[ATM] Sling study

[Tom Krajci]

>From: "Dan & Barb" <sunshine92 at insightbb.com>

>What is the definition of a thick mirror vs a thin mirror?
>I know about the 6 to 1 ratio but at what rario would you be
>in the thin range.

I don't think that's a useful way to approach the issue.  This is merely
labeling a mirror with the name thick or thin.

I think it's more important to speak in terms of mirror performance...in
this case probably stiffness, or perhaps the ratio of stiffness to weight,
or perhaps the amount of image degradation in the mirror as it sags under
its own weight.

Dmitry Maksutov, in his second book (Fabrication and Testing of Astronomical
Optics, 1948) discusses mirror performance in at least two chapters.

One chapter deals with thermal deformations and what materials are
better/worse in that regard.  The ideal material would have both low thermal
expansion, and high thermal conductivity...good luck finding that magic
material.  You'll end up making a compromise.

Another chapter deals with mirror deformations from its own weight, or
externally applied forces.  The ideal material is very stiff, yet very low
density...good luck finding that magic material...at least on an amateur's
budget.  You'll end up making a compromise.

At some point in the book Maksutov tries to combine considerations of
thermal deformation and stiffness, but I don't remember what his conclusions
were.

Anyway, a simple label of thick vs. thin tells me nothing.  A thick mirror
(whatever that is), poorly supported, will not perform well.  A thin mirror
(whatever that is), properly supported, will perform well.

I recommend focusing on performance, not labels.  For example:  what
thickness mirror is needed so that a Pyrex disk, resting on three axial
supports, still provides a good image (Raleigh criteria).  (Note that I'm
ignoring lateral support here, thermal issues, etc.)  If the mirror is thick
enough...you get good image quality.  If the mirror is too thin...image
quality suffers.

See:
http://grus.berkeley.edu/~jrg/MATERIALS/node4.html
Note the last statement:
"All of these materials can thus make excellent mirrors, although they
differ markedly in two important areas: stiffness and thermal behavior"
...which leads us to:
http://grus.berkeley.edu/~jrg/MATERIALS/node5.html
and
http://grus.berkeley.edu/~jrg/MATERIALS/node6.html


How's that for a simple answer?  ;-)

-------------------------------------------
Tom Krajci
Cloudcroft, New Mexico
http://overton2.tamu.edu/aset/krajci/

Center for Backyard Astrophysics (CBA)
http://cba.phys.columbia.edu CBA New Mexico

American Association of Variable Star
Observers (AAVSO): KTC http://www.aavso.org/
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