Return to Oz with a barn-door mount

Barry Wolfe

Published in theNOVAC Newsletter Jun/Jul 2003

Last fall (2002) I made a return trip to Australia with my 12.5" portable Dobsonian telescope so as to spend more time under the unique and beautiful southern skies during the Austral spring (Oct/Nov). Using the internet I had researched a number of bed and breakfast spots and found three promising ones to make reservations at. One in the wine country about 4 hours northwest of Sydney, one in the Warrumbungle mountains near the Anglo-Australian Observatory which was about 3 hours further yet and where I attended the Astrofest 2002 star party and, finally, one in the outback which was another 3 hours drive but the dark skies were worth it. In addition to my telescope, I brought along a newly-constructed barn-door mount with which to do some simple astrophotography. For anyone interested in these Australian bed and breakfasts and/or the southern objects that I was able to observe during the eight gloriously clear nights that I experienced, see https://www.angelfire.com/stars2/bwolfe and click on the 12.5".

Here, however, I will focus on how well the barn-door mount worked and what its limitations seem to be. This device can be built with a hand saw, a drill, and a screwdriver. The so-called barn-door mount is named because its basic function is to open like a door. There are many versions and the one described here is the simplest possible. This type consists of two 3/4" thick plywood rectangles (approximately 5"x13") joined at one end with a 4" piano hinge. To give a bit of space between the two pieces of wood, a spacer of 3/4" plywood is glued to the bottom board at the end on which the hinge attaches as seen in the side-view shown below. Approximately 3 inches from the hinge, 1/4-20 tee nuts are placed so that the camera can be attached (to the top board) and the barn-door tracker can be attached to a tripod (on the bottom board).

Through the bottom board, exactly 11.41 inches from the center of the hinge, is placed a 1/4-20 tee nut; and through this tee nut is threaded a 2.5" long 1/4-20 bolt which has a small rectangle (3/4"x2.5") of 1/4" thick plywood attached to the bottom of the bolt, held tightly in place with a nut, to act as a handle with which to turn the bolt. At the top of the bolt is a lock nut, hand tightened so it won't go on any further. The rounded end of the lock nut pushes against the top board when the bolt is turned causing the top board to rise and rotate around the piano hinge at the other end. If the hinge is pointing at a celestial pole, turning the bolt at one revolution per minute tracks the stars fairly accurately for about 10 minutes. The longer focal length of lens used on the camera, the more inaccurate the system becomes. I found that the best results were obtained with lenses of 50 mm in focal length or less. The alignment of the hinge with the celestial pole is aided by putting a finder on the top board. This is especially true in the southern hemisphere where there is no pole star. Note that if you are in the northern hemisphere the hinge is on the left while in the southern hemisphere, it must be turned around and the hinge is on the right. You gain a great deal of flexibility if you mount a camera ball-joint between the top board and the camera. This allows you to point the camera anywhere. Camera ball-joints are available from many places  (e.g. Slik at http://www.slik.com/bhe.html  or B&H at http://www.bhphotovideo.com search for ballhead). If you want to build a really accurate, motor driven, barn door mount, here is a good website to look at; http://www.astronomyboy.com/barndoor/index.html The barn-door mount attaches to a simple camera tripod. The setup is angled (with the help of the finder) such that the hinge points at the pole. The hinge here is on the left, the northern hemisphere version. From Australia, you have to turn around the finder and place the hinge on the right. To track the stars, the wooden knob on the bottom is turned at 1 rpm using your watch to time the minutes. The rectangular pieces of plywood are tapered from 5" at the hinge end to 3" at the finder end, to reduce weight.

So, what can you photograph with a 50 mm lens in 10 minutes? Here is one example of a great object, the Large Magellenic Cloud. The Large Magellanic Cloud (LMC) photographed with a 50 mm lens attached to my 70s vintage Pentax Spotmatic camera. This is a 10 minute exposure at f/1.4 with ASA800 film. The LMC is about 7 degrees long and 3 degrees wide and just chock full of visual goodies, including the Tarantula nebula. For comparison, the full moon would be about this big   o   at this scale. When I tried this same shot with a 100 mm lens, the stars became unacceptably blobby.

What about using a wide-angle lens? I found that my 28 mm lens gave sharp pictures at either 5 or 10 minutes exposure. For example, here is a shot that encompasses not only the LMC (on the left) but also the Small Magellenic Cloud (SMC) on the right (5 minutes, 28 mm lens, f/2.8, ASA800 ). For comparison, the full moon would be about this big:   o This circle is also about the size of the field of view in my 14 mm eyepiece at 118x on my telescope. The LMC and the SMC are smallish galaxies in orbit around the Milky Way. They are nearby (less than 200,000 light years) so the nebulae and clusters contained in them are easy visual objects in a backyard telescope. The LMC, in particular, is so rich with objects that you could spend weeks on it alone. The bright star in the upper right is Achenar, the head of the river (Eridanis).

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