Re: ATM Interpreting my readings help needed please......

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

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@nethere.com
NEW! http://bobmay.astronomy.net