Home Made Helical Crayford Focuser

Last Updated May 24, 2002

I chose to make a focuser because I like a challenge and I really wanted to have a low profile design. I wanted a focuser which would be rock solid (i.e. no image shifts during adjustments). I chose the helical Crayford design because unlike standard helical focusers this design covers the entire focus range with one simple turn. The bearings are mounted on a 4.5 degree tilt and drilled using a milling machine in combination with a rotary table. They are 120 degrees apart and the trick was to keep the bearings parallel to the optical axis. Fine adjustments of the optical axis can be made via shims. The eyepiece tube is 2" ID, 2.500" OD. The outer ring is 2.625" ID leaving about 1/16" clearance all around the tube assembly. The edge of the bearings must protrude about 1/16" past the edge of the outer ring. The bottom of the outer ring has 3 screw holes for mounting the focuser to the focus board. The total cost of this project was $50 ($35 of which were the stainless bearings). 

Bill of Material

1pc    AL Tubing 3" OD (focuser tube)

1pc    AL Plate 0.5" thick, 3.5" square machined to outer ring

4pc    SS bearings, 3/8" OD, 1/8" ID (carbon steel OK also)

2pc    1/8" hardened pins

1pc    Teflon pad 1/8" dia, 1/8" long

5pc    SS 6-32 screws 1/2" long (3 for mounting, 1 for Teflon pad, 1 for eyepiece)

                        Top view of the focuser ring containing the bearings and Teflon pad.

A small central mounted Teflon is used to apply force the focus tube to ride along the bearings. A screw directly contacts the Teflon pad while a spring situated between the top of the screw and the outside of the outer ring is used to keep the adjustment screw from moving during transit. The bearings were press fit onto 1/8" hardened pins.

                        Side view of the 2" focuser tube. The focus tube can be made longer if preferred.

                        Close up view of the bearings and on the 4.5 degree tilt.

        View of the complete focuser. Turn the tube to adjust the focus.

                            One turn is about 1.25" of total travel

 

                View Clip              Video clip of the helical motion. CAUTION - this file is 3.2Mbytes in size.

                                            Requires an AVI compatible JPEG viewer such as Quicktime. Can be downloaded from Apple.com