From: atm{at}misterg.fsnet.co.uk (Andy Gray)
To: atm{at}shore.net
Reply-To: atm{at}misterg.fsnet.co.uk (Andy Gray)


On Fri, 17 Jan 2003 14:25:34 -0500,  Aplanatic{at}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

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