I was inspired by the design in Astronomy mag at the start of '95. The mirror cell had two pairs of push-pull screws ninety degrees apart, and a pierced wooden bead as the central pivot.
I replaced the central pivot with a thick nylon bolt (6mm). It allows a few degrees of bending, but is too short to shear (that is, to prevent the mirror from moving side to side).
I replaced the pairs of push-pull screws with ONLY TWO (customised) screws as follows:
* Take a 6mm screw (about 1/4 inch). * Use a lathe or electric drill + file to reduce the diameter of the last 12mm (1/2 in) to 3mm (1/8 in). * Cut a 3mm thread on the thin part.
Each of these replaces a pair of push-pull screws, passing between the two lumps of plywood. Self-locking nuts (or inserts) are fixed to the wooden pieces - 6mm inserts on the lower, 3mm on the upper. This prevents vibration from affecting collimation.
As a screw is rotated, the up/down movement is simply the difference between the pitches of the two threads.
I've used this on my eight-incher, but it's too soon to say how long collimation lasts, as I only had first light last week.
Regards
Edwin Spector edwin.spector@att.com