Re: [ATM] Commercially available spherometer with MitutoyoDigimaticIndicator

["Bob May" <bobmay@nethere.com>]

I think that Peter lent you his plastic bodied spherometer.  That
one does have the possibility of meauring oddly at times.  The
mention of the bezel indicates that was the one lent ot you.
I'll also mention that some of the glass used for the reference
flat isn't that flat.  Check both sides of the glass for the flat
surface just to make sure.  Use another piece of glass (even a
window will work if you check to make sure that you've got the
flat surface by measuring the window in several locations as well
as other windows)  to find out what the 0 reference is.
Next is that the coarse grits won't give a very accurate
meaurement as the peaks and valleys of the ground glass will end
up with the spherometer sitting differently than with the finer
grit surfaces where the peaks are less different than the  the
valleys and their distance between them is a lot less.  Also, the
finer grits allow the surface to be a lot more spherical than the
coarse grits where the surface  is quite hyperbolic and you will
measure a lot longer ROC toward the edges of the mirror surface.
Lastly, to prevent the changing of the zero, don't slide the
spherometer across the glass as this erodes the feet and thus
change the zero point.  Rather put the spherometer down and tap
it lightly to settle the feet down.  This will tend to knock the
foot off of any high peak and get you a better meaurement.  If
the rod for the indicator sticks out the top of the indicator, I
also tap that one to make the meaurement more accurate.  It is
very easy to see a change of a thou or more when doing this.
Hope to see you when you come down again to Peter's place@
Bob May
bobmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net
Replace the obvious words with the proper character.

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