Re: [ATM] Transverse Error

[Alexis Cousein <al@sgi.com>]

Mike Lockwood wrote:
> Anthony.
> 
> Anthony Anconetani wrote:
>> I have a feeling this going to be a "Duh" question. But, what exactly is the
>> Transverse Error window in FigureXP supposed to be telling me?
> 
> For a particular zone, if TE is greater than +1 or less than -1, then 
> the light from that zone is being directed outside of the ideal airy 
> disk, which is the smallest spot that the telescope can form.
> 
Ray casting is a geometric approach, but it bears little relation to
physical reality (where even one photon tends to interfere with itself and
acts like a wave, at least until it hits your retina).

The transverse error reported outside [-1,1] just means that the *ray*
reflected from that zone (from an on-axis parallel bundle, and using the
assumed normal of the surface there) misses where you expect the central
portion of the Airy pattern to lie.

I'm going to be a nit and object to the term "being direct outside". It
does give you a *qualitative* appreciation for how little that error will cause
energy to be put in diffraction rings rather than the central disc, but
the error itself doesn't in itself direct real physical energy along the ray.

And if you want to do a *quantitative* analysis, then you need to use a
physical model that's based on wavefront modeling and not ray-casting:
light doesn't behave like ray bundles.

In the times of slow PCs (the time Rutten and Van Venrooij's book was
written), you couldn't do fast FFTs on PCs, so the geometric approach
was useful to determine when good enough was absolutely certainly
good enough (with some added data points from models that did
model wave diffraction, like the size of the central portion of the
Airy pattern for a given circular aperture).

That doesn't mean ray casting models a physical reality,
though.


-- 
Alexis Cousein                                  al@sgi.com
Senior Systems Engineer/Solutions Architect     SGI/Silicon Graphics
--
<If I have seen further, it is by standing on reference manuals>

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