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I got a 1/18 wave Antares secondary for my 12". Haven't installed it yet but 2.6" was the smallest I could go. 2.14" was too small with probable light loss.
Not sure why you want a 1/30 wave secondary though. I think its a waste of money if your primary isn't 1/30 wave. Hell the 1/18(which according to test documentation is 1/19) for mine is over kill and I only got a new secondary because the coatings on my old secondary was getting pin holes.
I'm sure there some formulaic calculation that can tell you the bare minimum the secondary needs to be depending on aperture to focal ratio.
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