I've been tinkering over the last few weeks and making progress towards constructing suitable hardware for the secondary mount and focuser.

With the mirrors, I received a secondary fixture with a round aluminium 45, topped with an elliptical brass plate to which the mirror itself is mounted. I epoxied three large washers together, marked, drilled and tapped three holes for collimation bolts and three further holes for the vanes (will use bike spokes). Pending bike spokes and re-coated secondary.

I've knocked up three focusers so far, all Crayford. First used small sections of plastic chopping board planed down to thickness and fixed to vertical blocks of wood - lacked the ability to fine tune. Second used four skateboard bearings mounted on an extruded aluminium section - too tall.

The third attempt is a Helical Crayford type. Four bearings angled at a couple of degrees, around a central hole with a friction screw opposite. Each bearing is secured with countersunk bolts, with two small washers to back the inner race and give the OD of the bearing clearance from the surface. The washers sit in counterbores perpendicular to the angled through holes. I set the angle with a small block of wood under one end of a platform placed on the drill press table - no idea what the angle is. I get something like 1cm travel for a full revolution but I don't imagine that even repeatability is that critical, provided it's secure, friction is more or less consistent and doesn't bind. I should just be able to screw the bolts straight into an appropriately sized through hole without the need to tap or use threaded inserts, and just epoxy the bolts back in if the threads come loose.

The whole assembly should mount to the OTA with three bolts to permit alignment. I can get this down to about 2 inches in height. Plus, it's only something like 9 holes and 4 counterbores. Will make it from two thicknesses of 13-ply birch plywood. One thickness would do if I offset the bearings from each other, but a greater vertical separation of the bearings should reduce any cam-effect in the event that the focuser tube is not 100% round along its length. Planning to use 39mm ID PVC pipe into which I can press fit and align the crappy 2x barlow I got with the Aldi dobson to serve as an EP holder, shortened to ensure no vignetting.

Preliminary design in newt-web with the 43mm secondary and 216mm primary in a 265mm ID tube gives me a tube length of about 1.6m, 20% obstruction, 17.5mm 100% illumination, 31.4mm 75% illumination and no vignetting.

Pictures of the secondary mount and helical focuser to follow.