{"id":727,"date":"2011-08-13T18:14:43","date_gmt":"2011-08-13T17:14:43","guid":{"rendered":"http:\/\/dianajuncher.dk\/astrophoto\/?page_id=727"},"modified":"2011-10-14T21:16:06","modified_gmt":"2011-10-14T20:16:06","slug":"barn-door-tracker","status":"publish","type":"page","link":"http:\/\/dianajuncher.dk\/astrophoto\/articles\/barn-door-tracker","title":{"rendered":"Barn Door Tracker"},"content":{"rendered":"<p>A barn door tracker is a simple, homemade device that can be used to track the stars as the Earth rotates, thereby making longer exposures possible. It is a very cheap alternative to an equatorial mount.<\/p>\n<p>Several types of barn door trackers exist. In this post I will just describe the basic single-arm barn door tracker as this is the one I intend to build. It can be made by attaching two pieces of wood together with a hinge. A camera is mounted on the top board, while the bottom board is fastened on something stable like a tripod. The angle between the two boards can be adjusted by turning a threaded rod or bolt.<\/p>\n<p><img loading=\"lazy\" class=\"aligncenter size-full wp-image-528\" title=\"Single-arm barn door tracker\" src=\"http:\/\/dianajuncher.dk\/astrophoto\/wp-content\/uploads\/2011\/02\/bdtracker.jpg\" alt=\"\" width=\"300\" height=\"205\" \/><\/p>\n<p>Like the equatorial mount, the barn door tracker needs to be aligned properly with the celestial north (or south) pole. The boards are then driven apart (or together) at a constant rate following the stars. This kind of drive is called a tangent drive and the angle between the two boards is given by<\/p>\n<p style=\"text-align: center;\"><img src='http:\/\/s0.wp.com\/latex.php?latex=%5Ctheta+%3D+2%5Carcsin+%5Cleft%28%5Cfrac%7Ba%7D%7B2r%7D%5Cright%29.&#038;bg=ffffff&#038;fg=000000&#038;s=0' alt='\\theta = 2\\arcsin \\left(\\frac{a}{2r}\\right).' title='\\theta = 2\\arcsin \\left(\\frac{a}{2r}\\right).' class='latex' \/><\/p>\n<p>The single-arm barn door tracker extends the possible exposure time from about 20 seconds to about 5 minutes for a 50mm lens. For longer exposures a tangent error becomes evident. This happens because the hinge angle <img src='http:\/\/s0.wp.com\/latex.php?latex=%5Ctheta&#038;bg=ffffff&#038;fg=000000&#038;s=0' alt='\\theta' title='\\theta' class='latex' \/> does not change at a steady rate (only the length <em>a<\/em> does). In the beginning the camera can track the stars almost perfectly for several minutes, but then it gradually speeds up, &#8220;overshooting&#8221; the stars and causing their images to trail.<\/p>\n<p><a href=\"http:\/\/dianajuncher.dk\/astrophoto\/wp-content\/uploads\/2011\/02\/error.png\"><img loading=\"lazy\" class=\"aligncenter size-medium wp-image-555\" title=\"Tangent error\" src=\"http:\/\/dianajuncher.dk\/astrophoto\/wp-content\/uploads\/2011\/02\/error-300x210.png\" alt=\"\" width=\"300\" height=\"210\" srcset=\"http:\/\/dianajuncher.dk\/astrophoto\/wp-content\/uploads\/2011\/02\/error-300x210.png 300w, http:\/\/dianajuncher.dk\/astrophoto\/wp-content\/uploads\/2011\/02\/error.png 600w\" sizes=\"(max-width: 300px) 100vw, 300px\" \/><\/a><\/p>\n<p>The most basic barn door trackers are manually operated, but an electric motor can be added to automate and improve the accuracy of the tracking process.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>A barn door tracker is a simple, homemade device that can be used to track the stars as the Earth rotates, thereby making longer exposures possible. It is a very cheap alternative to an equatorial mount. Several types of barn<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":713,"menu_order":4,"comment_status":"closed","ping_status":"closed","template":"","meta":{"ngg_post_thumbnail":0},"_links":{"self":[{"href":"http:\/\/dianajuncher.dk\/astrophoto\/wp-json\/wp\/v2\/pages\/727"}],"collection":[{"href":"http:\/\/dianajuncher.dk\/astrophoto\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"http:\/\/dianajuncher.dk\/astrophoto\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"http:\/\/dianajuncher.dk\/astrophoto\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/dianajuncher.dk\/astrophoto\/wp-json\/wp\/v2\/comments?post=727"}],"version-history":[{"count":4,"href":"http:\/\/dianajuncher.dk\/astrophoto\/wp-json\/wp\/v2\/pages\/727\/revisions"}],"predecessor-version":[{"id":737,"href":"http:\/\/dianajuncher.dk\/astrophoto\/wp-json\/wp\/v2\/pages\/727\/revisions\/737"}],"up":[{"embeddable":true,"href":"http:\/\/dianajuncher.dk\/astrophoto\/wp-json\/wp\/v2\/pages\/713"}],"wp:attachment":[{"href":"http:\/\/dianajuncher.dk\/astrophoto\/wp-json\/wp\/v2\/media?parent=727"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}