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You might find this useful:
The normal rule for a doublet is that the f/ratio needs to be 3-5x the aperture in inches for the f/ratio.
I.e. an 8" refractor doublet needs to be f/24-f/40 to be without chromatic aberration.
With some of the exotic glasses available today, as short as 2X the aperture for the f/ratio, i.e. f/16.
An f/8 doublet 8" scope is not a scope I'd want to own, no matter what glasses were used in the lenses.
It might be usable for widefield scanning with a strong minus violet filtration.
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