A Smart Barn Door Drive for Astrophotography

Posted on Feb 7, 2014

A tracking mount for trusty Minolta SRT-101. My untracked photos of Hyakutake had not done the comet justice; the exposures were too short. If one mounts a camera to a sturdy tripod and opens the shutter for several minutes, the developed picture will be trailed, with every object in the frame creating a short or long streak depending on the length of the

A Smart Barn Door Drive for Astrophotography
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exposure and the part of the sky being imaged. Because of the Earth`s motion, the positions of the stars shift during any long exposure, thereby limiting the practical exposure time for a fixed camera. This limit works out to around 30 seconds for a normal wide-angle lens. Of course, very long exposures exploiting this effect have their own artistic appeal, but To counteract the Earth`s rotation and allow longer exposures, some kind of drive system is needed. If an equatorially-driven telescope is available, it can provide the tracking with the camera mounted piggy-back  on the side. For beginning astrophotographers seeking a more economical approach, a barn door  mount is a simple way to track a camera for wide-angle pictures. The simplest barn-door drive consists of two pieces of wood, hinged at one end. One piece is mounted to the tripod and has a threaded rod going up through the end opposite the hinge. This drive screw is turned by hand at regular intervals, or driven by a motor, to gradually push the boards apart. If the hinge axis is aligned with the north celestial pole, very near the star Polaris, a camera riding on the top board will have the Earth`s motion canceled out, resulting in sharp images of the sky. This basic configuration is sometimes called a tangent arm drive, because the drive screw and the boards form a right triangle. If the screw is turned at a constant rate, errors accumulate rapidly. A...

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