Barndoor Mount
Barn door EQ mount
The target I had in mind was to build the smaller and lighter barndoor equatorial mount possible. I wanted to bring it to a travel in South America where I was very limited on the weight and volume of my luggage. This design is not the best combination of size and precision you can build. The target was really to reduce the size to a minimum ... even if I had to loose a bit of precision.
The biggest challenge has been to keep the whole thing balanced. In longer barndoor mounts, the camera sits in the center of the upper arm. This is not possible with a smaller model because of the center of gravity of the camera. When positioning the camera in the vertical position, it's center of gravity was moved outside the mount limits. The worst case was when the camera ended on the opposite side of the hinge, creating a torsion force that was pulling on the rod instead of pushing on it. The solution, as displayed on the two first pictures, has been allow the camera to be attached at both ends of the upper arm.
To enable vertical positioning of the camera I added a removable extension to the base of the ball head. The extension is not required when using the camera in landscape position. The third picture displays the camera mounted without the extension.
The hinge is a short steel tube that fit thigh in a 13/32" hole. This allows polar alignment without the need of an extra polar scope. This is more accurate than a standard hinge to align on the north pole.
Following the advises of George Anderson, a member of the APML, I decided to go with the curved rod model. This allows longer exposures than the straight rod and is easier to build than a type 3 or type 4 barndoor mount. I target exposures between 5 and 10 minutes using a 50mm lens. The size of the mount results in a rotation speed of 1/2 turn per minute. This mean I have to turn the bolt 1/8 turn every 15 seconds. This is where I loose precision compared to a longer mount where I could use a 1 turn/minute speed.
The camera tripod is fixed into a 1/4 nut on the bottom of the mount. The nut is a special kind that can be screwed in the wood. The outside of the nut is threaded like a wood screw and can be screwed using an allen-key.
Finally I added a red LED powered by two AA batteries to be able to read my watch and run the mount simultaneously in the dark. It is attached to the mount using Velcro straps.
Dimensions: 7" x 3.5" x 1.5"
Weight: 1 pound, including the ball head and the batteries.
Tripod: The tripod is a Compact Slik (1lbs5oz, 14" folded)
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