From: "Tom Krajci" 
To: 
Reply-To: "Tom Krajci" 



From: "Nathan D. Spaulding" 

>> If you make a spherometer with a 6 or
>> 8 inch spacing of supports (for this 12 inch mirror example)...you
can
>> center it on the mirror and take readings...then off-center...and if
your
>> mirror is spherical you should get the same readings.

>I agree, though I do not have the expertise to make a spherometer.  You
are
>100% correct!

See:
http://www.megspace.com/science/sfe/i_fa_sph.html
Can you make this?

I made a similar version...out of scrap hardwood...wood screws for all
points except the, ahem, 'precision measuring' screw...which was a hardware
store 1/4-20 bolt, that ran through a hardwood hole that was drilled and
tapped slightly undersized...it was a snug fit.  I could repeatably measure
to about 0.0005 inch on fine ground surfaces.

If you want help with the math, I'm here.  All I need to know is the
spacing of your three in-line points (two fixed, and one of the *outer*
ones is the 'measuring' point).  Please make place the middle point as
equidistant as possible between the two outer points...to 1/32 inch is good
enough to check focal length in most amateur cases.  (That makes the math
easier for me.  ;-)

If you make your spherometer with a wider spread of points it's better for
measuring longer focal lengths accurately.

Hope this helps,
Tom Krajci
Tashkent, Uzbekistan


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