Re: [ATM] Another Astigmatism question, but at the sphere stage

["vladimir" <svla@isp.com>]

John Sherman wrote:

>And "stigmatic" meant that your scope produced a round, 
>pinpoint image of a star (within the limits diffraction gives). 

I'd speculate that "stigmatic" in its optical use relates to geometrical,
not physical optics. It literaly does mean "point-like". As we all
know, no actual optical instrument is strictly stigmatic, diffraction
or no diffraction (afterall, "point" by definition has no physical
dimensons, thus no physical existence as well). So, in the original
sense, all of them are "astigmatic", regardless of the cause.

No word has meaning inherently related to a certain phenomenon.
We just agree upon the meaning, and not seldom such conventional
meaning doesn't agree with what would be a strict original meaning.
"Achromatic" should mean w/o colors; not exactly reflecting - or
refracting - the reality.

Like you said, there is specific meaning agreed upon as to what
"astigmatism" and "astigmatic" mean in optics, and it relates to
both particular surface deformations, and their effect on the
point image. It wouldn't help clarity if we start using these words
for other surface deformations and their effects, just because it wouldn't
be in disagreement with the original general meaning. 

But it does make for an interesting chat...

Take care,

Vlad


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