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


At 02:08 2003-04-13 -0400, Scott Donaldson wrote:

>The question which remains in my mind is whether the ray
>optics can yield useful predictions of optic quality.
>Especially of interest to me, is whether ray optics has sufficient
>'resolution' to enable the ATM to set specifications for optics.

Geometric optics is certainly important, particularly, as Mike Peck
mentioned, in optical design.  Geometry says that the parabola is the shape
we have to get close to.  Also, ray tracing is the way we find from various
quantitative amateur test readings what the actual mirror profile is.

>It seems to me that at some point a mirror
>cannot be further improved to deliver a better image.   What point is that?
>Can that point be described geometrically?  When that point is reached, what
>is the nature/magnitude of the residual surface errors?  Or is it the case,
>that continual improvement in the diffraction pattern can be realized by
>further reduction of surface errors until the 'perfect' paraboloid is
>reached?

Lord Rayleigh gave the requirement - "A quarter-wave of primary
spherical aberration reduces the irradiance at the Gaussian focus by
20%," and words to effect that more aberration causes serious loss of
image quality.  The 20% has been rephrased as the .80 Strehl ratio, which
is now regarded as a passing grade for all aberrations.  Of course, good
students want to do better than just pass, but we burdened old-timers,
having taken Econ 101, get deep into cost/benefit ratios.

Ray optics tells us what the surface profile is, and therefore what the RMS
is relative to the best-fit parabola, so all we need from wave optics is
the RMS threshold below which it's "if you have to keep trying, < 1
minute spells and don't overshoot..."

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

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