[ATM] Re:How much of cell induced deformation is too much?

[Vladimir Sacek <vladis.2@juno.com>]

Hi John,

I finally got to answer. By definition, 50% Strehl means that central
intensity of the Airy pattern is reduced by 50% (as a result of wavefront
deformations; so the c.obstraction effect is in "informally"). Encircled
energy may vary around that figure, but can be expected to be similar to
the Strehl, on average. That still leaves us with ~50% energy out of the
Airy disc. It should be noticeable in the appearance of focused
diffraction pattern, but not necessarily terrribly obvious. The pattern
is roughed up by the seeing, and the energy is smeared around, increasing
both brightness of the rings and that of dark interspaces. If we'd take
out the seeing error (which would result in ~0.70 Strehl) to re-compose
the pattern, one would still need to have a clear mental picture of what
the pattern of a near perfect aperture for the particular magnitude looks
like, as a reference for determining evident changes in the in-focus
pattern appearance.

Back to cell induced error, seems that one needs to find some sort of
balance between different factors. As mentioned before, computer model
gives deformation error based on an idealized, perfect cell. Actual
error, induced by an actual (imperfect) cell, is likely to be greater,
and the "unplanned" error component may be significantly larger.
Considering this, it may be at least as important to have a cell that can
be made to highest standards, as it is to have one that gives the least
amount of deformation based on the number and distribution of supporting
points alone. 

Vlad
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