Introduction

This page is an adaptation of the article which originally appeared in Canopus May 1995. Eric Brindeau produced the fine engineering drawings you see here, complete with the arresting 3-D rendered images.

Focusers built carefully to the design as presented should function every bit as well as commercial models costing several times as much. The design is the product of a great deal of hard work that embodies some innovative aspects. The construction is not difficult; the drawings speak for themselves and the 3-D images should help clarify any conceptual difficulties you may have. While access to a machine shop really helps, it is possible to construct the focuser purely with hand tools.

Drawing quick view

Hints:

• Click on the table headings to display the associated view.

• Go to the Drawings page to get high-resolution images to print

History: The ASSA Jhb. 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 several 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 sometime before 1993. 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) soon 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. Finally the project was complete. Many examples of the "ASSA Crayford Focuser Mk. 1" have since been constructed.

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