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You won't like my answer:

First, to state the obvious, the mirror thimble should have an extremely
close tolerance to the central baffle. Only a thin film of grease can fit
between. Those who complain of excessive mirror shift might have one of
several problems: 1). Excessive play between the parts due to wear
(inadequate grease) and age, or improper machining from the factory
requiring a total rebuild; 2). A mechanical problem resulting from damaged
or improperly tightened cork and/or silicone seals around the mirror stays.

Some mirror shift (albeit minor in the best at high power) is inevitable and
unavoidable, otherwise the mirror couldn't move. Even Questars have this
problem. A tensioning system can be devised, that would load the mirror
thimble, but this still might not entirely eliminate mirror shift during
focus, as the focusing design in an SCT is an inherently low-cost approach
and the tensioning would induce minor torsional stress. However, it might
work beautifully.

A mirror locking mechanism can be devised which would prohibit the mirror
from shifting position, and it could be integrated with or without the
tensioning system rather nicely, however it is unrealistic to expect that
the mirror can be locked down without causing *some* movement of the mirror,
as we are talking about extremely small (less than a thousands of an inch)
movements beeing detectable at the eyepiece.

This would, however, permit smooth focus and with little or no free play,
and would allow the mirror to be locked into place with a minimum (or no)
movement that is possible, that would resist any outside forces once set and
would be easy to implement. A crayford focuser with micrometer fine focus
would probably be advised as you apparently intend photographic application?
I wonder though, what low levels of vibration you are referring to? In
practice, a target object would be acquired and focused upon and the mirror
locked. Then the micrometric focus of the crayford focuser would be used
along with the ccd software to attain the optimum pixel value. The crayford
focus would also allow for periodic fine refocus adjustment between images
to compensate for thermal changes as the telescope cools without causing any
image shift or loss of centering.

The ideal solution would be to create a three-point focusing scheme which
would focus (move) the mirror on all three internal points of support. They
could be tied together by a belt-driven DC servo motor. It could be a
digital, software-driven or analog, manually electrically operated motor,
and this would drive the mirror focus with inherent stability during focus
and would negate the need for an mirror lock. It may be possible to
simplify the design so that a single focus knob would drive the entire
mechanism manually.

Wayne E. Gondella
AFA Engineering Company
Pittsburgh, Pennsylvania