14 inch 1 inch thick plate mirror

I'm about to replace the 13 f-5.6 inch mirror in my homemade dobbie with a 14.5 inch f-5 using a 1 inch plate blank.

I've read (don't agree necessarily)that plate is harder to work and needs a sealed tube.

The BVC mirror is 45mm thick. Works well but I get minor astigmatism when it cools that settles. I have an open bottom mirror box in a truss tube telescope.

Do I need to make a sealed mirror box? or is it better to have the open bottom design which surely means faster cooling with the plate mirror?

Also is plate more difficult to figure than Pyrex in this size mirror??


I refuse to believe its harder than BVC which was a nightmare to figure. I've done Pyrex in a 13 inch mirror it turned out very well and was far easier than BVC to figure.

Phil, I have worked three plate glass mirrors, an 8, a 10, and a 16. I would say from my limited experience that the plate is actually somewhat easier to grind and faster to polish. However, you will discover that you need to wait at least a half hour to an hour to make final focault readings because it takes a little longer than pyrex to stabilize after polishing sessions.
Many years ago our club built a 14" plate glass dob that was an f/5. It was a great scope and gave very nice views in the summer months, but in the winter it took several hours to equalize to the point reasonable images were seen. That was in a climate that would get down to below zero C often and the mirror would be transported from a warm environment. We learned to set up several hours before dark!
I say go for it and enjoy the experience and the views!

Walt