i bought a parks mirror several years ago based on their good reputation. i felt that mirror testing was beyond me, so i would pay the premium price and not need to test. dont make my mistake. after literally years of disappointing views and enough fiddling that i am an expert at collimation i have finally built that tester. it took one day to build, and the test took minutes. how a six inch f/8! could be so badly zoned is beyond all comprehension. if it had been left spherical it would at least have been NEAR the rayleigh limit. i am willing to believe this is a one time freak phenomenon. but making a tester is easy compared to the troubles and disappointment of a bad mirror. i will never buy a mirror without testing ever again. no matter who makes it.