Re: [ATM] Astigmatism on thin blanks

[Nils Olof Carlin <nilsolof.carlin@telia.com>]

Mark,

>>>If you test with Foucault a sling will do as the test stand sag is 
across the horizontal.[...]
The thin mirror folds over because it can.  (OK so far?)

This seems to be a very popular idea. It has been challenged by 
Schwesinger, someone who has apparently taken pains to figure it out 
(see Wilson's Reflecting Telescopes - I am ever grateful to the member 
who once sent me a copy of those pages!), and a 180 deg sling, if doing 
its job properly, does not introduce astigmatism. 

I like to explain this by the sling squeezing the mirror side-to-side, 
thereby neatly compensating for the top-bottom sag. The short of it is 
that neither the sling nor the 2-point 90-deg support introduces 
primary astigmatism, but the sling is better as far as the higher-order 
deviations are concerned.

Recently, Robert Houdart and also Mauro daLio have done FEM 
simulations that verify this:
http://www.cruxis.com/scope/mirroredgecalculator.htm

The trouble is of course making the sling (or for that matter the 90 
deg point support) behave as intended when in a practical application. 
One crucial factor is supporting at the plane of the COG - not so 
easily done on a fully vertical test stand. If you tilt it ever so 
slightly to face upward, you need a proper back support (the idea of 
supporting a flat backed mirror on a flat support has been discussed 
previously) - perhaps not dimensioned to support the mirror when 
horizontal, but nonetheless a floating support, to make the forces that 
support the back and edge respectively *fully* ortogonal - if not, 
there will be bending.

Nils Olof
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