SCOTCH MOUNT

Motorized equatorial mounts allow for accurate guided tracking of a celestial object's apparent movement in relation to Earth's rotation. Mounted on a telescope or upon the mount itself, the camera remains fixed on the subject and can gather light for hours on end from even the most faint, remote objects.

For stunningly beautiful astrophotography, they are worth every cent of their cost which can range from hundreds to thousands of dollars.

But those on a tight astro-budget, needn't despair: Using some spare hardware and lumber it is possible to build a manually-operated tracking device which allows for quite accurate time exposures of nearly a half hour.

The mount shown below was adapted from the Scotch mount detailed in Philip S. Harrington's fine book Star Ware. Outlines for similar trackers, known also as "barn door" or Haig mounts, can be obtained from various other sources.

The Scotch mount, shown here attached to a tripod, is named after its inventor, George Haig of Scotland.


The Scotch mount consists of two 6x12" pieces of 3/4" plywood joined at one end by a 6-inch-long piano hinge. If a 6" hinge is not available, two smaller, carefully aligned hinges will do.

 

Join the two plywood boards with a piano hinge.


Measuring along the center of the bottom board, make a mark which is exactly 11.42" (290mm) from the center of the hinge. There, drill a hole to fit a 7/8" coupling nut. Hammer in the nut and, through it, thread a 4-inch 1/4-20 bolt. The upper board will ride against the bolt head.

 

The 4" bolt extends through the coupling nut which is positioned in the bottom board,11.42" from the center of the hinge. The board shows scars from a tee nut I originally used but removed in favor of the coupling nut.

It is necessary to drill three more holes, one through the bottom board where the mount will attach to the tripod, and two more through the top board where the camera will attach to the mount.

In the center of the bottom board drill a pilot hole measuring 0.3125" and pound a 1/4-20 tee nut into the top of the bottom board.

A rubber lid gripper glued to the bottom of the bottom board will improve its grip with the tripod mounting plate surface.

Install tee nuts between the midpoint of the top board and both long edges. Using an adjustable tripod adapter, the camera can be attached to either nut, making it easier to look through the camera viewfinder regardless of where the camera is aimed.

A tee nut is used to adapt the mount to the tripod and the camera to the mount.


Toward the bottom of the 4" bolt which "drives" the Scotch mount attach a clockwheel. A round, door handle backplate, outlet concealer or similar plate from a lighting or electrical fixture will suffice. Check with the hardware store. (Don't even bother trying to explain what you need it for).

Draw a clockface on the plate, then drill a 1/4" hole in the middle of the plate and attach it to the end of the bolt, "pinched" between a pair of 1/4" nuts.

 
Photo shows the clockface and a small metal plate positioned to reduce friction where the bolt head meets the top board. A rubber band lightly stretched across screw eyes at the edge of the boards will prevent them from swinging apart unexpectedly.


As the clockface is rotated the specified distance at the specified intervals, the bolt will drive the top board (and attached camera) around the axis formed by the piano hinge at a rate corresponding to the Earth' rotation.

How? It's all in the trigonometry. Don't worry about it—the mount is complete and it's time to try some cut-rate guided exposures.  

 

home