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What are you hoping to gain?
By my calculations, the width of the light cone from a 12" f/5 scope when it reaches the secondary is about 63mm. What size is your secondary mirror? A 70mm secondary allows 3.5mm of play either side due to imperfect allignment. By reducing the secondary obstruction from 70mm to 63mm, the light gain is only 1% due to areas being relatively smaller at the centre of the primary mirror.
A 63mm secondary is most likely going to spill the light off edge and therefore lose more than was gained by the smaller diameter of its obstruction.
Regards, Rob.
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