From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


Your stroke is a bit long - the histerically proper stroke length is 1/3
total stroke - and this is causing a faster change of curvature than is
needed.  In other words, a longer stroke makes the curve change faster than
a shorter stroke.
As to flipping the mirror and tool, go ahead and do it.  With the longer
stroke that you are used to, the curvature will change fairly quickly. 
Stay that way until the spherometer shows a constant reading over the whole
mirror.  I usually see the spherometer change by about a tenth of a thou.
over the surface when at the 120 grit level.  By the 320 grit level, that
should be to no more than a narrow needle width of change. When flipping,
the best time is at a grit change as this means that the mirror and tool
are in good alignment and both to the same ROC so there won't be a hole or
hill that needs to be gotten rid of. Since you are already at the ROC area,
you will be needing to keep checking the ROC as you proceed.  You can get
the final ROC to within a quarter inch without problems and I suggest that
when you finish grinding that you are about 2x the difference between the
difference in readings of the Foucaut test center and edge longer than you
want.  In other words, if the Foucault will read that the edge zone is
.120" longer than the center on a 120" ROC, you add .240 to the
ROC for the last stage of grinding and that would be 120.24" for the
final reading of the spherometer.  A half of that quarter inch will be
taken up in the inital polishing and the rest will be done in the figuring
as you dig the center out by .12"! Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

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