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Old 30-04-2012, 09:13 AM
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Satchmo
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Quote:
Originally Posted by Peter Ward View Post
I'd be interested to see a ray trace of a 1/2 wave spherical error....but so far I've not manage to track any down.
Peter, I think a 50% obstruction produces modification of the Airy pattern similar to a Strehl ratio equivalent to 0.5 wave or so spherical aberration in a perfect system. When you get down near the diffraction limit geometric ray trace diagrams are of intellectual interest only because of course the real image is subject to diffraction effects and the Strehl ratio tells us a lot more . I'll look into it for you.

Off the top of my head the geometric ray spot bundle at best focus for a 1/8 wave of pure spherical aberration optic ( wavefront ) is about half the theoretical airy disc diameter , and a 1/4 wave wavefront system has all it rays within 3 Airy Disc diameters.

Casual onlookers must keep in mind that in general discussions about wavefont quality vs utility for visual or imaging we are always talking about simple pure spherical aberration at those tolerances ( to keep things simple ) , and not other kinds of errors, which may or may not have similar effects on the Strehl ratio. The famous 1/4 wave `Rayleigh Criteria' which many mistakenly call `diffraction limited' referred only to pure spherical aberration, and even then was really a generally tolerable lower limit. Lord Rayleigh added that the effect on visibility of planetary detail was `already decidedly prejudicial ' at 1/4 wave.

I personally define the meaning of Diffraction Limited to mean that the only disturbance to the Airy Pattern attributable to the optics being the diffraction effects itself, and that won't happen until spherical aberration is reduced to 1/8 to 1/10 wave in the system . This caveat was first expressed by Danjon and Couder in the 1930's. Unfortunately few optics display SA in its pure form , but have generally more complex errors. In all the commercial optics I've looked at I find figure of revolution problems to be the greatest issue, and pure astigmatism itself is rare in favour of more complex irregular shapes.
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