Re: Telescopes - Diffraction limit

[John Mallinckrodt <ajm@CSUPOMONA.EDU>]

>         Without working through this carefully, my first
> thought is that the angular separation is what is
> involved in the deification limit.  The primary
> mirror or lens has increased the angular
> separation viewed by the eyepiece.

God or no God, I think Roger is right here.  The angular
magnification of the scene provided to any element in the optical
system is

    M_ang = f_obj/f

where f_obj is the focal length of the objective lens and f is the
focal length of the element.  The angular resolution of the system
should, therefore, be determined (primarily) by that of the weakest
link after correcting for the angular magnification factor for that
link.  That is

   angular resolution = maximum[(1.22 lambda/f_obj)*(f/D)]

where the expression in brackets is evaluated for each element using
its focal length f and diameter D.

For instance, in the case of a telescope with a 150 mm aperture
operating at a power of 50, the eyepiece would need to be at least 3
mm in diameter to prevent it from degrading the angular resolution.

--
   A. John Mallinckrodt        http://www.csupomona.edu/~ajm
   Professor of Physics      mailto:ajm@csupomona.edu
   Physics Department         voice:909-869-4054
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