Re: ATM B.V.C. concerns

[David McCarter <mccarterd@claven.fanshawec.on.ca>]

>Some members of our club are going to grind some mirrors this winter
>and my thought was to use the BVC blanks offered through ASM in Canada.

One of the local ATMers has now made several mirrors with BVC, and is
currently polishing a 24" to about f4.5.

>Questions I have relate to its ease of grinding,

It is quite soft, more like plate than pyrex, and grinds quickly. It is
also more prone to fine scratches if you are not careful in maintaining a
clean work place, and keeping a fine bevel on the blank edge.

>ability to determine when polishing is complete

Because it is quite black it is easy to see any pits. The polished surface
seems to have a depth like a multi coated varnish on walnut. Pits do stand
out quite well, but of course will not be a problem because you will do
adequate fine grinding!

>will it respond well to foucalt testing

Certainly does, and because the material is opaque you do not see the back
side of the blank, the way you often can on pyrex or plate. The reflection
looks brighter for some reason.

>, can you use a ceramic tile tool,

That would be the prefered method, although using a large steel tool to hog
out, and then make a plaster tool makes a lot of sence. ASM will pregrind
to the depth you want if you want to avoid the sweaty work on a large mirror.

>will the coaters have a problem aluminizing

Seems to accept coatings as well as any other glass. No problems there.

>and was the ASM prompt responding to orders.

ASM does not seem to respond to their email as quickly as they answer the
phone, so that is surely the prefered approach. Their lead times are, the
last time glass was ordered here, quite reasonable, although they are
getting bussier as their product gets better known.

>Also any other peculiarities with the stuff.

It's black. Wierd looking, but makes a great mirror. Seems to cool down
quickly in the field, but that may just be subjective and not reality.

Dave