To reiterate, aluminum expands and contracts, carbon fiber does not. The
more aluminum the more the difference. Therefore the possibility exists
that the carbon tube might reduce or mostly eliminate one of the major
causes of the need to refocus. However, there are many sources of focus
shift, and you cannot detect them visually as the camera is more sensitive.
What looks sharp to the eye, may not cut it for the camera. When observing,
one refocuses with every eyepiece, person and new object. In imaging, tube
material aside, one is always recommended to check focus repeatedly
throughout the night. Why would go through all the expense and effort for
astrophotos (expecial good ones) and not do a simple focus check every so
often? Cheap insurance.
Sure, carbon fiber might help with focus drift, but it is an expensive
option, it cracks rather than dents, is essentially unrepairable if it does
crack, and is not available on many scopes. Nor is it usually needed. Look
at Ed Graftons pictures. CCD exposures are relatively short, and the
software provides for focus recalibration.
Also, depth of field is greatest at the prime focus (f/11) where the scope
was designed to operate best.
All a moot point, unless you want to custom make your own C14 tube.
Wayne E. Gondella, CGA
South Hills Coordinator
Amateur Astronomers Association of Pittsburgh
AFA Engineering Company
Pittsburgh, Pennsylvania
