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Sorry Houghy I should elaborate on the Strehl ratio a little. It is basically the % of light that is concentrated into the airy disk from the theoretical maximum of 100%.
In Bob Royces table at the very bottom of his article he indicates that a strehl of .94 is very good and anything over .95 is excellent. Remember of course that in Bob's table he is talking about the primary mirror only (1 optical surface) . That Mak Cass has a "system" strehl of .94 after encountering 3 optical surfaces, the corrector plate, the primary mirror and the secondary mirror, so you can't ask for any better than that, its right at the top of the tree.
CS-John B
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