Updates to the Barndoor Tracker

So after two runs outside, I found a couple of places where the tracker could be improved. I will detail the issues and solutions from my experience, as well as the final results.

Click here for the original build log. 

Polar Alignment

The most noticeable issue I had was with aligning the tracker to the pole. If the hinge wasn't pointed exactly at the pole, I got a lot of trailing and loss of tracking after an hour or so. It was actually bad enough that the stacking software did not recognize any stars in any of the images.

Yeah, those stars don't cut it

This issue is due to user error, and with enough training and practice, I am sure it can be done reliably and accurately. However, I wanted to make my life a little easier and make setup faster, so I needed a way to point the tracker towards Polaris with good repeatability. To do this, I salvaged a cheap spotting scope from a $20 telescope, and attached it to the hinge. After zeroing it, it made the alignment process far easier, and the results have gotten much better.

3D printed mounts

Battery Issues

Another issue that manifested itself was that the motor required a stable voltage to maintain the correct turn speed. What happened was that as the night went on, the AA batteries lost voltage from their use, and that in turn slowed down the gear to the point that it was off by four seconds every minute. The batteries weren't even used to capacity, as they measured 1.46V using a multimeter, indicating that the motor is quite sensitive to voltage changes. Another issue was that my camera battery also ran dry pretty quickly in the cold, averaging only about three hours of use before dying. I didn't have a good solution to fix that, aside from swapping batteries and limiting exposure times to under three hours if left unattended. 

To fix the motor battery problem, I replaced the batteries entirely with a standard DC power supply brick. One thing I did learn is that the wall wart bricks you typically find with most appliances are unregulated, meaning that they don't actually output exactly what's stated on the brick. In my case, I used a 6V brick thinking it would be perfect, but I found that it actually applied closer to 10.4V. This made the potentiometer relevant again, and I was able to dial it back so that it output exactly enough voltage to spin the motor at 1RPM. The downside to this system is that I am now constrained to a wall outlet, but it works out ok for me taking pictures in my backyard. I imagine a more portable solution would involve bigger batteries with a greater voltage overhead, that you can adjust on the fly to make sure the voltage going to the motors is constant. 

Results

Following these changes, I tried again another night. This time, the polar alignment was far easier and faster with the scope. I was able to acquire Polaris very quickly, and offset it to more or less true polar alignment in a snap. The motor speed needed a little fiddling to get it exactly spinning at 1RPM, but once it was dialed in, I could simply unplug it and leave it set for next time. I think the results speak for themselves, albeit with terrible post processing because I haven't quite figured out that step yet.


M31 taken with my stock D3100 and the tracker

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