re:Re: re: glass scale expansion vs steel

I had a good reason for using BK7 as a sample glass rather than pyrex. What
follows is a discursion into glass..
A grating scale used in transmission had better be free of bubbles, down to
at least the size of the grating period. While no glass is guaranteed to
have no bubbles of any size, BK7 is one of the best glasses for having a
small number of bubbles and good homogeneity. Hence its almost universal
use for flat optical work. When lower CTE is required fused silica is used,
but BK7 has much better bubble characteristics and homogeneity, except is
the ultra expensive grades. BK7 also has pretty good chemical properties,
and is available in large pieces. It is the least expensive of the quality
optical glasses.

Bubble content in glass is dependent on the viscosity of the glass. BK7 has
a moderately low melting point, and can be held at high enough temperature
to give it a chance to get rid of bubbles without devitrifying
(crystalizing). Devit in glass is as bad as bubbles, it is lots of little
crystals floating around in the glass.

Optical scales are probably made from a specialty glass in a continuous
process. There are specialty glasses for almost every conceivable purpose.

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'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.

The glasses with much higher expansions have other undesirable properties.

A reflection grating is a totally different beast, and could be made on
either metal substrate or lower quality glass.

Elliot Burke

>Message: 17
> Date: Sat, 18 Mar 2000 12:49:59 -0500
> From: Ken Lowther <lowther@...>
>Subject: Re: re: glass scale expansion vs steel

>Elliot Burke wrote:
>
>> From: "Elliot Burke" <elliot@...>
>>
>> For curiousity, I calculated the temperature change needed to cause a

length

>> measurement error of 0.0001"/ft between glass and cast iron.
>> The glass is BK7, a typical borosilicate glass used for many purposes in
>> optics, including scales. CTE=3.8 10^-6 /°F.
>> Cast iron CTE is taken to be 6 10^-6 /°F, per my materials book.
>> Then a temperture change of about 4 °F causes a 0.0001"/ft differential
>> expansion between the glass and iron.

>Schott glass has an online catalogue with info for a lot of different
>glasses, including CTE. I don't know the site off hand. BK7 would have
>the advantage of being a "common" glass, though it is still relatively
>expensive. I think pyrex would have a closer CTE. More common and
>cheaper. Depending on the setup, you might be able to do some effective
>heat synching between the two units.

>Did a quick search before posting. I don't do windoze much anymore and
>the catalogue is actually a windoze program.
>http://www.schottglasstech.com/catalogs.htm

--

>Ken Lowther
>Youngstown, Ohio
>http://www.atmsite.org
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