Re: [ATM] Sling study

[Jarvis Krumbein <jkoptic@juno.com>]

On Wed, 6 Dec 2006 19:55:34 -0700 "Tom Krajci" <tom_krajci@tularosa.net>
writes:
> >From: "Dan & Barb" <sunshine92@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.
> 

The following oridinated with Richard Schwartz and was on the atm_free
group.  

(Note from JK- I have made an 7-1/2" 5/8" thick mirror from one of
Richards slumped blanks that turned out very well.  Richard has been
playing with glass for a long time and the following might be a guide as
to what's realistic.)

Every time you double the area, you must double the thickness.   So you
can imagine the series of numbers, 4, 5.6, 8, 11, 16, 22, just like
camera f-stops, each requiring double the thickness of the previous size.
  This keeps the "span" constant.   Since I was able to do an 8" x 3/8",
the following table results:

Diameter            Thickness
-------------            ---------------
   4"                      3/32"
  5.6"                    3/16"
   8"                      3/8"
  11"                     3/4"
  16"                     3/2"
  22"                     3"

This table is based on my experience with one thin mirror.  Note that Mel
Bartels once did an 8" x 1/4" mirror, so his thicknesses would be about
2/3 of mine.   I am not that good at glass polishing.  But anybody out
there can do what I can do with my modest skill level.   

Also, note from the above table that for a given focal length, the
required thickness is closely related to the sagitta.   That leads to the
following table, independent of mirror diameter:

focal length          sag/thickness
----------------           --------------------
    40"                        0.26
    50"                        0.21
    60"                        0.18
  100"                        0.11

The effective thickness of a mirror that has a flat back and a concave
front will be something between the maximum and the minimum thickness.  I
propose a "W.A.G."  ***  that the effective thickness is half way between
these two numbers.   So, the consequences of this second table is that if
your focal length is less than 100", it is a good idea to slump your
glass, and this does not depend at all on the mirror diameter.   (Based
on Richard's skill level with a span= d^2/t of 170".  Obviously Mel could
go to a shorter focal length without slumping, a beginner would need
slumping at a longer focal length.)
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