Re: [ATM] Draw Tube Length

["Don Clement" <don_clement@charter.net>]

Vignetting is a concern of a long "drawtube", particularly with a fast
system and large format ccd camera. Another is baffling. But with a long
"drawtube" there is also the possibility of protrusion of the tube into the
light path. With a design like the crayford  in order to get more travel one
needs a longer fixed length "drawtube". My design has no " "drawtube" at
all. Travel does no depend a longer "drawtube". One can also add baffling
below the focuser in the form of a conical baffle, shaped so as not to
vignette. I use a collapsible bellows instead of a fixed length "drawtube"
that has deep internal pleats which act as a light traps. One can add
additional baffling, but it is difficult to remove the protrusion from a
fixed length tube. The bellows are made from industrial rubber, which has a
flat black interior coating. One of the problems with anodized aluminum
typical of most fixed length tube focusers is that not much light is
absorbed at the longer wavelengths where a silicon CCD is most sensitive. If
there is an anodized aluminum internal surface, (e.g. an adapter) then I cut
sharp V threads on the internal surface. I try to use geometry for baffling
as much as possible and not depend on black coatings for baffling as at some
wavelengths coating that appear black are not black at all.



One more aspect of the long tube is mechanical. The longer the tube, the
more overhang and torque that will be applied with heavy eyepieces and
accessories. Shorter tubes are more rigid for the same diameter with the
load closer in.



Don Clement

Running Springs, California


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