Re: [ATM] Spherometer Accuracy

[Jeff Rowe <jroweapp@swbell.net>]

Dear Friend
I could be wrong but I believe the only thing that is measured is the 
difference between the the indicator center point and the radius 
support. Put a spherometer on a "flat" or something that is not quite 
"flat" . There is a measured difference. The indicator could care less 
as to the figure of revolution.
Regards
JR
Mutalib Abdallah wrote:

>Ultimately, the accuracy of a spherometer depends on one consideration that 
>nobody has mentioned: the measured surface must be a sphere!   Sometimes it 
>is not.
>
>The spherometer will give the same reading everywhere on the surface, at all 
>possible rotation angles of the instrument, if, and only if the surface is a 
>sphere.   Furthermore, the curvature measurement will not depend at all on 
>the size of the spherometer base if the surface is a perfect sphere.
>
>To characterize a more complex curved surface by sampling it at only three 
>points is nonsense, just like a two-zone foucault test would be.   We 
>frequently encounter seriously non-spherical surfaces during initial 
>hog-out, when we first dig a hole in the center of a relatively flat 
>surface.   Spherometry is not accurate there.   Also, since we have been 
>talking about extremely accurate instruments, we might need to be careful 
>about the difference between a sphere and a parabola on deeply curved 
>mirrors.
>
>(My apologies.  I'm getting out my fire extinguisher.)
>
>
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