Re: [ATM] Back Focal Distance design considerations

["Bob May" <bobmay@nethere.com>]

You really don't need that much length in the tube as the bearings for the
Crayford can be only 1/2" (12mm) apart and do the job well.  That makes a
40mm travel tube only about 55mm long, a lot shorter than what you are
thinking.  Remember that the bearings only need to have a flat surface on
which to roll and the tube is flat all the way to the end.  In addition, it
is generally the knob shaft that provides the stop for the travel of the
tube so you only need a short distance beyond that to insure that the
bearings stay on the flat surface.
Long focal length EPs are a problem at one end and sometimes you just need
to put a collar around the barrel of the EP to put it further out of the
focuser to relieve that problem.  The other end, with the camera, is one
that sometimes just needs to use a barlow lens to bring the light out
further without problems.  This is a problem tho if you are intending to
take wide angle photos and the only solution there is to setup the primary
so that you can move it when you want to do photos and live with the slight
degradition of the brightness of the image at the edge of the film when
doing so.  You may lose a half magnitude of brightness at the edge doing
things this way.
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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