Re: [ATM] my ongoing war story (polishing and figuring)

["Rodney Mayes" <rgmayes@bellsouth.net>]

----- Original Message ----- 
From: "Vladimir Galogaza" <vladimir.galogaza@zg.htnet.hr>
To: "ATM List" <atm@atmlist.net>
Sent: Wednesday, February 15, 2006 2:58 AM
Subject: Re: [ATM] my ongoing war story (polishing and figuring)


> Rodney,
>
>>That's right, a central hill while parabolising.  I would not have thought
> it possible to do this.
>
> Why not. Imagine a sphere and you happened to start digging, say at zone
> 50%. You end with depressed
> 50% zone ant it all may look like central hill in a valley.

This appears to be the case.  I would have thought a central hole would have 
been a more likely scenario for me due to my inexperience and belief that 
the center of the glass would see more wear in the initial figuring stages. 
The longer I continued, the worse the hill became.

>
>>I began to notice mild astigmatism in the ronchi bands
>
> Recently we had long discussions about how to detect, or is it even 
> possible
> to detect
> astigmatism by Foucault (or Ronchi) test. Not to speak of "mild" one.
> Which feature of Ronchi image you found was telling about mild 
> astigmatism?

The unusual curves near the ends of the bands (top and bottom), although not 
extremely noticeable, looked very similar to some photographs I have seen on 
the web of ronchi images where experienced persons have determined the 
existence of astigmatism.  If you go to http://www.johnstelescopes.com/ and 
look for "astigmatic" under "mirror testing," looking at the bottom photo 
(not the top photo thank goodness) gives a good example of what I (think) 
was seeing.  It is possible however that I was seeing a bend in the lines 
due to the emergence of a zone created by my poor figuring.  But, I was 
concerned enough to stop and go back to sphere.  Thanks very much for your 
input and the input of all those who responded.  Best regards.  Rodney
>
> Regards
> Vladimir.
>
>
>
>
>
>
>
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