Re: ATM Corning optical glass

[atm@misterg.fsnet.co.uk (Andy Gray)]

On Fri, 17 Jan 2003 14:25:34 -0500,  Aplanatic@aol.com
wrote:

>Rick,
>
>Try looking up Abbe Refractometer.  This will give you some idea of the method.  Are you joking about the precision?
>
>-Dave-
>
>> OK, fine.  Now, will someone explain to me how Schott and
>> Ohara to this to better than 1 part in 10 million?
>
Hello, all (1st post...)

Abbe is only good to about 10^-3. Pulfrich or Chance Hilger
(vee block) is good to 10^-5 in routine or 10^-6 under
carefully controlled conditions. Beyond this you are into
spectrometer measurements on a very accurate prism (e.g
minimum deviation method) which is good to 10^-6, or 10^-7
under exceptionally controlled circumstances. Various
interferometric methods are also good for 10^-6 /-7
precision, but surprisingly few give absolute measurements.
Indices of commercial optical glass are commonly quoted to
10^-5, with individual "melts" being controlled to +/- 5 or
10 x 10^-5 as a matter of course (all relate to n(sub)d).

I think that there's more information than this on the
"measurement services" bit of both Schott(Germany) and
Ohara's web sites.

[Brief intro:
Have just discovered this list after harbouring a latent
desire to make a mirror ever since reading a "How to.." book
years ago (can't remember the exact title, but it was from
1950's or earlier, pocket sized with a green cover). I've
been vaguely involved in optics (IR and visible) & coatings,
and now find myself in a factory making optical glass
(formerly Chance Pilkington). It's almost a uniquely
favourable position to be in if you want to make a mirror!
Having seen the time, trouble and equipment used to make
commercial optics, the prospect of making a sub-wavelength
accurate glass aspheric in my garage is just outrageous!

I have to admit a passing interest in astronomy (more a
curiosity, I suppose) fuelled by the gift of a small
telescope to my son this Christmas. 

]

Back to lurking and learning...


Andy

Andy Gray, N.Wales, UK