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My guess? When they were experimenting with how to machine mass produce these mirrors the made a really good mirror for the 12" and 10" at those F ratio's by sheer luck and said "yep! Save those settings for those apertures from now on!"
Reasoning is because GSO also makes mass produced mirrors but they are at clear F ratio's F4 F4.5 F5.
Otherwise I've always wondered why too.
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