> Hi,
>
> With regards to mirror flop, I had an idea and was wondering
> if in fact it is a feasible one.
>
> Recently, I had my 12" completely disassembled and was quite
> surprised to see how small the diameter is of the spring located
> behind the primary mirror. Hence the seeding of my idea.
>
> Would the scope still function with a spring which exerted the
> same amount of force on the mirror but which had a large diameter.
> In other words, a spring mabe 12" in diameter. Seems to be this
> would catch the slack of the mirror/baffle tube evenly around the
> outside of the mirror rather than on a small fraction of it near
> the center which is what allows the mirror to pivot as the scope
> moves in RA. Of course I'm not an engineer, so I thought I would
> throw this one out there for further scrutiny by the experts.
>
> Regards,
>
> Chris Sauer
The problem is that _anything_ that pushes on the mirror, introduces the
potential to generate visible distortions. It is critical to understand just
how little force is needed on a mirror to generate such problems. You could
use a larger spring, by introducing a 'ring' cell behind the mirror, with
perhaps a dozen adjustable mounts (similar to the design of a Newtonian
mirror cell), and then pushing on this. However there is no advantage to
this, relative to simply pushing on the mounting tube, in term of stopping
mirror slop. The central spring does _not_ push on the mirror, but instead
pushes on the outer 'slide' tube, over the baffle tube. It's function is not
really to get rid of mirror shift, but rather to minimise backlash in the
focusser mechanism (a seperate problem), and to keep the focussing action
from being too heavy. The problem, comes about, because of the extremely
small tolerances that are involved. You have a mirror assembly, supported on
a tube, which itself slides over another tube. The outer tube than has an
arm, going to a 'pin joint', on the end of the focusser adjustment rod. If
you visulise the situation with the focusser arm below the baffle tube, the
entire assembly (of tube, and mirror), will rock, pivoting on the focusser
pin joint, till the outer tube rests on the inner tube, at it's front upper
edge. If you add a pressure spring, the rocking is reversed, with the system
resting on the rear upper edge instead. Repeat with the system inverted, and
the rocking is in the opposite direction in each case. The actual motion in
use, is very complex, forming a rolling action round the inner tube, with
the one fixed point, and with the motion damped by the grease.
The way to reduce mirror shift, is to reduce the gap between the inner and
outer tubes. The 'collet' system on the latter Meade scopes, serves to
tighten the gap at the rear of the tube, and _reduces_ the problem (however
because the front is still loose, there is still slight detectable shift).
The Celestron scopes, are usually slightly 'better' than the unclamped Meade
units (their tolerance on the tube sizes seems to be a little tighter - this
is probably why Meade felt they had to introduce the clamp). The 'travel
bolt' locking systems, work by adding a second fixed point onto the arm,
applying some force to the entire system, and deliberately rocking it in one
direction. Ideally, what is needed, is a tighter collar, both in front of
the mirror, and behind it, but you then have the problem that anything in
front of the mirror, may introduce either diffraction spikes, or if round,
increase vignetting...
It is possible to make Maksutov's, and SCT's, with no detectable mirror
shift at all. The problem is that the low profile linear bearings used, are
very expensive.
It would be possible to make a design similar to the existing units, which
would not add too much to the cost, and would give very good results, by
replacing the screw at the front of the baffle tube with an adjustable
'collet', with an inner PTFE ring, and have a similar adjustment at the
rear. Part of the setup of the scope would then be to adjust both collets
till the motion is as tight as practical, without sticking. The 'downside',
is that it adds another assembly operation (cost), and as the scope ages, it
will become necessary to re-adjust the collets. I have a Maksutov, that uses
such a system, and shows image shift, of only a couple of arc seconds.
One other metod, is to go away from the single focussing knob system. If
instead, you had a knob that turned a ring gear, and three focusser shafts,
each going to a separate arm behind the mirror, the system again becomes
properly supported.

Best Wishes

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