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I would think that a 12" f10 would be a good planetary scope but the focal length will be very long (3.05m) . The 8" f10 that you have now is pretty good as a planetary scope, what about getting a 10" f8 scope. focal length is only a bit over two metres.
A 12" f13 scope is 4 metres long, and not usable. You might want to consider grinding a 8 with a long f ratio. To work out how long your focal length will be simply times your diameter of your mirror in mm by the f ratio. as in 305mm (12") x f10 = 3050mm (that is your focal length).
So as you can see the larger the F ratio the longer the focal length.
Personally I would keep the f10 that you have and do some flocking modes to it. See Mikes project on this.
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