If you look at the test data from the 1st add, you will see from the lot of 6 20" the Strehl numbers are .956, .980, .961, .954, .954 and .980. Looks like for the extra premium, you can have one of the .980 mirrors. If I understand correctly, the interferometric testing is costly and time consuming. The optician makes his best mirror (and no two will be exactly alike) then tests them. If they meet the minimum criteria, they go on sale and for a premium, you can pick the best of the lot. I don't see a problem with this at all. I have to confess I have one of those mirrors, it came on a used scope though so I did not have a choice up front. The seller though was asking a little more because of it though. I'm not sure if I am expert enough to tell the difference, but I am pleased with the views. Just my $.02...
J. Tucker
J. Tucker
