Re: (ATM) Chasing Shadows

[randysas@scn.org (Randy Johnson)]

>
>Hi All,
>
>I=B4ve been figuring my mirror (6", f6.5) for some weeks now. Gotton close=
> to
>some good
>results but =B4lose=B4 them again as I try to correct a particular zone -=
> due to
>inexperience
>I guess. But it=B4s fun and I=B4ve learned so much. Anyway, some things I=B4=
>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=B4ll sharpen up when the curve is just
>about right ?????
>
Carl,

I'll take a crack at this first one on your list because I think I've 
been there and done that.  My mentor is here on the list and he can 
correct me if I am giving too much bad info.

I wouldn't be concerned at all about the sharpness of the shadow if I 
take sharpness to mean intensity of contrast in the shadows.  Indeed, if 
the intensity of the shadow is strong in a moderate light cone like your 
f/6.5 then your likely leaning toward a hyperboloid.  Suttle shadows are 
your friend.

In your testing, you are looking for the slightest change in intensity to 
define the zone.  The real challenge is not in matching the shadow to the 
textbook shadowgrams but in detecting where and at what amount of 
k.e.travel the zone is defined.  Record this data, plot it (or enter it 
into the data reduction program) and see where it falls.  Do not let 
yourself be seduced by what you think the picture of the 50% zone should 
look like.

That said, I'll climb down and let someone else take the other 
questions.  (Peter, if you're listening, did I tell him right?)

--
Randy Johnson                   randysas@scn.org
Seattle, WA  USA