Astronomical Society of Edinburgh
Journal 40

Cheap Astrophotography - a Variation on the Theme

It was great to see Dave Gavine extolling the virtues of the good old "Scotch Mount" ( A.S.E. Journal No. 39 - Summer 1999, Cheap and Easy Astrophotography). I built my drive as long ago as 1982, but my experience with hinges has suggested a variation leading to a more stable platform and a smoother drive.

Hinges tend to have lots of play in their movement leading to inaccuracies even during short exposures. I have had stars appearing as short corkscrew trails due to what can only be described as a cycloidial wobble in the hinge. In which case how about keeping the Haig Mount with its "magic number" of 229, but have the camera weight bearing down on sliding melamine surfaces, one moving over the other which is fixed. These I cut from an MDF shelf.

Instead of the polar axis formed by the long axis of the hinge, a solid nut and bolt joins the two surfaces. The upper "plate" carries the camera and is driven over the lower by the standard ratio screw at its pre-calculated distance from the centre depending on pitch. By cutting the two plates to the pattern in the drawing, I was able to fit in easily the drive screw and the tension spring. I am sure it can made even neater.

This particular version is designed to be bolted to the top of a solid tripod, but there is no reason why the Haig idea of a table top base made from a solid block of wood should be ignored. This version will not give longer exposures than the classic Haig Mount since it uses the same short screw, but it is very stable indeed with the weight bearing down on the surfaces and the centre of gravity, and there is little worry about the turning surfaces going out of alignment. It will certainly be easier to mount a polar alignment telescope, perhaps even through the middle of the polar axis if this is made hollow.

design for the 'Ford Slider'

Back in 1983 I used my first "Scotch Mount" to photograph Comet IRAS-Araki-Alcock. I got a nice photo but it showed some drive errors which troubled me and set me thinking.

I made my "slider" in one afternoon at a rate of knots, because the sky that day was perfect and Hale-Bopp was to be at its best that night. It was in position by nightfall, but ironically the skylighting was so bright that exposures longer than 10 seconds were impossible, and no drive unit was required

Comparison of the Ford Slider and the Classic Haig mount

Both cameras are pointing towards the Celestial Equator

George Haig wrote a paper in the "School Science Review" September 1974 describing his methods of astrophotography for physics students, and this seems to be where the "Scotch Mount" first appeared. When I spoke to him years later when the term was on everyone's lips, he was amazed, he had no idea that this was now its name worldwide, or that this gadget might be considered another landmark Scottish invention. In this paper, and in a J.B.A.A. paper, George Haig also describes how the stars can be made to "trail" slightly for spectroscopic photography by putting the drive screw in at say 95% of the calculated distance from the pivot. Truly a masterpiece of simplified engineering, but of course it does come from a master.

Harry Ford