[glend (Glen)]

Thanks Allan. Those central obstruction ratios and Contrast Factor calculations, really cannot be used to compare different scope designs IMHO. If you were comparing two similiar sized visual newts to make a purchase decision, then yes they might be good to know.

When I look through the MN190 it is a wonderful view compared to looking through the 10" Imaging Newt, with pinpoint stars, great contrast, good shape, coma free to the edge, etc where as the Contrast Factor numbers say the 10" newt should be the better. That's the problem for me. It's not apples to apples. I'd be happier comparing the MN190 to a equivalent clear aperture APO (Something like a 6" APO), because the view I am seeing is closer to what that APO provides. Trying to get a central obstruction down to 18% in a newt to reach refractor central spot parity is only relevant where your are comparing equivalent apertures. IF you accept that you need to have more aperture in the newt to negate the obstruction differences then your getting to the heart of the matter. Re the Mn190 verse the 10" I can only assume the difference lies in the mak corrector front end, and the rest of the optical design that is not brought into consideration in that paper's Contrast Factor determination, which is a newt to newt comparison factor.