Re: ATM Astigmatism on 20 inch thin mirror

[Dennis Rech <dennisrech@mirror-o-matic.com>]

At 11:59 PM 12/24/02 -0600, David Lonard wrote:


>http://www.geocities.com/lonard3/mirror.jpg
>
>The blank was ground and initially polished by machine,
>then polished by hand using a 16 inch tool.  Lately, I have
>been using a star lap (pictured at the above http link),
>to keep the figure from wanting to assume a more spherical
>or oblate spheroid shape.
>
>So far, I realize that I have a severe problem with
>astigmatism.  In the two images, the mirror has been
>rotated 90 degrees along the axes of the minimum and
>maximum amount of parabolization.  I'm presently attacking
>the problem by concentrating strokes along the more
>spherical axis using mostly COC with a slight amount
>of W stroke.  It appears to be working to some extent,
>but is resolving very slowly.  More recently, I have also
>been using a shorter stroke to attempt to correct what
>I perceive as a broad rolled edge along the more hyperbolic
>axis.
>
>I would like to know if I'm doing the right thing.
>Can this be fixed without going back to grinding?
>I don't know how I managed to wind up with the astigmatism.
>On the machine, the mirror was not rotated to avoid
>generating a new center.  Upon hand polishing, I have
>been rotating the mirror and have kept a wet thick teri
>cloth towel underneth it.
>
>Thanks for any tips.
>
>David Lonard

Hi David,
Just out of curiosity, what did the Ronchi look like when it came off the 
machine after polishing. Where there nice straight lines or what we see now?
Its really hard to make a Ronchi pattern like that on a machine if things 
are turning, the mirror is centered, the tool is 80%, and the overhang is 
anywhere near correct (center to edge stroke,15% tool overhang).
If the machine is still available, I would just go back and start polishing 
again. I would repour the pitchlap and just make a nice standard 
symmetrical grid on the surface. Run at 40 or 50 rpm with 20 pounds of 
weight and an eccentric speed of 5 or 10 rpm.
Its probably going to take 2 or 3 hours to straighten the figure out back 
to spherical. Check the Ronchi every have hour or so just to see what is 
happening.
When you get something close to spherical with a unturned edge, check back 
and we can go through parabolising.
I suspect that you were trying way too many new ideas.
Lots of luck,
Dennis