Boring!! (a hole in an existing mirror)

[koputai (Jason)]

Guys,
I'm contemplating boring a hole in an existing 8 inch Newt mirror (1 inch thick) to make a classic Cassegrain.
I know, I've read Texereau, and he says don't do it. But, I have a spare mirror, and think I might give it a try. Am I absolutely going to ruin it? Is it even worth a try?

Cheers,
Jason.

[Merlin66 (Ken)]

With some care and attention it can be done. We used to make mirror blanks from old porthole glass using galvanised sheet wrapped round wooden formers as the cutters and #80/ #120 grit.
The starting and break-through are the critical parts.

[sheeny (Al)]

Never done it myself but I've seen it done on bottles and other glass objects...

Use a copper pipe the diameter of the hole you want and valve grinding paste and grind by hand. It's a very gentle process - not sure how long it would take you to do a mirror.

Al.

[Merlin66 (Ken)]

It used to take about 20-30 minutes to cut a 200mm diameter disk, 25mm thick.

[Satchmo]

Originally Posted by koputai

I know, I've read Texereau, and he says don't do it. But, I have a spare mirror, and think I might give it a try. Am I absolutely going to ruin it? Is it even worth a try?

Unless the glass is very well annealed boring the hole after the mirror has been finished may release strain, thereby warping the mirror. I guess the issue is whether you can spare losing the glass or not.