Re: [ATM] RE;1/8,1/16-1/20th debate.

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

James Lerch presented a couple of examples of mirrors, discussing the Strehl
at the center and edge of the FOV assuming various degrees of
under/overcorrection.

I'm no mathematician, but AFAIK correction error (low order sph abb) and
coma off axis are expressed by orthogonal Zernike terms - they are
independent and add by their mean square error - the root is extracted
afterwards to get the total RMS error (there are of course other terms in
general cases).
The Strehl ratio of 0.8 may make some sense as a design goal, but it is
arbitrary, and the difference between 0.78 and 0.82 is not likely to be
noticed. But calling one "diffraction limited", the other not, is
essentially misleading - the term is most unfortunate, at least as it is
widely (mis)understood.

Nils Olof

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