4.9 Focuser

The focuser is a modified Crayford type. The basic design is similar to the Crayford in that the 3" diameter draw tube rides on four bearings. The modification is that the focus rod has a pinion and the draw tube has rack. The traditional Crayford uses friction on the draw tube. The pinion is spring loaded into the rack in a similar manner to the Crayford. Figure 4.9.1.

  a  b

Figure 4.9.1. the modified Crayford focuser. The draw tube has a rack on it instead of driving the draw tube directly. The 4 bearings that the draw tube rolls on are mounted in a round housing.