MIG,
No offence taken.
Doug Miller is a small time Amateur Astronomer and mirror maker in the USA. That statement is his opinion which he is entitled to have, doesn't mean its correct. He makes good mirrors but does not have the reputation of people like Carl Zambuto, Bob Royce, Steve Swayze or Larry Hardin. If you also look at the properties of BK7 glass on the Melles Griot website you will notice some of the figures are significantly different to what Doug quotes, particularly Knoop's hardness which Doug quotes at 520 and Melles Griot at 610. This compares with plate glass at 470. A couple of other numbers worth looking at are Steady State Thermal distortion and Transient Thermal distortion where BK7 fares infinitely better than plate glass.
http://www.mellesgriot.com/products/...mp_3_1.htm#BK7
FWIW Bob Royce indicates that he quite likes BK7 as a primary mirror substrate but goes on to say that he has also made a number of excellent plate glass mirrors in the past.
What does all this mean, not a lot really other than "who makes the mirror" is infinitely more important than "what its made from" which is what I also indicated in my previous post.
CS-John B
