Re: [ATM] Foucault and diffraction

[Jim Burrows <burrjaw@earthlink.net>]

At 2005-01-16 12:30 -0600, Michael Peck wrote:

>According to Wilson the tolerance for astigmatism is about 1/3 wave PV 
>(for 0.8 Strehl).

Had to dig out the pocket calculator to check this, with A22 the amplitude 
of primary astigmatism (PA), and the surface RMS (everything in nm):

PA wavefront PV = 4*A22/550 = 4*sqrt(6)*RMS/550 = 4*sqrt(6)*20.67/550 = .368

Wilson selected a good "about".  The 550 is the standard visual wavelength 
and the 20.67 comes from solving Mahajan's Strehl formula at 0.8 for the 
required surface RMS.

>For investigating diffraction effects the easiest thing to do is download 
>one of the FFT based simulations of knife edge (and similar) tests.

If you "visit" (standard ad language) my web site and go to the ATM 
section, you'll see a simulated Foucault image of a mirror with just 
passing PA.  As Tex sez, p. 85, "... the shadows are difficult to 
interpret...".  Tex recommends a bench star test.  On the web page, next to 
the Foucault image, is a simulated image of the bench star test - it's also 
a little subtle.

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

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