Re: [ATM] over&under correction.

[Michael Peck <mpeck1@ix.netcom.com>]

At 18:59 7/10/05, Jim Burrows wrote:
>At 2005-07-10 06:32 -0700, John Moore wrote:
>
>>Mirror A;is 1/4 wave-undercorrected.
>>MirrorB;is 1/4 wave-overcorrected.
>>
>>1.which optic would perform btter under the stars?
>
>overcorrected with b = -1.3025 has .25 wavefront, 20.4 nm surface RMS and 
>.805 Strehl (Mahajan approximation), all relative to 550 nm 
>light.  Undercorrected b = -0.6975 has exactly the same numbers!  So no 
>difference.  Shoot, I suppose all the math-obsessed ATMs knew that.
>

Not only is there no difference in summary quality measures, the in focus 
diffraction patterns are exactly the same. Also, the extrafocal diffraction 
pattern in one case exactly matches the infrafocal diffraction pattern in 
the other for the same amount of focus travel. So, based solely on optical 
theory there's no difference at all between the two cases.

Of course I defer to others on which condition is easier to fix.

Mike Peck

_________________

Michael Peck
email mpeck1@ix.netcom.com
Wildlife photoblog! http://wildlife-pix.com
Amateur telescope making http://home.earthlink.net/~mlpeck54/astro/astro.html 

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