Re: ATM How to find indices of refraction and dispersion for glass

[Anthony Stillman <atmer@flash.net>]

Guy,

Facing exactly this problem, I did a bit of research and found several
methods to measure index and dispersion.  One of the most sensitive methods
to measure index is to form a prism from a sample of the glass to be
measured.  That prism is then used in what amounts to a two pass
photo-optical balanced-bridge circuit(1).  Of course I don't care either to
sacrifice aperture or make a percision prism.  A far less sensitive
means(2) to measure index is to measure the optical and michanical
thickness of the piece of glass(3).  This can be done by using a long focus
microscope to focus on the front and then through the glass to the back,
and recording the microscope's translation.  This is then repeated in
several colors to derive a dispersion.  Of course care must be taken to
insure that the top surface of the glass is reasonable flat and
transparent.  And that the translation of the microscope is normal to the
glass.  Some use can be made of an index matching fluid(4) for a ground
glass surface.

Consider though, that flint is in general rather heavy and hence a
measurement of the glasses specific gravity(5) can be quite telling.  This,
and the Schott Catalog(6) can go along way in guessdifing the glass.

Now I haven't made an achromat, yet, but I've read, and heard, that its a
good idea to just assume you only kind of know the indicies and
dispersions, grind and polish the lens, then test and repeat as needed.
I've seen spec. sheets on glass with several indicies identified to five
and even six places.  While glass with a paper trail isn't cheap, I'm
certain it would be less expensive(7) to order a known quanity, especially
for a small aperture, than to work an un-known.  Naturally, I perfer to do
it the hard way.  Besides a piece of optical glass has a fundamental right
to become part of a telescope.


Anthony


1) This comes from the front section of a printed Schott Catalog and
   an ~'49 issue of The Optical Soceity of America.
2) Three places, maybe four.
3) Light travels more slowly in glass.  The optical thickness, of the
   sample at a given wavelength, devided by its mechanical thickness
   is by definition the index.
4) I use Refractol ~1.567 but oil works in a pinch.
5) Place a container larger enough to hold the sample into another larger
   container.  Fill the sample container with ice cold water to the brim.
   A drop of soap will help with surface tension.  Put the sample into
   the smaller container, spilling water into the larger.  Put the sample
   continer aside, then weigh the larger container with and with out the
   spilled water.  The ~density of water is 1.000 gr/cm^3 at 4 degrees C.
6) You can download this from their site.  It doesn't work under NT.
7) Ah the time-money balancing act.  Some day I may consider my time
   working, to be more valuable than my time making telescopes.  Some
   day I may win the lottery, if I buy a ticket.