Where I live seeing is rarely much better than 2 arcseconds and I have never seen it better than 1 arc second (as estimated by the doubles I can split with a well collimated scope with good optics).
Our club is contemplating buying a 30-32" mirror for our dob and I was asking myself the question 'how good a mirror is good enough?'.
To explain a little. If we go ahead with this project, the scope would be pretty much restricted to being used within 50 miles of here, so we won't be taking it to locations with super seeing (we do have dark skies nearby though). If the seeing is always lousy, visual images will be dominated by the seeing. If that argument is correct, the only reason to pay for a 1/10 wavelength PV mirror is if your seeing is good enough to show extra detail when using that mirror, over a say 1/4 wavelength PV mirror.
I realize that some people will always want to have the best mirror they can, so that they can take advantage of nights when seeing is excellent. But if your seeing is NEVER better than 1 arc second, how good a mirror do I need? The scope would of course largely be used for DSO's, and if we wanted planetary we would use an aperture mask.
Is there some analog to the 'adding errors in quadrature' method that would let me calculate that? If so, what is it and what is the conversion from seeing to PV error?
Maybe I'm thinking of this completely wrongly, in which case would appreciate someone explaining it to me .