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The general definition of a Apochromatic optical system is one which brings three wavelengths to a common focus (we hope!)
In traditional lens designs this would require a minimum of three different lens/ glass specs.
I'd say any optical system, whether it has three (or four in the Petzal design) which produces an APO image is a Apochromatic instrument.
( I still use my Genesis, s/n 1007)
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