Glass
I will start this post by stating that I have no knowledge of the glass or optical business. I can relate my experiences with finding glass for my two projects.
For the 11" plate I was able to obtain from a optical friend a BK7 plate that was a clear glass with a coating used as a filter. The wedge was only .0002" and the coating ground off as the plate was ground. No problems here. As for the 16" plate I was not that lucky. In this case I needed a 16"x 1/2" plate. Plane parallel BK7 would do the job. In this size I was quoted 1K plus dollars which was out of my low budget range. I talked with another glass supplier that had a glass called water white. He said it was plane parallel and was for optical windows and filters. It sold for $100.00 per sq. ft. but only came in 1/4" thickness. Next I located a glass dealer that makes glass table tops. I ordered a 16" table top of a glass that the dealer called crystal white. Also ordered was a 16" disk of plate glass. Both were unedged or edge polished ( big error as the edge had 1/8" chips in the disks). The crytal white was almost white and cost twice as much as plate glass but it had .015" wedge! As not being up to grinding .015" wedge from a 16" disk I ordered a another 16" plate glass edged and polished disk. This one had only .0004" wedge. Yes in a 1/2" thick plate it does have a green tint but in my Schmidt camera this is no problem.
I think any good glass can be used in a plate if the thickness is in the correct range and the wedge is .001" or less. (more if you don't grinding the wedge out). The index of refraction need to only in the ball park. How about those solar filters? The coating will grind off. For you low budget people remember the original 48" Schmidt had a plate glass corrector. A lot those SCT's out there have green glass correctors also (look like window glass to me).
My next post will cover mounting the plate to the pan and saging it. Bob Pfaff