Re: [ATM] Detecting Astig in Ronchi images

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

Six or seven months ago I was properly advised by Mike Peck not to confuse
local defects with astigmatism, that astigmatism was a shift in focus with
the long and short being at a 90 degree angle. Not just long and short but
at 90 degrees.

Can some one inform me which defines the axis? Is it the long radius or the
short one? If it can be either then it is easier to determine the
orientation because you could only be 45 degrees off at the most.

I see what I would have previously called astigmatism. And I can see what I
might call an axis, but then I can see that I could just as easily chose the
other at 90 degrees to that and call that an axis. And then some in between
too!

I think the truth of this mirror is closer to something like.... 
That is some baaaaad mirror. The local defects are of so many different
sizes and shapes from millimeter to inches wide, that the best description
would seem to be something like megaripplepolyrandomfoil.

I think it is not even reasonable to try to describe the surface or
orientation of the defects because they are too gross.

While I think this ultra-thin has been quite a learning experience for us
concerning Ronchi, I have to say that the best course for David would be
rather than trying to learn to see astigmatism in Ronchi, that a better
course would be for David to first learn to make the surface smooth. 

When the mirror becomes smooth the astigmatism will naturally become less.
I'm sure there will be many who disagree. But even if I am wrong, smoother
would still be better. When the mirror is smoother astigmatism will be
easier to see. With the present roughness the Ronchi lines are bending all
sorts of directions as they passes over local highs and lows on the glass.

I don't mean to be negative.  The intent is to be positive. I think the
right thing is for David is not to work so hard on astigmatism but to
concentrate on SMOOOOOTH!

Jerry






-----Original Message-----
From: Dale Eason

I'm trying to help David Davis identify astig in
ronchi images.  Can any of you see signs of astig in
these images?   They were taken every .05 inches
starting about .2 inside ROC.

http://home.comcast.net/~doeason/DDavis/ronchi_scan.htm

If you can identify what it is that tells you.  Can
you also estimate the major axis and/or the magnitude?

I'm sorry they are not full height but that is the
best the web cam can do on the computer controlled
platform.  If that is not good enough then say so but
I can't get better until Edmund gets the lens I need
in stock in 2 weeks.

Dale Eason



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