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Richard
I ran your numbers and the 1.83" will give a 6mm diameter fully illuminated field and the 2.14" a 15mm. The light drop off ( vignetting ) is very gradual in an F5 scope so either size will work fine. Edges of diagonals can sometimes have issues so I would go for the larger diagonal.
Compared to mirror figure quality and surface smoothness, central obstruction is one of the most overrated aspects one can tweak. Anything below 25% obstruction will perform very well. I would recommend at least the 1/12 surface diagonal. Spherical errors on the secondary do not bring down the Strehl ratio nearly as much as errors like spherical aberration on a primary.
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