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Version 1 - simple tangential arrangement. These pictures are mostly of the construction and prototype stage, but you can see from the angle of the tripod head that balance was a problem...
After a while the drive screw became loose, so I tried to fix that and the balance with the next version.
More robust, and it now has it's own tripod. I built in dual-latitude capability using removable vertical struts at the North side in case I took it to Trinidad (No room in suitcase as it turned out). Fine-adjustment for latitude and bumpy ground is done with the vertical screw.
The springs on the stepper are my low-tech solution to an eccentric hole in the big gear wheel - the drive screw stays still and the motor gets pushed back and forth - better than version 1 where the axis was pushed causing errors in tracking.
This version was beginning to work well when the loose cable for the motor gave up the ghost through repeated flexing. As it was a sealed unit and matched the gear a complete redesign was required.
Rather more complex with two hinges for the camera arm, and one for the flexible motor mount, but a lot more stable as the drive screw is securely held at top and bottom. In this version the drive screw rises and the "nut" is rotating with the gear within the rectangular box at the opposite end to the camera.
Thanks are due to Brian Brooks (Mr. Astroparts) for constructing the drive nut to fit my dodgy spec and flakey design. It sits inside the box at the eastern end of the platform and the black drive screw rises up as the nut is rotated by the stepper.
Using this configuration the drive axis no longer forms part of one simple right triangle, but is the base to an isoceles triangle, with the length derived from 2 times the sine of half the angle between the two arms (!).
Flexing in the motor cable is limited by bolting down the connection end, and the somewhat unreliable universal camera mount is abandoned for a home-made two-axis job.
The second piano hinge allows a greater range of latitudes to be accomodated.
Experiments on this version :
As geometry students will be aware, the hypotenuse of the right-triangle extends as the angle increases. In V1 & V2 the wing-nut rides up the drive screw positioned in a slot in the camera arm, but there is some rotation of the nut which reduces accuracy. For this version I hacked (literally) the head-postioning slider mechanism out of a 5 1/4 inch floppy drive - it may be as ugly as a sow's ear but it's motion is as smooth as silk !
Previously I sighted on Polaris down a straight edge near the hinge (the red-dot finder from V2 was never a runaway success), but here I re-used a little pocket telescope - and another floppy disk component! The spring clip was exactly the right size to hold the scope.
If you notice some extra holes - it's not to reduce weight, but parsimonious re-use of the same bits of aluminium angle from earlier experiments. That, and the odd mistake or two!
Due a blunder by the purchasing dept (me) buying Fujifilm instead of Fujichrome I got prints instead of slides so was able to scan a couple to prove this odd setup can work (treated slightly in PaintShop Pro to cut down the overdose of green).
Useful links on the same subject.
