I purchase nearly all of my borosilicate glass mirror blanks from Newport Glass Works in sunny California. These folks are great to work with, and they are SPEEDY! The most common glass I use for mirror making is called Supremax-33. On a typical 20" mirror blank, Newport will create and fine anneal my blank in three to four weeks. That may sound like a long time, but in the glass business that is very good service. After all, a blank spends days and days in the oven, annealing and getting all those atoms lined up just right.
The thing I really appreciate about Bill and Ray and Christene is their willingness to make up special blanks for me. For example, we have our big elliptical secondary blanks thinned down quite a bit from the standard. This gives us a lighter mirror, and a lighter upper tube assembly. That's something that is really nice when you are trying to balance a telescope with a giant "pineapple" eyepiece in the focuser! And this service lets me customize blanks for any client's specific needs. If somebody wants a 1" thick blank that is 15" in diameter, it's not a problem. The same way, a client might order a 20" mirror that is 1.75" thick. Again, not an issue. This saves huge time in fabrication, and means that I can produce more mirrors with less work.
Newport almost always pre-generates and machines my blanks. The round blanks start out sort of "mildly" flat. Usually I have the back of the blank "Blanchard ground" to a very flat surface. Then Newport puts the glass on a big diamond grinding machine and they generate the curve on the front to my specifications. This is an operation that saves huge amounts of time and abrasives. For example, I recently purchased some 10" blanks pre-generated to a 40" radius. That's F2.0! When they arrived, the radius Newport generated was within 0.5" of specification. And the diamond tooling they use left a surface that was so smooth I was able to start grinding with 25-micron abrasive! That means avoiding hours of grinding with the old 60/80/120/220/320 sequence of silicon carbide abrasives. Of course they can edge any blank, and bevel it, and do just about anything you might need on a mirror blank.
Thanks to everybody at Newport. Their blanks are terrific!