[ATM] Is this Sphere good enough?

[Don Bates <dbates59@yahoo.com>]

I am in the process of re-figuring a 10" f/6.6 x 7/8"
thick plate glass primary. It originally had a bad
turned edge which I have pretty much eliminated as
best I can tell. I have read several posting regarding
the importance of reaching a good sphere before
attempting to parabolize. But exactly what "good"
means is unclear to me:
"We now have a rough sphere, either slightly oblate or
prolate with a decent edge and a very low angstrom
roughness. It has been checked for figure of
revolution (astigmatism) and is ready for the figuring
machine. "   - Carl Zambuto

"The mirror does not have to be wonderfully spherical
before we start parabolizing, but it does need to be
regular. If the mirror is irregular, with let's say a
high center, low 50% zone, and high edge, then we
continue our smoothing by lengthening our stokes
slightly"  - Mel Bartles

I ran FIGURE using a 5 zone mask with a Sphere as the
reference with the following results:
P-V Error - 1/2.75
Strehl Ratio - 0.825
Surface RMS Error - 19.3

The graph shows a low center zone, with a high peak at
the 45% zone that slopes down at the 75% zone, then
dips back up to the edge. The Foucualt shadows show a
very shallow hole in the middle, and an otherwise
featureless surface. The Ronchi seems to indicate a
very slight turn at the edge (turned UP?). 
Here is my question: Is this sphere good enough to
start parabolizing, or should I continue to try to
improve the sphere? When running the same numbers
against a Parabola I get the following:
PV Error 1/1.25
Strehl Ratio 0.054
Surface RMS 63.5 
The graph indicates a high center and high edge, with
a low dip around the 70% zone. Therefore, it SEEMS
like I need to lower the center and the edge
(parabolize). Am I thinking clearly here, or
completely off-base? Thanks for the feedback!

D Ray
dbates59@yahoo.com




		
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