Re[4]: ATM Airy disk frustrations

[Dwight Elvey <elvey@hal.com>]

"Tony Dunlap" <tdunlap@odot.dot.ohio.gov> wrote:
> 
> Hi Dwight
> 
> How can you have a focal ratio less than one? wouldn't F1 be basically, half
> of a sphere? I think I'm missing a fundamental here.
> 
> Thanks

Hi
 With a parabola, one can have a f/.25 with all of the light
coming from one side. Beyond that, the light would
begin to come from the other side. Infact, one could make
a f/.002 and use only the light coming from the front
side as is done in one of the XRAY satelites( not sure
of the f ratio but it is something less than .25 ).
There is a difference between what one can do with
a sphere of fixed diameter and a parabola that can
be any size.
Dwight