The following plans are modifications of Haig's original plans. In keeping with the spirit of a simple equatorial mount I do not intend to offer plans to motorize this simple machine.
Most of the material needed may already exist in a garage or basement. With the exception of the ball swivel head for the camera, all items can be found at a general hardware store. Expect to pay less than $15.00 for all needed hardware. The swivel head will cost anywhere from $25.00 to over $200.00, depending on your tastes.
1- piano hinge cut down to 4 inches or a door hinge
1 - 1/4 X 20 X 4 tangent bolt w/ wing nut at one end
1 - 1/4 X 20 X 1 bolt
2 - 1/4 X 20 T- nuts, internal threads
2 - small nails
1 - tripod swivel head
The top board will have one 1/4 inch hole drilled. Center the hole on the board. The swivel head will attach to the top of this board with the 1/4 X 20 X 1 bolt.
Next, take the Bottom and drill a 1/4 hole centered on the board and attach the T-nut. Ensure the smooth side is facing the bottom. A tripod will attach to this side.
Next, measure from the end of the board 11.42 inches (29cm). This is the distance from the edge of the board to the center of the hole which will house either a 1/4 X 20 T-nut or 1/4 X 20 nut for the tangent bolt to travel. If you use a nut, glue or epoxy it in place. Ensure it is perpendicular to the surface of the board.
The side view looks like this.
Why does this work? The earth turns 360 ° in 24 hours. (15 °/hr or 5 °/20 min.). A 1/4 X 20 (1/4 in. dia. w/20 threads/in..), when turned @ 1 RPM (revolution per minute) travels one inch in 20 minutes.. When a 1/4 X 20 screw is 11.42 inches from a pivot point and turned 1 rpm the triangle's hypotenuse will move at the same rate as the sky.
Place a camera at the same point and it will match the sideral rate. Presto, a hand driven equatorial mount.
Use a fast film such as Fujicolor 800 or Ektachrome 1600.
Use the fastest lens you have (low f/stop number such as f/1.8 or f/2.8) A lens with an f/stop of f/5.6 or f/8 is not a fast lens and not useful on a barndoor.
Focus the lens on infinity.
Once you are aligned to Polaris find your target. Focus on infinity. Open the lens (f/stop) to the maximum aperture. Set the shutter speed to B for Bulb. Automatic cameras will not work for this.
You will also need a shutter release cable to open the shutter and keep it open for a set amount of time. Once you open the shutter begin counting or start a timer you can see in the dark.
Turn the screw 1/4 turn every 15 seconds. You must not jar or move the mount while doing so. Make a range of exposures, three minutes, five minutes, 10 minutes, etc.
The exposure time is limited by the focal length of the lens. Here is a rough guide for maximum exposure times with various focal lengths.
|Wide Angle - 35mm or less||180 ° / 30 sec.||35 min.|
|Normal - 40-50mm||90 ° / 15 sec.||20 min.|
|Telephoto - 70 - 200mm||30 ° / 5 sec.||10 min.|
Practise with the mount. Remember, aligning to Polaris is the most crucial element. While some very exotic and refined barn door mounts exist, the simple hand turned model may be the most challenging and offer the most satisfaction when you are done.
This shot was taken at the 32nd Riverside Telescope Makers Conference using a mount of this design.
This is a ten minute exposure using K-Mart 400 film (talk about being cheap!) and a Tamron 28 - 200 mm zoom lens at f/3.5 (28mm focal setting). The foreground objects were "painted" with light from a flashlight which had a red cover over the lamp. Scorpius is seen rising in the background. Taken May 27, 2000.