Re: [CAD_CAM_EDM_DRO] re:Re: re: glass scale expansion vs steel

Elliot Burke wrote:

> Hence its almost universal
> use for flat optical work.

But optical reference flats are usually Zeodur or ULE. ;^) I have an
old fused silica flat claiming to be good to .000001 inch flat per
side.

> Pyrex is a borosilicate glass, but not of optical (transmission) quality.
> It is full of bubbles, and is rarely even specced for bubbles. It makes
> good kitchenware, and is used for low end telescope mirrors, but that's
> about it.

I was thinking that the grating would be etched into the surface. I can
see the advantage of having it on the bottom. Can you selectively
focus through the glass to ignore surface scratches that may happen
while the unit is in use?

>
> I've been using BK7 in my optical designs for 25 years, and have yet to come
> across a rejectable bubble in it.
>
> The Schott catalog has been available as a downloadable database for some
> time, I prefer the printed version. Also have Hoya, Ohara, Chance, Corning
> of France, as well as a few filter glass catalogs. Kodak used to be in the
> business, but no longer.

Only wanted to make sure you knew the alternatives. Guess that was
excreting my futility. Yes I scratch out telescope mirrors from lowly
pyrex. Molded precision annealed blanks will have the occasional
bubble. I have never worked with sheet pyrex. There has been a lot of
grumbling from knowledgeable telescope makers that pyrex isn't the same
quality as it used to be.


My initial question, in my mind, was why you didn't use something with a
closer COE. But, you have adequately answered that.

--
Ken Lowther
Youngstown, Ohio
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