Last updated 1st May 2009

The 'BarnDoor' (also known as a 'Scotch' or 'Haig' mount), is basic star tracking 'platform' that can be built using simple wood-working skills and used to mount a camera with zoom lens (or even a small telescope) for astrophotography.

Why a 'star tracking' mount is needed

It is quite possible to photograph the Moon using a standard zoom lens, so long as a tripod and a cable release is used (to eliminate camera shake).

To photograph stars requires much longer exposure times - and unless some means of tracking the stars is used, your photos will show visible 'star trails' (streaking).

The max. exposure time before visible streaking starts to show depends on lens size, f/ number and ISO ('film speed'). A summary is shown below :-

**A full discussion of moon photography is beyond this article, however a 300mm lens fitted to a digital camera (at max zoom) is about the minimum required to get decent shot (filling about 15% of the frame).

The general 'rule of thumb' for full*** Moon exposure setting when using a 300mm lens is the 'sunny16 rule' (1/100 ths second @ f/16 and ISO 100).  This gives speeds from 1/15th sec. (thin crescent) to 1/250th sec. (full moon) at f16, 400 ISO.

It is, of course, impossible to use automatic focus - and you will soon find how difficult it is to focus on the Moon manually. It is thus recommended that you choose the highest ISO possible and then 'stop down' the lens by at least 1 f/ stop. An 'out of focus' result is far worse than any ISO 'noise', so f32 @ 800 ISO will usually be better than f16 @ 400 ISO.

*** for a quarter Moon this means either 1/25th seconds, or f/4 or ISO 400.

Basic tracking platforms

The operating  principle is shown in the Basic 'tangent' drive diagram below (to skip this overview and check out the final design I decided to do for, go to page 2 ). 

The two green 'arms' (planks of wood) shown below are connected by a Hinge. The lower or Base arm is the fixed (or static) arm -  it will be mounted at a fixed angle, 'tipped up' towards the viewer (in the diagram below) as we will see later.

Accuracy of the Basic Tangent Drive.

Close examination of the diagram will show that the Basic 'tangent' drive system suffers from 2 problems. Both are associated with the simple nature of the screw rod Drive system.

1) As the Drive rod is turned, the top arm moves in an arc (as shown by the blue arrows in the diagram above) centred on the Hinge. As a result, the top end of the Drive rod will be 'sliding' against the bottom of the Top arm - and this can cause sufficient vibration to ruin your photos.

This first problem can be solved by the use of pivot mounts (grey blocks, shown in the Isosceles Drive diagram right). In the diagram, the top end of the Drive screw bolt is fixed (eg. via a ball race) into the top pivot block. The Drive screw bolt is threaded through the bottom picot block and turned (manually) from below. As the arms open (or close), the pivot blocks rotate allowing the Drive screw change angle (and avoid any slippage against the top arm)

In the 'full size' system example left, from <web site ref here>  manual operation is replaced with a servo motor (this can just be seen under the pivot block on the bottom arm). Since the Motor is fitted to the bottom end of the screw rod, this now becomes the fixed end. So the Top arm is threaded through a fixed nut in the top pivot black allow the arm to 'ride up' on the rotating screw rod. Note - it doesn't matter exactly where the camera is mounted - it just has to be turned through the correct angle.

2) A more difficult problem is that, if the bolt is turned at a constant speed, then the Top arm will actually slow down as the 'BarnDoor' opens (or speed up as it closes). This means that, even if the system is correctly calibrated at 1 rpm for some specific start position, the longer it is driven, the more the Top arm will be in 'error' at tracking the stars.

The tracking error can be reduced by adding a second Top arm - which is 'driven' up and slides along the first - thus creating a Double Arm (Haig) Drive. However building a Double Arm Drive is no longer the simple proposition we started with. An alternative to Double Arm Drives is to vary the drive rod rotation rate - again not so simple if you intend to 'drive' the system manually.

Arc Drives

Returning to basic principles, we see that both the 'sliding' and vibration caused by the rotating screw bolt (and the drive error) can all be avoided if a fixed, curved rod is used - and movement achieved by turning a Drive 'nut'. In the diagram shown (right) the curved rod is fixed at the top end arm. The arms will be pushed apart as the Drive (nut) running on the curved rod is 'unscrewed'. Any friction between the rotating nut and bottom arm is minimised by fitting (oiled) washers between the (nut) and the lower arm.

The example from <web site ref here> shown left is a 'half size' system. This makes the unit highly portable. Since all the dimensions have been halved, the Drive Nut only needs to be turned at half revolution per minute (the rotating 'nut' is fitted with a card 'wheel' that will need to be turned once per 2 minutes).  Note the fixed angle brackets between the bottom arm and the tripod - this 'tips over' the arms to required Latitude angle (so that the Hinge can be more easily aimed at the North (pole) star). One disadvantage of the half-size design is achieving the required the drive rate. In the full size design, 'one revolution per minute' is simple to achieve - you only need to 'track' movement of the second hand of a watch. Achieving half a rev. per minute is a little more difficult (especially in the dark !)

So much for the basic design proposition - now for the actual calculations ...