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Hi Rod,
A Dall-Kirkham shouldn't be made faster than f/15 because of the excessive coma. Mine is f/16 so the focal length is 6500mm.
For planetary imaging aperture rules and I consider 10" as entry level but 12" gets you close to the cutting edge. If you accept that, then you are left with the proven C14s, the large Newtonians and the large D-Ks. The D-K being more of a DIY option as I don't know of any available commercially.
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