ATM Tyro questions

[lindberg <lindberg@olywa.net>]

> > Subject: mirror testing
> >
> > I am very interested in grinding a mirror  & have read Telescope making but
> > don't quite understand the math in testing. I think I understand how to read
> > the test to get a spherical shape but don't quite understand how to determine
> > the couder mask window sizes. 

The size of the windows is not critical, but the average radius of the
windows must be known. (That is r, or hm in your knife-edge position
calculations). Texereau recommends 5/8" minimum width. Too narrow and
diffraction along the edges interfers with the readings, and if too wide
the zone will not appear sufficiently even in brightness. 

> >correct me if I'm wrong. Once the couder mask is
> > made one checks the individual zones & note the micrometer thimble reading
> > where each zone grays evenly.

That's correct. 

> >If this is correct than how does one determine
> > what the correct measurements are. 

Use the formula:
 
        r(squared)/R =  ideal micrometer reading for the zone under test.

where: r=radius of the zone under test (hm in Texereau's nomenclature),
and R is the radius of curvature of the mirror. This formula is for
Foucault testers with stationary light sources, with only the knife-edge
being moved by the micrometer, such as those shown in Texereau's book.

> >Please any guidance here would be
> > appreciated. I'm a designer & tool maker by trade so making other components
> > aren't a problem. If I can understand this part I think I will be ready to
> > order the kit.
> >
> >                             Thanks!!
> >                                         Rusty Burnett
> >                                         arbpjbjlb@aol.com
> 
Rusty:
        By all means, order the kit and get yourself grinding. Help, if you
need it, is only your keyboard away.
        Regards, Steve.