Re: (ATM) Chasing Shadows

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

At 11:01 20.10.1995 +1000, Mark Suchting wrote:
>
>Hi Carl
>
>1   Foucault shadows only have sharp borders for fast (ie < f4.5) 
>mirrors. Usually the shadows look quite gentle. photos have a habit of 
>increasing contrast.
>
>2. Sounds like you have a very mild turned edge, if you can see a 
>diffraction ring all around your mirror.
>
>3. the ronchi grating is not so good for evaluating  TDEdge because of 
>the inherent diffraction effects at the edge of the mirror.
>
>hope this helps
>
>mark
>
>At 17:57 19.10.1995 -0700, Randy Johnson wrote:
>>
>>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
>
>
>