Re: ATM polishing advice sought

["Frank Schiralli, Jr." <schiralli@mindspring.com>]

At 03:24 PM 7/5/99 EDT, you wrote:
>>To analyze the Ronchi patterns we need to know if you were inside or
outside 
>the RoC of the mirror.  

Sorry.  You are absolutely correct.  Forgot to mention (and I took it for
granted) that everyone would understand I was inside the RoC for the
purpose of the description.  Outside of the RoC, the lines reverse into the
classic "hook" pattern as far as directions go.  It's not so severe as to
be descriptively accurate to call it a "hook", though.  Merely a subtle
change in direction.

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.

This "relatively long" edge is too subtle for the KE at this point, it
seems.  For whatever reason (including the fact that I'm testing in a
darkened room in the daytime, rather than a DARK room at night), it's not
showing up in the KE test.

> 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.

Agreed!! That's why 1) I took a "premature" look, and 2)  I asked for help
before digging myself further into a hole from which there was little hope
of escape w/o extensive hours of unnecessary additional labor.  I am still
of the opinion that it's a result of exclusive MOT polishing, but will
monitor and evaluate the situation every couple of hours of polishing.

BTW Dave, as an aside, I want you to know that the spherometer I made
pursuant to your advice and web page instructions has proved itself to be
so accurate on this project as to be downright unbelievable.  When I
measured the RoC w/ a metal tape (Scott helping) it was less than 1/3" from
calculated!!  1/3 of 1 inch out of 152 inches.  Super!!  Many thanks.



Frank Schiralli, Jr.
Member, Astronomical Society of Long Island (ASLI)
N 40* 54' 28"
W 73* 19' 22"
E-mail schiralli@mindspring.com
Web Pg. http://members.tripod.com/~AstroImager/index.html