To: atm{at}shore.net
From: Charles Mitchard 
Reply-To: Charles Mitchard 


Greetings all

I would like to explore further the observations of Vladimir regarding the
stability or flexure of the glass and mirror making processes.

Caveats - I do not know anything regarding the "numbers" or math
involved to prove or refute any answers but put these ideas forward just as
thoughts emanating from what I hope may be the logical progression of
statements as made on this forum. Conversely they may be just pure garbage
and a waste of time. Please bear with me.

In search of the perfect support.

There is a lot of effort into producing in some instances rather complex
engineering solutions to solve the problems of flexure or distortion in the
support of the mirror when in the telescope. It seems that these systems of
various point load supports are fine (maybe) when pointing at the zenith
but must have different effects upon the resulting image when pointing in
different directions from the original test setup.
If we can calculate the stresses arising from a finite number of these
points (Plop?) to arrive at a computed solution that gives acceptable
results then would it not be better to have more points of support equally
spread out over the whole of the back surface?

If this is the case then why not a million point support in the form of say
short pile bristle carpet where each bristle will only place an extremely
small pressure point before deforming to spread the load further? Maybe
that is where the complex engineering solution could come into play as the
support for the carpet would also have to be very flat and non flexible so
as not to allow the mirror to follow said flex. Mind you would not
2x1" thick ply sheets with say a grid work of 2'x1/4' ply sandwiched
between them to make a honeycomb not be more than stiff enough? Or
aluminium honeycomb?

What about when the mirror is pointing close to the horizon? (a rare
occurrence probably)
If the mirror is stuck to these various supports with silicone does the
weight of the glass give rise to distortions where the back of the mirror
is held firm but the front is hanging down from gravity?

If this is the case then surely this is an instance where a solid edge
support is required utilizing maybe these multi point supports (carpet) so
as not to apply localized pressure points.

If the mirror is free to "slide" on its supports then some sort
of sling/edge support is still required and again one that will not allow
the mirror to lose its collimation and yet still supply adequate non
distorting support.
On its own the sling has been accused of introducing astigmatic distortions
in the mirror due to its weight. What would happen if the mirror was
supported from below with a non-flexing base with a solid ring of the same
material approx 0.25" higher than the thickness of the mirror all
covered with previously mentioned carpet so the mirror slowly sinks into
its support structure? The only way the mirror can fall out is to turn the
cell upside down.
Would neoprene be a better material than carpet? What about small cell bubble wrap?

I do not want to get into the argument of thermal equilibrium yet as I feel
that can be worked upon later.

Mirror grinding.

We can see that small pressure points will produce a visible difference in
the shape of the glass. There must be no "tight" methods to hold
the mirror down when grinding.
There are instances of physically distorting glass with vacuum pull or
distorting harnesses to obtain an otherwise difficult shape. How does glass
distort under pressure? If we say place a constricting band around a
cylinder of rubber from its base to approx half way up and apply more
pressure the cylinder will deform into a for want of a better description a
mushroom shape, that is the center would rise with a drop in the edge and
an outward curve. If a similar exercise were applied to a mirror is this
what would happen also? If this distorted mirror were then ground and
polished in the usual manner would the outer edge upon release of the
constriction rise back up again, possibly converting the inevitable turned
down edge into a turned up edge or better?

Maybe the only answers are to try it and find out but I welcome any replies.

Charles
(who should be polishing but is writing this instead)

I just got lost in thought. It was unfamiliar territory.

--- BBBS/NT v4.00 MP