Here's the translation of optical tests on two GSO 8" RC .
Scroll down for interferogram and star tests, ronchi and phase contrast photos ( to see surface roughness ) on two different instruments .
Strehl ratio of 0.56 and scary level of surface roughness on these instruments proves the old adage " you get what you pay for". Compare the smoothness of the ronchi bands ,phase contrast test and star test , with the refractors on this page. How much this effects prime focus photography I can't say but it is not desirable.
http://tiny.cc/XSmyf
Mark