The double-arm barn door tracker
Easy methods to construct a double-arm barn door tracker
Sources: Sky & Telescope April 1989 (p436 – p441) [very good]
Sky & Telescope February 1988 (p213 – p214)
Original concept by Dave Trott
A single-arm barn door tracker, pushed by a straight screw, accumulates tangent error as time passes. Most of this error could be eradicated by including a second hinged arm to the usual association. There are 4 varieties of double-arm trackers, every with a unique geometry. A comparability of amassed error (in arc seconds) and building parameters is given beneath:
[Apparently Type 1 is very bad and not worth constructing?? The two Type 4 drives vary in beta. This results in shifting the region of maximum error over the course of two hours of tracking. Beta, the ratio of b/c, is unitless and must be preserved if you use different dimensions.]
Fundamental Anatomy
Double-arm drives have three primary platforms: the bottom board, the drive arm, and the digital camera platform.
The base board is the underside board and hopefully would not transfer round through the course of the publicity.
Each ends of the drive arm are linked to the bottom board. One finish is linked to the bottom board with a hinge that enables the drive arm to swing shut. The opposite finish is linked to the bottom board by way of a threaded rod (the drive screw). The speed at which the drive arm closes is decided by the speed at which the drive screw spins.
The digital camera platform can also be hinged to the bottom board on one finish. The opposite finish of the digital camera platform slides alongside the drive arm. The digital camera platform closes with and slides alongside the drive arm.
The hinges for the drive arm and digital camera platform are parallel to one another.
The drive arm, base board and drive screw kind an isosceles triangle whose base angles change because the drive arm strikes. No matter association is made to mate the drive rod to the bottom board ought to enable the drive screw to pivot because the drive arm swings closed. The diagrams in Sky & Telescope additionally present the drive motor connected to the tip of the drive arm by itself hinged extension to stop binding.
The drive screw should journey at three inches per hour for the above measurements to be legitimate. 1 / 4-twenty rod pushed at one revolution per minute will work.
Parameter Definitions
r = the gap from the drive arm hinge level to the drive screw (measured alongside the bottom board).
b = the gap to the sliding contact level between the drive arm and digital camera platform. The hinge you measure from will depend on the drive geometry since this distance should not differ because the arms shut. For a Kind 1 and Kind 4 drive, b is measured from the digital camera platform hinge level to the sliding contact level. For a Kind 2 and Kind 3 drive, b is measured from the drive arm hinge level to the sliding contact level.
c = the gap between the drive arm hinge and the digital camera platform hinge (measured alongside the bottom board).
beta = b/c
Diagrams
An artist I’m not. I’ve finished one of the best I might…
Type 3 Drive (click to get an almost-to-scale drawing)
Type 4 Drive, Elevation View (click to get an almost-to-scale drawing)
Type 4 Drive, Plan View (click to get an almost-to-scale drawing)
In Conclusion…
I hope this helps. I’ll add the drawing for the Kind 2 drive if sufficient individuals ask, though I relatively doubt it as a consequence of its poor efficiency. Different solutions for this web page are welcomed! Comments about and photographs of a Type 4 drive that I have built can be found in your viewing pleasure.
Clear skies!