Re: ATM Spherical mirrors

["James W. Burrows" <burrjaw@earthlink.net>]

At 08:17 2001-08-13 -0400, Guy Toutant wrote:

> From "How to Make a Telescope" (Jean Texereau), Second Edition, p.19 :
>"... A. Couder has given a formula for the minimum required focal length f 
>of any mirror of given diameter D that is to satisfy this condition : " 
>(Rayleigh's rule)
>
>           "... f3 = 88.6D4"    (f cubed, D to the fourth)

The exact expression for a quarter-wave mirror is

         f/ = [cur(D/L) + cur(L/D)]/8

focal ratio f/, diameter D, reference wavelength L, cube root cur().  For D 
in mm, L = the usual 550 nm, and dropping the last tiny term, one gets the 
cute formula

         f/ ~ 1.526 cur(D)

 From Couder's formula, fiddling w/cube roots and 25.4 mm/", I get the 
factor 1.516; maybe different L.

For D = 16", it's f/11.3.

         -- Jim Burrows
         --              mailto:burrjaw@earthlink.net
         -- Seattle      N47.47233, W122.36620 (WGS84)