Re: [ATM] Spherometer & Hogging with Diamonds

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

Ah, yes, the ring will sit on three points but the thing that is
nice is that those three points will be anywhere on the ring hat
thappen to be the points on the glass.  The diameter of the
spherometer will be held valid with any of those three points so
it is possible to put the three points all on one side of the
measuring point and still have a valid reading.  A three point
spherometer, on the other hand won't allow for the measuring
point to be all the way out at the edge of the glass as the three
points are, at best on a chord with the measuring point in the
case of a two point spherometer with a vertical support point out
at 60 degrees from the beam.  Most of us tho do build the three
point spherometers with the three points at 120 degrees from each
other which means that the measuring point is even further from
tthe edge of the glass when two points are at the edge.
If all you are doing is checking to see if the sphere is the same
out to the edge, the 3 point will do the job but the accuracy of
what the error is is decreased by the two points not seeing wha
tthe surface further in is.  All a bit technical but that is the
way that it is.
If you are really interested in measuring the edges of the glass
accurately, you can always build a 3 point spherometer with the
three legs on one side of the measuring point but that isn't
really a stable instrument unless you add some weight so that the
sphderometer stays on it's three points when sitting there.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net


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