Re: ATM Perferating Cassegrain (slightly off topic)

["Thomas Janstrom" <tomjan@ozemail.com.au>]

----- Original Message -----
From: <StarHoper@aol.com>

> What is the correct procedure when making a cassigrain type mirror, to
core
> first before grinding or after polishing?
>

Actually, David you couldn't be more on topic;)

It's best to core before grinding and either wax or plaster the core back in
so you don't get a turned edge around the hole. But you can core when you
are almost finished grinding too, only in this case you should not cut all
the way through from the back stop about 1/8" from the front, finish
grinding, polishing and figuring, and then cut the core out from the front.

This way you don't get any edge effects and you have, at least in theory,
removed any strain in the glass prior to figuring. Only two risks in this
method, one the glass could have so mush strain in it that it fails when you
get about half way in from the back, and two that you will scatch the
polished surface when you cut from the front. Remember you are putting 400
grit on a polished surface in this case.

If I had to do this again I would cut the core out compleatly before
beginning grinding, and plaster it back in and then remove it after I had
finished the mirror.

Cheers, Thomas.