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The current 61EDPH is a 60mm fpl53 doublet f/5.5 with a large diameter 3 element reducer/corrector bringing it to f/4.5.
It looks like they've fitted a bigger focuser to the already existing f/5.5 scope and designed a reducer to suit.
Also AFAIK the Redcat isn't strictly a Petzval (4 elements in 2 groups); the Redcat has 4 elements in 3 groups.
Both designs produce flat fields.
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