The ASSA Crayford Focuser Mk1.

The Crayford focuser concept (named for Crayford Manor House in England, home of the astronomical society whose members invented it), has taken many years to penetrate the astronomical community at large. Only in the last few years have successful commercial models become available, albeit at a hefty price! This is strange, because the concept, based on kinematic principles, enables one to produce a focuser that works really well yet is tolerant of construction inaccuracies and can be made from scrap materials.

I first tried to sell the idea to participants of the ASSA Transvaal (now Johannesburg) Centre’s telescope making class some five years ago or so. Only Peter Bosman had the courage (he modestly referred to it as "ignorance") to follow the suggestion. Armed only with a hacksaw, a drill and an arc welder, he put together a focuser from scrap bits of flat and angle iron, a chunk of PVC drainpipe and some odds-and-ends scavenged from who knows where. Despite its initial deficit of beauty (it has since been painted and otherwise tarted up) his device worked impressively well. So well, in fact, that others (notably Keith Liddle) followed suit with "junk-box specials" of inordinate ingenuity, using all sorts of scavenged junk (sorry, "junque") in the grand tradition of telescope makers everywhere. Jan Schut then shamed us all by virtually hand-carving from a solid chunk of aluminium a truly remarkable example. In refurbishing his 12-inch, Eric Brindeau later produced a really nice model in brass.

In later years others in the class wished to emulate these feats, but felt that they were insufficiently inventive, lacked the requisite copious junk-box, considered themselves mechanically inept or for whatever other reason preferred to have some set of plans that would essentially guarantee the desired result. Recognising the difficulty of obtaining a good focuser at reasonable cost as a widespread stumbling block to the aspirant telescope maker, and having themselves experimented with variants on the Crayford concept, Andrew Leigh and Walter Baccio (with occasional input from myself) embarked on the production of a suitable focuser design. The design criteria were: minimal cost; use of readily-obtained materials; ease of reproduction; minimal tooling and machining requirements; attractive appearance; and high probability of success (i.e. the damn thing must actually work once you have built it!). About two years of fairly intensive part-time effort went into the project. Many approaches were tried, much experimentation was performed, a lot of discussion ensued, manufacturing costs, techniques and a host of materials were investigated, bucks were spent. And thousands (well, at least hundreds) of ales were sunk.

Now finally the project is complete. Several examples of the "ASSA Crayford Focuser Mk. 1" are under construction in the current class and Eric Brindeau has produced the fine engineering drawings, complete with the stunning 3-D modelled images, that you will find below. Missing from the parts list are the two Nylon washers on the drive spindle, and the two dished spring washers on the pressure plate mounting screws. (I would have corrected these oversights had I the time.)

One variant with which you may wish to experiment, is to replace the pressure plate mounting screws with longer ones, and the dished washers with proper compression springs. It may not look quite as good, but should make the design more tolerant of imperfections in the drawtube. I would appreciate feedback on the results of any such experimentation. A note of caution: although the design concept is very tolerant, it is still necessary to be very careful when marking out and drilling the holes, to ensure everything lines up properly and that the drawtube does not bind. As with all other aspects of telescope construction, care at every step will be richly rewarded in terms of final quality. Should you be considering the construction of a version suitable for 2-inch eyepieces, you will be pleased to hear that the design has been proven to scale well.

I believe that focusers built to the design as presented should be every bit as good as commercial models costing several times as much. It is the product of a great deal of hard work that embodies some innovative aspects which are probably worthy of patents. Here they are being placed in the public domain. The construction is not difficult; the drawings speak for themselves and the photos should help clarify any conceptual difficulties you may have. I encourage you all to go out and build one, but advise that you should probably view one "in the flesh" first, and talk to the constructor to get some helpful hints. Good Luck! And remember to show it to me when you finish. If you have any ideas for improvement, let’s hear them.

I would like to thank everyone who has contributed to the project in any way; this includes all who have merely taken the time to comment. The early pathfinders "with the courage of my convictions" (!) showed us it was all worth pursuing and spurred us on. In particular, Walter, Andrew and Eric deserve a big round of applause for the serious effort and careful thought they applied, and especially for their tenacity in seeing it all through to completion. Well done, all, and thanks again.

Chris Stewart

D R A W I N G S
(will be re-scanned at higher resolution.)