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


The actual value of the ROC can be fairly inaccurate as to length (go play
in TEX for a while and see what the change is for a set of readings!).  I
use the technique of laying a tape measure behind the tester and mirror (I
do my testing on a long workbench and the access is from the side or the
testing apparatus) and put the 0 mark at the tester KE location and let the
mirror fall where it may for getting readings of the right location.  After
getting the location for the central zone, a look across the edges of the
mirror will give the ROC to 1/8" or so and is quite sufficiently
accurate for most work.  If you need more accuracy, having a second parson
hold the tap to the KE and you measuring to the edge of the mirror will
give the ROC as accurately as you can measure it.  Note that you want the
tape on the working side of the KE, not the backside and the measurement
will have no trig errors as the edge of the mirror will be at the same ROC
as the center is - no compensating the measurement from the edge to where
the center is supposed to be.
FWIW, I also support more than 3 zones for measuring.  I recently tested a
mirror with just the odd zone mask and came up with something that was
close to 1/10th wave but once the odd zone mask data was included, the
quality of the mirror dropped to only 1/5th wave!  The problem was that the
mirror was zoney and just measuring the surface at the zone points gave an
incorrect result.
Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

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