours with this one. When used on telescopes, these are also called equatorial mounts.

Before use, the hinge axis must be pointed accurately at the celestial north pole.

Star Tracker Schematic (northern hemisphere)

This style of equatorial mount is called a Haig Mount (after the inventor) or a “barn door” mount (because the platform turns on door hinges). A 9V battery drives the electronics and stepper motor (surplus, reclaimed from a broken vending machine) that moves the camera platform. The boards are from a 6×1x48 pine plank that I had the hardware store cut into three pieces. The lowest board has a 3/16 nut so the mount attaches directly to Bogen tripod legs. The top-most board has a Bogen ball-head for the camera.

This barn-door tracker is different from the common ones: the screw is a circular arc, which means there is no “tangent error”; exposures can be up to 3 hours long (limited by the size of the screw). Since the screw is not straight, the motor needs to spin the nut (and not the screw as in most designs).

For more on barn-door trackers, see Pete’s Astrophotography.

The Sidereal Tracker

The bipolar (4 wires) stepper motor I’m using has 48 steps per rotation, and the final gear ratio (the mini-bicycle-chain drive) is 2:1. Since the final drive must turn at 1 rpm, the stepper motor needs to be stepped 96 times a minute or 1.6/s (1.6Hz). I use a conventional 555 timer circuit to generate a 1.6Hz square wave. (Some day I may replace it with a crystal-controlled oscillator.)

555 Timer to drive stepper motor controller

The stepper motor driver is the MC3479 from ON Semiconductor. The datasheet provides the circuit to use:

MC3479 stepper motor controller

The completed electronics

The final drive is a delrin “mini bicycle chain” (from Small Parts Inc.). The drive nut fits into a hexagonal hole in the drive sprocket:

Chain and sprocket drive

The 0.5″ drive sprocket hub is extended slightly on one side. This extension fits into a brass tube of inner diameter 0.5″ that is glued into a half-inch hole in the board.

Drive sprocket upside down

For longer exposures it’s very important that the hinge be accurately aligned with the North Celestial Pole, which is about 0.75° from Polaris, the Pole Star. For that, a Telrad sight is also mounted to the tracker, very carefully aligned with the hinge. (The Telrad clips into a bracket, and is removed after alignment is complete so the camera has complete pointing freedom.)

The Telrad sight mounted on the tracker