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Tracking the stars

Listening to a photography podcast last week, I heard a segment about night photography and they mentioned star trackers and diy star trackers. I did my research and decided to make a motorized barn door hinge start tracker.

Its a pretty simple design. The jist of it is its a hinge that opens at the same rate that the earth rotates. The jist of the math is its a lever that a point 23 cm or 11.43" from the fulcrum should travel at a rate of 1/20" per minute. Sounds complicated doesn't it.

Rotating a nut around a 1/4-20 threaded rod at a rate of 1rpm accomplished that travel rate. The threaded rod has to be bent at a radius of 23 cm. I bought a set of gears with a ratio of 3:1 and a DC motor that rotates at 3 rpm along with mounts for the motor.

I made a simple hinge out of 2 boards and a door hinge. Drilled a hole through both boards at 23cm from the fulcrum point of the hinge. Bolted the end of the threaded rod to the top board. I bought a hub for the gear I got that had a 1/2-20 threaded center. With the gear on the threaded rod I positioned the motor and its gear and figured out where it needed to go.

After the motor was attached I found out that at 12 V the motor spins more at 3.5 rpm. I initially was going to use a potentiometer to adjust the speed of the motor but I found a Pulse Width Modulator on amazon for a couple bucks and wired that to the motor to dial the motor in. I calibrated it so the main gear rotates exactly at 1 RPM and was ready to test it out on a clear night.

On a clear night I decided I would try it out in the backyard. When I tried it out the first night I aligned the pin of the hinge toward the north star. Turned it on and recalibrated the gear and snapped a few photos. I eventually just picked on an area of the night sky and took 10-15 mins of minute long expusures over and over again. I figured I should be able to over lay them and the stars will always align on top of each other.

It looked like on my LCD screen it was a success. It wasn't until I uploaded the files to the computer that I realized that my attempt falled. The stars were moving in each photo.

I plan to add a finder scope with either a cheap red dot or with a small piece of tube I can look through to make it easier to align with the north star. I might also attach a cheap compass to it. Looking forward to trying it out again and trying to master it and capture some good photos.

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