Re: [ATM] Reducing the outer zone in figuring

["Jerry" <wa4guu@verizon.net>]

The Classic method:
Zone 3 growing means 2 is the one that is getting the work. That is from the
sides of the W. It is the parts of the W nearer the center that work zone 3,
and that is what makes correction 3 to 4 larger. So with the "Classic"
Texereau MOT parabolizing stroke you want more stokes near the center of the
W and fewer on the sides of the W. That work on 3 needs to feather across 4
to make a smooth curve across the mirror. This is the normal standard common
classic usual way of figuring a first mirror. Adjusting the concentration of
strokes in the W either more in the center of the W or more in the sides of
the W depending on whether more correction is needed on the outer zones or
the center. It is a very basic method that works rather well for its
simplicity. This method assumes basically a two zone mirror, inner zones or
outer zones.

A slight variation would be a long W about 1 1/2 to 2 inches inches wide
(3/4 to 1 inch each side of center. This is just leaving the sides of the W
out since the center is over-cooked already.

The reason both of these are a little ineffective is that the part that
works the outer zones it is so close to centered that there is no
significant overhang force to get the lap to bite the zone you want worked.
It just makes you so mad you want to press down on the overhang to make it
bite!

And that is a good variation on the Classic method to correct the outer
zones. Put some pressure on the overhang side of the W. Not much just a
little on the narrow W as above.

You can make a pretty good, even a very good or excellent mirror using only
that "Classic" method. But it does lack some direct control over smaller
zones. If it all goes smoothly to the parabola you get a good mirror. If
zones develop the fix in keeping with the Classic method would be to make a
sphere and come at the parabola again.

There are more direct ways. Personally I would use accented pressure TOT.
But not as in the example you point to in Texereau.

Don't do what you just thought of. That will go the wrong way. But when you
say zone 2 you might not be thinking of the right way to work 2. It would be
good if you are wrong about how to work 2 in this case. You really want to
work zone 3. That will make the 111% go lower and the 32% go higher.

I have said what methods I have used for zone correction, such as would work
well (for me) in your present case, but have been told it is too much for
inexperienced beginners to understand or use. Before I say how I would do
what I would do, are you an inexperienced beginner? If so, do you want to be
an inexperienced veteran?


Jerry


-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
David Steinhauer
Sent: Tuesday, May 01, 2007 10:31 PM
To: atm@atmlist.net
Subject: [ATM] Reducing the outer zone in figuring

I'm having a hard time reducing the outer zone of my 6" f/5 mirror.  
I've tried the standard full-size tool parabolization strokes, from full 
width W to 3/4 diameter center-over-center and everything in between.  
The 3rd zone's correction grows and grows, but I can't seem to do 
anything about the 4th zone.

Here are my current Foucault measurements:

zone    R-R1 (inches)  Ideal R-R1    Correction
---------------------------------------------------------
 1          0.000
 2          0.009        0.021       Z2-Z1 = 0.009 =  43%
 3          0.030        0.040       Z3-Z2 = 0.021 = 111%
 4          0.036        0.059       Z4-Z3 = 0.006 =  32%


The center zone is a piece of cake to reduce.  It's the 4th zone that I 
can't seem to correct without doing twice as much to the 3rd zone.  I 
know the next step is to bring it back toward a sphere, but then what next?

If at all possible, I'd like to be able to do it with the mirror on 
top.  I've tried reducing the edge with the tool on top (Texereau's 
stroke C-II on p. 91), but I ended up with a turned down edge that took 
many hours to eliminate.

One thing I haven't tried (that I just thought of) is spending more time 
on zone 2, to keep zone 3's correction under control.  Is this what I 
should do?  Still, I've had a hard time reducing zone 4.

David Steinhauer
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