Home made kit:

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An all brass 2" Crayford focussing mount:

I though this might amuse those with experience in working with brass. The original idea was to make a very heavy focuser for my 6" refractor. The intention was to help the balance of the telescope tube which is very nose heavy.

I threw the whole thing together entirely from brass sections found in my workshop scrap bin. This eyepiece focussing mount weighs a ton but offers smooth movement. When I first made it the focussing tube tended to roll downwards under its own weight. So I fitted a length of plastic fuel tube on the drive axle for more grip. This quickly grew flats so had to be removed. Further tightening of the spring adjusting screws eventually solved the problem once I fitted the flared brass plugs which sit in the end of the springs. These threaded plugs are self locking to the springs allowing the screws to be adjusted without slip and without removing the focussing mount from the OTA.

I copied the form of the hand-wheels from an old brass microscope using a home made radius bar with round nosed cutting tool in the lathe. The knurling wasn't a huge success and should have been straight cut rather than the diagonally crossed pattern the borrowed knurling wheels actually produced. I couldn't get remotely enough pressure to make a clean and deep imprint on the yellow brass.

The focuser has been knocking about in my unheated workshop for ages so has lost much of its original shine. The 2" Vixen rack and pinion focuser fitted to my 6" refractor has served its purpose well enough. Though I would have preferred an alignment adjustment in its supporting baseplate. Not a difficult task. Just another of those roundtoit jobs.

Here's a 5" (125mm) achromat doublet which I made many years ago. It uses BK7-F2 (crown and flint) in an approximately equi-convex crown element design. The fourth surface is slightly convex. It was used oiled and taped. I wrote my own BBC Basic ray tracing design software to optimise the colour and spherical correction. The orange stain of the Cerium oxide polishing powder is still visible on the edges of the lens. I also made mirrors from 4" up to 16" back then on various simple reciprocating grinding and polishing machines. It would have been far quicker and quite possibly better to have worked entirely by hand but I liked building and using my machines. Far more than getting slowly dizzy walking round and around a barrel or water filled oil drum.

My old spherometer has recently been upgraded with a Mitutoyo digital head bought from eBay to replace the old Imperial and Metric mechanical dial gauge heads I used back then. The lens really deserves a new cell and tube. The original tube was rolled from aircraft ply around a series of turned baffles. Beautifully light and stiff, it survived for years until the damp eventually got into the laminations. The push-pull cell was made from simple rings of plywood with aluminium rings front and back. All sawn out and smoothed with hand tools.


Here's an old snap of the 5" achromat in a 6" PVC drainage tube behind a 12" which I also made around that time. Both use Dobsonian mountings. Named after John Dobson, who popularised these mountings, they were a relatively new concept back then. They used tabs of PTFE (Teflon) riding on Formica sheet for their azimuth bearings. PVC pipe and spaced strips of PTFE were used for altitude bearings. These mountings provided incredibly stable, buttery smooth movement at very low cost compared with even the very cheapest commercial mountings.

I made the entire 12" at cost for a friend of a friend under heavy pressure to finish it. The plate glass mirror was far from perfect under the knife edge test but gave a decent view of the moon before I parted with the completed instrument. It taught me that one shouldn't waste time on plate glass mirrors in my unheated workshop. Nor try to make a living from telescope making.

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