Re: [ATM] Crazy simple mirror cell idea

["Olivier Biot" <olivier.biot@gmail.com>]

On Jan 9, 2008 11:30 PM, Mike Carambat <mcaramb@rocketmail.com> wrote:
> Finally building my 14" scope with a primary of
> moderate thickness (about 1.75"). Biggest scope I've
> built so far is my 10". Now, I've found that these
> larger mirrors need more complex mirror cells (9,12,18
> points, etc)... to keep from warping due to gravity,
> pressure, temperature, etc..

You will do fine with a simple 6 point flotation cell - even with a
mirror as slim as about 1"  for a diameter of 14".

> I'm just wondering why you can't just insert one into
> a 14.5" diameter, 2" high x 1/8" thick cylinder, with
> a wooden base, lined with foam rubber (I suppose,
> kinda like that memory foam which adjusts to fit your
> shape, giving equal pressure to all points).
>
> Cut three large air holes in the base for air
> circulation, then put in your mirror. Fix the
> arrangement into a regular mirror cell system. DONE.

Some scopes have no cell but only a bubblewrap mat between the mirror
and a flat wooden plank. One notable example is the UK built "Dark
Star" dobsonians which perform remarkably well. However I would advice
against the principle of a retaining ring for a couple reasons (note
that I never did one so it's more a mental exercise).

> Sure, it would fall out if turned upside down and
> shaken, but if the right size and tightness, in
> general it would be quite stable with the foam on the
> sides providing friction to hold everything in place.
>
> The cylindrical band would be sturdy enough to not
> allow deformation when the scope is near horizontal,
> so hopefully, the force of gravity wouldn't all be
> focused on the very bottom, which could cause a pinch
> point there. All in all, I'm assuming about an 1/8" of
> foam around the mirror and it's base would suffice to
> give enough play for potential expansion.

It'd be interesting to see how a couple of readily available foam
materials would behave when used in this type of setup. Apart from
insulating the mirror I only see loss of collimation due to the play
of the foam as being a potential show killer.

> My only thought is that the friction from the foam on
> the base may be uneven,  causing "hang points" which
> wouldn't allow the mirror to move during expansion at
> those points, thus causing pinch points. This is the
> apparent drawback with using silicon glue to hold
> mirrors down. It doesn't allow the mirror to move at
> those points. My idea would allow movement and equal
> friction everywhere (hopefully).

The silicon has another other effect that was computed a couple months
ago on other boards. From these computations it appears that bonding
your mirror to the cell with RTV (silicone caulk) yields the worse
lateral support. If your mirror is sufficently stiff (read: thick)
this should of course not be a problem ;)

Cheers,

Olivier
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