RE: ATM Scratch during initial polishing...question

[Dominic-Luc Webb molmed <Dominic.Luc-Webb@molmed.ki.se>]

On Wed, 23 Aug 2000, Ken Bertapelle wrote:

> Well, I finally got to polishing my 8" f/5.5 and as dumb luck would have it
> after my second spell of polishing I noticed a hairline scratch about 1/8 of
> an inch long on the mirror.  It's about 1 inch in from the outer edge.

This is about as common a problem as wrinkles 
on your skin with aging. I have tried getting 
absolute perfection on my mirrors, usually 
without success. Although, I am getting really 
close on a 12.5" mirror I am working right now.
However, I can say that the scratches are nowhere 
near as detrimental as I would have thought when 
I made my first mirror. I found that errors in 
figuring and possibly aluminizing and coating 
are vastly more problematic because they can 
affect the entire field of view. Scratches on 
the periphery on not only very difficult to 
remove, demanding many additional hours of 
further grinding and polishing to deal with, 
you might also end up having difficulties getting 
both a scratch-free surface AND exactly the 
right figure. The outer edge of the field is 
often not used anyway. For instance, most 
amateurs are using 35 mm cameras or CCDs. These 
rarely ever capture the edge of the mirror. 
6 x 6 cm cameras can in some instances get closer. 
My own observation leads me to the conclusion 
that attempting to completely remove minor defects 
at the edge should be viewed as an interesting 
challenge, but not necessarily needed for 
optical performace.