To: atm{at}shore.net
From: Jim Burrows 
Reply-To: Jim Burrows 


At 18:59 2003-03-09 -0800, Jeff Baldwin wrote:

>I understood that the Raleigh criteria not only included 1/4 wavelength
>correction, but also included a 1/14 wave RMS.  MIght be wrong, thought this
>was the ticket.

What Rayleigh (1879) said was, "One-quarter wave of primary spherical
aberration reduces the irradiance at the Gaussian focus by 20%, and more
aberration than that causes significant deterioration of the image." 
Further study has most opticians saying the 20% is a more general (it has
become the .8 Strehl ratio) definition of "diffraction limited"
and applies whatever the aberration.

The 1/14 wave RMS equivalence to .8 Strehl ratio came later (Mar‚chal, 1947).

         -- Jim Burrows
         -- mailto://burrjaw{at}earthlink.net
         -- http://home.earthlink.net/~burrjaw
         -- Seattle N47.4723 W122.3662 (WGS84)

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