Re: [ATM] Reducing the outer zone in figuring - I did it! My first mirror is done!

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

That is some great looking mirror!

You will have to figure out what it was you did right on the 12.5".

I would guess the center strokes smoothed the correction across zone 2, 3
and 4 and the center work as Jim recommended increased the conic. All
numbers moved in the correct direction.

Jerry

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
David Steinhauer
Sent: Monday, May 07, 2007 10:55 PM
To: atm@atmlist.net
Subject: [ATM] Reducing the outer zone in figuring - I did it! My first
mirror is done!

I don't understand how I finally got the outer zone to cooperate, but I 
did it, and my mirror is done!  According to FigureXP, it is has 1/23 
P-V wavefront error, and a Strehl ratio of 0.995.  Here are the numbers 
(yes, verified again and again and averaged):

Diameter: 6 inches
Focal length: 29.6 inches (f/4.9)

Zone   KE reading    Ideal reading   Delta
1        0.0000         0.000
2        0.0190         0.021        0.019 = 90%
3        0.0395         0.040        0.021 = 107%
4        0.0563         0.059        0.017 = 88%
 
After my last email I did 1/6 to 1/4 diameter center over center strokes 
with the mirror on top and lots of downward pressure, trying to bring it 
back closer to a sphere.  I did the Foucault measurements, and couldn't 
quite believe it, so I did the measurements two more times.  Yes indeed, 
the outer zone correction grew from 37% to 84%.  Can anyone explain 
why?  Perhaps it could be the manner in which I was pressing down on the 
mirror.
 
Zones 3 and 4 were great; I just needed to reduce the center a bit.  I 
did just that, and with a lot of luck, the end result is what you see above.

Here is a knife edge photo:
http://www.atmlist.net/contrib/desteinhauer-at-verizon-dot-net/070507_6inch/
070506-KE-final.jpg

And a Ronchigram:
http://www.atmlist.net/contrib/desteinhauer-at-verizon-dot-net/070507_6inch/
070506-Ronchi-final.jpg

This is my first mirror, and it took me a long 20 months to complete.  I 
learned many lessons, though.  I spent most of a year trying to get rid 
of a stubborn TDE.  It turned out that the problem all along was (go 
figure) poor contact.  I eventually realized I had an exceptionally hard 
pitch lap which didn't get in good contact no matter how long I cold 
pressed it.  I learned to heat it up in a 90 degree (Fahrenheit) bath 
and hot press it before every polishing session.  What a pain that was.  
I got rid of the TDE that way, though.  I did 1/4 diameter strokes with 
the tool on top, until the diffraction ring was faintly visible on the 
right side (knife approaching from the right).  Then I did 1/4 diameter 
strokes with the mirror on top.  I kept my polishing sessions to 5-6 
minutes before I pressed again.  It took some time, but eventually the 
right side diffraction ring was nearly as bright as the left.  Forget 
about accented pressure to eliminate such a narrow TDE - that just made 
things worse.
 
Eventually, when I was struggling with figuring, I made a new pitch lap 
and wished I had done it months earlier.  What a difference - I could 
cold press just like everyone else.
 
Anyway, time to celebrate by taking my family out to dinner!
 
Next project: a 12.5 inch f/5 (and of course, finishing up the rest of 
the 6 inch telescope).  My 9 year old son will be grinding a 4 inch 
mirror while I am doing my 12.5 inch.  He is very excited.

David Steinhauer
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