ATM armchair theorizing.

[Charles Mitchard <charlesmitchard@iinet.net.au>]

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.