Re: ATM polishing advice sought

[Aplanatic@aol.com]

Frank Schiralli wrote:

> The figure looks like a gentle "compound bow", as in bow and arrow, with
>  the "tips" of the bow indicating an incipient TDE.  Another way to describe
>  the shape would be the way a guy would move his hands to indicate a
>  curvatious female figure a la Mae West.  Woo. Woo.  You get the idea.
>  (Joanne, et al...Please don't trash me for this description, I'm only
>  trying to get an idea across).  :-)

To analyze the Ronchi patterns we need to know if you were inside or outside 
the RoC of the mirror.  In addition, I find the Ronchi test for turned edge 
hard to read because of diffraction effects.  In my opinion, it is easier and 
more obvious to check for a turned edge using the knife edge.  Adjust the KE 
so that the outer zone of the mirror is nulled and look for a uniform, thin 
circle of light surrounding the mirror.  If the pinhole/slit side is brighter 
than the KE side then you have a bit of TDE.  If it is a lot brighter then 
corrective action is called for even at this early stage of polishing.  Don't 
worry too much about the figure developing in the mirror but DO worry about 
turning the edge.  It requires a lot of hours to fix later and is therfore 
good to avoid now.

Dave Rowe
Torrance, CA
Medium Format Astrophotography:
http://members.aol.com/aplanatic/photos/astro.html