Astronomy: A Barn Door Mount

This is an early project, made without some of the sophistication that I can bring to the later projects.  You can make this with some relatively simple woodworking tools, and parts from a floppy disk drive.  The older the drive, the better. 
I'm regernerating the text from memory, so this will not be as detailed as most, hopefully, the pictures will speak for themselves.  The critical dimensions are the hinge to the major arm pivot, and the secondary arm distance to the pivot and length.
Here's the overall project.  The blue box is the stepper motor controller, there's some old (and removed) gell cell batteries, and the driver for the stepper motor is the same floppy board that was in the drive.  Even the sensors came from the drive.   This all came from a 5 1/4 inch floppy. 
I used a piano hinge to make the whole thing sturdy.  The aluminum angle is used to give the bearings a smooth place on which to ride.
The stepper is mounted below (details later) and the drive screw (1/4 x 20) goes up through the piece of oak.  The bearings on the drive allow the nut block to pivot freely.  There's a threading insert in the oak that the brass screw rides in. 
The sight scope (for Polaris) is mounted on a rather kludged together mount that still works well.  Again, most of this can be done with standard woodworking tools....  Some degree of neatness beyond this is recommended.
The inner platform rides on bearing wheels made from more floppy drive bearings.  This moves smoothly, and if the stepper screw is wobbling, there will be less motion transmitted to the camera.  With a good machine shop, you wouldn't have this problem, but the setup actually works around this.
Using a hole saw twice, you make a round section.  It's pivoted the same way the nut is pivoted, so that the stepper motor (mounted below) can maintain a true tangent arm angle to the main arm. 
Detail of the scope mount.  It'll pivot up and down (use a spring for the up and down part) and slotting it does left and right.
You can see the spring for the up and down.  The slot allows the whole bracket to pivot left and right, which gets you the coarse centering.  The crosshair adjust on the scope will take care of the fine adjustments.
Inside this box there's an 8741 microprocessor (very old), it homes the arm (you use the home sensor and a little piece of brass), and runs up about an inch, then starts going slow.  You can select slow up, home, and fast up.  Next to the controller is a surplus lead-acid battery charger.  It'll want 18 volts or so (12 volts AC) to charge the battery.
The original drive board is used for controlling the sensors and the stepper.  Since the controller is wirewrap, there were no printed circuit boards needed for this project.  Replace the terminating resistor from the drive with a 2.2K or 4.7K resistor pack. 
In the foreground is the little brass flag that tells the processor that "home" has been reached.  You'll have to reverse two of the wires to one of the stepper coils to get the motor to reverse itself.  This is because the little controller on the stepper drive board has enough smarts to obey a "home" command.  The drive worked in the opposite direction, so flipping one will fix that.
A small piece of aluminum bracket holds the power switch, pilot, and the power input.  It's polarity protected, so no problems here.
A trifle rough in construction, this is still good for 5 minute plus exposures.  There's too much light pollution here to try for much more, and you're limited to the size lens you can put on the camera.  The more telephoto the lens, the more critical the arm is. 

To align this, you point the axis at a fixed point in the distance.  Move the arm up and down and adjust the scope mount so that the center (crosshairs) point does not move.  More is contained in the other sites that talk about scotch arm (barn door) mounts.  This is more of a construction project.