Barn Door Tracker (Scotch Mount)
For wide-field DSLR photography, a Barn Door tracker or 'Scotch Mount' can provide sufficiently accurate tracking to follow the stars. A fixed tripod will only allow about 15 seconds exposure before star trailing becomes noticeable.

A small DC motor drives an M6 bolt at 1.3 revs per minute. The pitch of the thread is 1mm. The thread passes through a small furniture bolt in a perspex housing which raises a hinged platform. The platform or 'arm' will follow star movement for a while but the error in tracking steadily increases because the threaded bolt is straight and provides a tangential motion.

Using the driven arm to raise a second arm improves the tracking accuracy enormously, and in theory it could be possible to get good tracking for 45 minutes.

The camera is attached to the second arm. The hinge position of the second arm is carefully chosen to give optimum tracking.

The second arm hinge is about 5cm to the right of the main arm hinge. A spring and bolt arrangement allows the distance to be accurately adjusted because it is only at one particular distance that accurate tracking is produced.

Gravity holds the second arm against the main arm and there is a sliding contact between the two. To prevent a sideways force acting on the hinge a small block also supports the second arm, with PTFE between the contacts to provide a relatively friction free motion.

The motor is attached by 4 bolts to a perspex square. The bolt can pivot freely so that the threaded rod is always tangential.

Underside view of the motor.
'L' denotes the threaded rod. The motor pivots at the lower end of L. The linkage to the main arm pivots at the top end of L.

The main arm is a 'U' shape, and the second arm makes a sliding contact at the point marked by the arrow.
(Diagram not to scale)
When the four lengths d, a, c and e are optimised, the mathematics predicts a phenomenally low tracking error for the first 45 minutes.

In reality this will never be achieved. The motor will not rotate at a perfectly uniform rate, and the distances cannot be precisely set. The arm hinges and motor pivot would also need to be exactly in line.

Errors of only a few microns have a dramatic effect on the tracking accuracy, but fortunately the image scale of the camera will be of the order of 70 arcseconds per pixel at a focal length of 18mm.

The tracking accuracy over 70 minutes.

Tracking over 49 minutes.

We cannot start from zero because that would imply that 'L' is zero and the motor pivot and arm linkage would occupy the same place. A more realistic starting point starts with L = 23mm

19th October 2010

The first image. 30 seconds exposure with a bright Moon behind the holly tree at the right. Camera lens at 18mm focal length.
Cassiopeia at lower centre. M31 over to the right. Still only 30 second exposure.

A 2 minute exposure. Massive Moon glow and light pollution, but the stars are quite sharp with very little trailing.

The camera at 55mm focal length. The reduced field of view diminishes the light pollution effect. Perseus should be recognisable at the right.

The bright star is Capella. If your eyesight is good you may be able to make out a slight green blur as marked by the arrow.
This is comet 103P/Hartley 2. Clouds prevented me taking a more centred image.

An enlargement. This is not the brightest comet! Distortion in the stars is perhaps due to a coma effect rather than poor tracking, as the comet was right at the edge of the field.