Double Arm Barn Door Tracker. For some of my longer exposures I use this device, which can track the motion of the stars across the sky while the image is being made. To use it the tracker is set-up in the field so that the hinges are aligned with the Earth's axis of rotation. The 6x30 mm finder scope is parallel to the hinges, so it can be used to centre Polaris and make the alignment. The crank on the far side is turned at one RPM which lifts the board with the camera at the same rate as the Earth rotates, and the camera follows the star motion.
The design of this tracker is called a double arm because of the two boards that are lifted as the crank is turned. Single arm designs (with the camera mounted on the board directly lifted by the crank) also work, but they lose tracking accuracy after several minutes. The double arm design remains accurate for much longer. The lower board is also hinged near the crank so that it can keep in line with the upper board as it slowly swings back during the exposure. A swivel ball tripod mount allows the camera to be pointed anywhere in the sky.
It seems strange that this contraption of scrap wood, a carriage bolt and some hinges can make long exposure astro photos possible, but it works remarkably well. Here's an example of a twelve minute exposure made using the tracker.
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