(ATM) Chasing Shadows

[Carl Brennan <carl@brennan.pb.eunet.de>]

Hi All,

I´ve been figuring my mirror (6", f6.5) for some weeks now. Gotton close to
some good
results but ´lose´ them again as I try to correct a particular zone - due to
inexperience
I guess. But it´s fun and I´ve learned so much. Anyway, some things I´ve
wondered about
during the figuring and interpretation of the Foucault shadows are :

 1. No matter how I try I can never seem to get the shadows (doughnut shape
at the
  50% k.e.) sharply defined - certainly not as sharp as  the photographs in
all the Tel.
  Making Handbooks. I suppose they´ll sharpen up when the curve is just
about right ?????

  2. My Foucault setup has the light source on the right-hand side and the
k.e. on the left.
   When I cut the cone of light at the 50% k.e. zone (70% Mirror zone) the
extreme right-hand
  side of the mirror ( a band about 1cm from the edge inwards around the
mirror to the
   vertical axes) is appreciably brighter than the rest of the mirror. I get
the doughnut shape
   - albeit not so sharply defined- and the lip of the mirror has a bright
line running all the
   way around - left side is not so bright as the right side. Is it normal
that the right-hand
   side has this brighter band of light ?? Instinct tells me that I probably
have a turned edge
   but a Ronchi test shows the lines running straight over the edge of the
mirror without
   bending. The fact that the lip of the mirror has a thin line of light on
the left-hand side
   when the cone of light is cut tells me that the edge isn´t turned down.
Other than that
   I´m running out of ideas !!  What am I missing ????? 

   Comments and suggestions gratefully accepted !


    Carl Brennan,  Paderborn (Germany)