Re: ATM Astigmatism on 20 inch thin mirror

["Ken Hunter" <kb7h@onemain.com>]

David,

In looking at the latest photo's, specifically the diagram of the strokes...
 
Does the stroke lines represent the center of the tool's travel? If so, you 
are taking strokes that are too long. At the most, your tools center (or
the center of the mirror if on top) should NOT EXTEND past the edge 
of the bottom piece. If the top piece (tool or mirror's center) extends
past the edge, you are asking for a severely turned edge. I know that you
have been given advice to extend the stroke but that advice should have
included the above information also...

Otherwise, you are proceeding well...

Ken Hunter


----- Original Message ----- 
From: "David Lonard" <lonard@hotmail.com>
To: <atm@shore.net>
Sent: Saturday, December 28, 2002 7:58 PM
Subject: ATM Astigmatism on 20 inch thin mirror 


> 
> 
> Hello Group,
> 
> I would like to thank all those who have given advice.
> So far the biggest consensus that I have received thus
> far has been to extend my stroke to bring the sphere out
> further to the edge, which I have been doing and seems
> to be working well.  I have still been concentrating a
> lot of strokes along the more spherical axis since I
> still think that the edge is higher along this axis.
> Along the more hyperbolic axis, I am using a longer W
> (or V?) stroke.  I have an updated ronchigram photo at
> 
> http://www.geocities.com/lonard3/mirror.jpg
> 
> I have also made what I hope is a more reasonable star lap
> as was suggested.  Several of you have said that I should
> wait on the star lap, but I have tried using the full
> lap and things go wildly oblate spheroid when I do so.
> At any rate, the ronchi bands are getting rather straight
> with this current lap so I think it is currently useful.
> 
> To answer your question about machine work Dennis, when
> the surface came to polish, the ronchi pattern was packed
> with multiple nasty zones, too many and too deep to honestly
> know if I had astigmatism at that point.  The 12/24/02 image
> is the product of probably 20 hours of subsequent hand
> polishing.  I actually did put it back on the machine,
> but strong zones started to return, so I stopped.  To
> be fair, my machine isn't very mechanical sound, made
> out of wood and was practically destroyed during the
> violent hogging out process.
> 
> I'm sure that there is still plenty of interest in a
> 20 x .875 inch mirror so I will try to give more
> updates.
> 
> David
> 
> 
> 
> 
> 
> 
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