Posted by
opticsco on Tuesday, April 21, 2009 11:15:40 PM
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.