[ATM] New? Idea for mirror support

[salguod at minbaritm.com (Doug Williams)]

The thread on mirror support got me to thinking.  That is dangerous, because
I came up with an idea.  But nothing is new under the sun (or stars) and so
I was wondering if anyone had tried something like this before.  Here goes:

The consensus is that a wiffle tree offers great vertical support for the
mirror.  The problem then becomes, how do we support the mirror
horizontally?  How do we keep the mirror from sliding off of the wiffle tree
when we point the scope toward the horizon?  While several solutions have
been tried, the most common answer is the sling, but how about this?

Gluing or RTV?ing the mirror gets a bad rap, because it doesn?t allow the
mirror to expand and contract uniformly.  But what if we glue only the very
center of the mirror?  Just put one pad of RTV at the center to keep the
mirror from sliding off the wiffle tree?  If there is any distortion, it is
at the center of the mirror, which is in the shadow of the secondary anyway.

But wait!  We can get rid of even that!  Glue a flange to the center of the
mirror on the backside.  This flange holds a support post perpendicular to
it.  So now we have a small support post on the center backside of the
mirror that we could (if we were that foolhardy) lift the mirror with.  But
we are not designing it for that much stress.

We now slip this rod through a hole machined in the center of the mirror
cell or tailgate.  The hole has just enough slop to allow for thermal
expansion (very slight, if we use the same material for both the post and
the mirror cell).  Now when the scope is lowered toward the horizon, the rod
restrains the mirror from sliding off of the wiffle tree.  The rod is
performing the same function as the sling, but the stress (if any) is on the
center of the mirror where it is not optically important.  Note that since
the support post is free to travel up and down in the hole, the support post
is providing ZERO vertical support (just like a sling).

Has anybody tried something like this?  Is this just another crazy idea that
won?t work?


Doug