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Any objective mirror faster than f10 needs to be figured to a parabola in cross section, a spherical mirror at f5 would suffer from severe undercorrection and would be quite unusable. I have a ten inch GSO reflector which shows excellent correction using the star test; the diffraction pattern is the same inside and outside of focus which means the mirror has been well figured to a parabolic shape. Any deviation from a parabolic figure would show up under the star test.
The term Parabolic mirror should not be used as a selling point as it should be a given in any usable reflector regardless of brand.
Malcolm
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