[ATM] Figuring from Center

["Jason Hissong" <jhissong@gmail.com>]

Hey guys,

It has been a while since I last posted.  I am working on another fast
mirror: a 12.5" F4.3 BVC mirror and I had some questions about figuring from
the center.  I had read from various sources that with fast mirrors,
figuring from the center is advisable (essentially using the edge as the ROC
you are shooting for).  Especially if you are using a sub-diameter tool.  So
I have started to do just that.  My tool is a 6" tool using soft acculap
pitch.  Here is where I am at so far:
Zone : Ideal : R-R1: Correction
1: 0 : 0
2: 41 : 37 : 111%
3: 73 : 93 : 33%
4: 108 : 63 : 29%
5: 143 : 82 : 54%

As you can see, the slope between zone 1 and 2 is over corrected.  When
figuring from the center, how does one avoid the large correction in the
center of the mirror using a subdiameter tool?  I am thinking my next move
is to overhang the tool so that the edge of the tool concentrates on the
area between zones 3 and 4.  The other edge would not be on the mirror.  The
correction between zone 4 and 5 will flatten out because the tool will be
over those zones the most.  I am hoping this synchs up the outer zones.  If
the tool does not touch zone 2 at all, does the correction stay the same
between zone 1 and 2?  I am thinking it would because the tool working the
outside zones like that will essentially wear the outer zones down to meet
up with the overcorrection in the center.  Correct me if I am wrong :)

The mask that I am using was created by using the Couder Mask program.

I am really close to figuring this all out... I think I need a few more
steps in my mind to have a better understanding of it.

Thanks!

Jason Hissong

BTW.. it is good to be back on the ATM list.  I am working on a 12.5"
ballscope similar to the 8" one I built a couple years ago.
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