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Hi Trav. Do you use any 2" eyepieces? If so the required focal plane illuminated size would need to be bigger than that needed for a 32mm plossl.
Rule of thumb is to calculate for 70% illumination at edge of field for the field of your longest fl eyepiece. To calculate this you need to know measurements from your primary to the centre of your secondary, focal length etc.
One thing not often considered is that the worst part of any secondary optically is at the edge of it. The chase for planetary contrast could have you being worse off if you downsize too much and rely on the edge being good even for a centred high powered view.
Best thing is to download a cope of newtwin.exe and plug in the numbers.
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