The Crayford Eyepiece Mount (CEM) or Crayford focuser was invented by John (“Jack”) Wall in England. The name “Crayford” comes from the Crayford Manor House Astronomical Society (UK) to which he belongs.
The Crayford design moves the focuser drawtube by applying high pressure on a metal drive shaft that in turn presses against a flat surface machined into the metal focuser drawtube. The drawtube is held in place by sets of bearings on the opposite side of the drawtube from the drive shaft. This direct metal-to-metal and surface-to-surface drive mechanism eliminates the problems found in the rack-and-pinion focusers found on many telescopes, such as backlash, gear slop, and side to side shifting of the drawtube as the focus knob is turned.
The Crayford design allows for very fine adjustments, with tolerances up to 100 times better than conventional rack-and-pinion focusers. Its zero image shift and zero backlash makes it outstanding for visual and photographic work and a must for CCD imaging.
JMI recognized the benefits of this design and was the first company to bring it to the amateur market in a commercial product. Many telescope accessory companies have used the design for add-on focusers since JMI first adopted it and it is now being incorporated into many telescopes as standard equipment, as well.
The illustration shows one of Jack Wall’s original design drawings for the first Crayford focuser.