Poor Man's Crayford Focuser
Written by Toshimi Taki
December 23, 2002
Revised: November 22, 2003
Crayford focuser is considered as a focuser of sophiscated design. Standard design has four (4) rollers to support draw tube and a friction roller to move the draw tube. Building a Crayford focuser seems to need precision metal work. However, if you apply kinematics which Dobsonian Mount is based on, you will have a very simple Crayford focuser. You can build it from wood. It needs neither metal work nor expensive parts.
Simple Crayford Focuser
My design does not use roller bearings to support draw tube but teflon sheets. Friction between the draw tube and the teflon sheets is small enough. I use a friction roller (rubber o-rings installed to a shaft) to move the draw tube. Pressure from the roller gives friction between the draw tube and the roller to drive the draw tube. Metal parts are not necessary.
Photographs of Simple Crayford Focuser
I made this focuser for my 8-inch Newtonian reflector. As you see in the photographs, I kept the design as simple as possible. I found that the focuser worked very well without a rail (shown in figure 3) between a friction roller and a draw tube. Adjust the pressure between the draw tube and the friction roller with the screw. Note that two additional o-rings are placed on both side of the other five o-rings to keep the shaft in the proper lateral position. To prevent the draw tube for falling off, just put a stopper at the end of the draw tube.
Parts for the focuser. 10mm thick hardwood and PVC tube are the main materials. Eyepiece holder is a commercial product. The white tube is a spacer made of paper, which is put between the eyepice holder and the blue drawtube.
Epoxy adhesive is used to bond the wood parts together. Rubber o-rings for 5mm dia. shaft (OD=7.0mm, ID=4.0mm) are used as friction roller.
The focuser weighs 150 grams. The focuser has 26 mm of travel.
Figure 1. Simple Crayford Focuser
Figure 2. Base Assembly
Figure 3. Draw Tube Assembly
Figure 4. Focusing Roller Assembly Focuser