Barn Door Tracker

I don't think a barn door tracker works this way.

In an attempt to get longer exposures of deep space objects, I decided to build a barn door tracker. This would help track the stars so there is no trailing created by the rotation of the earth, The general concept behind the tracker is that the camera is mounted on a hinge, and the axis of the hinge is pointed toward the celestial pole (the apparent axis of rotation for the earth; it's near the star Polaris in the norther hemispehere). The hinge would rotate at the same rate as the earth, so to the camera the stars appear to be fixed in place. About a month ago I tried making a hand-cranked one, but there many severe problems with it. I didn't have a second ball head, so I couldn't aim the camera, and the setup lacked any rigidity, so any attempt to compensate for the rotation of the earth was negated by the wobble created by my hand. Not to mention I didn't have a real way to properly align with the celestial pole.

Attempt 1: Eh.

My second attempt was to use a motor to drive a screw that would open up the hinge. Without my hand involved, the setup should be more stable, not to mention the hinge could be constantly driven for indefinitely long periods of time (as long as the battery still had charge).

I already a relatively low rpm gear motor with an encoder and I bread boarded up a quick circuit to run the system. For the drive mechanism, I first wanted to do a trapezoidal drive using an old lead screw from my CNC mill, but the pitch on the screw was too high and I couldn't run my motor slow enough. After failing to get the trapezoid working, I changed to a curved screw that was positioned at just the right distance from the hinge axis to drive it at 1 rpm to match the earth's rotation. I used a 1/4-20 screw, and it was driven by the appropriate nut attached to some Lego gears. I had to used a 6:1 5:1 gear ratio (I just realized the teeth count on the gears, which is proportional to the diameter of the gears, is 8 and 40, not 48.) since the motor was slow, but not slow enough to match the 1 rpm needed to drive the system. Running the motor too slow introduced cogging, where the amount of power I was providing through PWM wasn't enough to reliably overcome internal friction.

The hardware setup. (I lost some initial images which showed the circuit
before I took it apart to use on another project)

I used a PID algorithm to control the speed of the motor, with the encoder providing feedback. The Arduino program (great for fast prototypes) ran on a Teensy 2.0 microcontroller board. I've probably over complicated the whole setup, as other in the past have simply used a motor connected to a variable voltage supply to fine tune the angular velocity, and others have used stepper motors. Ah whatever.

Everything appeared to be working: I calibrated the rotation rate to 1 rpm with my watch, and the portable phone power bank that I was using to power the setup was charged. I headed outside and did a very bad job aligning with the celestial pole (I still don't have a way of properly aligning, and the light pollution didn't help.). I turned it on and pressed the shutter...

It was just as bad as before. I was seriously confused. I tested with the tracker off, then on to see the difference. I thought I had the setup rotation the wrong way and was actually making the streaking worse. I flipped it around, and back again. Everything was inconsistent. I just couldn't figure it out. At one point the threaded rod got stuck, but after clearing that up and watching that the hinge was rotating, the setup still didn't work.

Close up of the gears. The threaded rod just passes though the wood,
and the threads are likely getting stuck on the wood.

I brought the setup inside and aimed it at a doorway to verify that the system was really moving. And it was. I'll have to debug this another day and make it simpler/create a PCB for the drive. Probably need to get better celestial pole alignment too.

Tracking on.

Tracking off.