[ATM] refractive index of air

[Dominic-Luc Webb]

On Wed, 8 Aug 2007, Richard wrote:

> R> there's a Sellmeier representation
>
> Actually that looks wrong to me, I think there may be a typo. It is of
> the form of Sellmeier Type 4, but it should be N^2, not N. A quick
> numerical check will soon tell you. If it isn't a typo, then it isn't
> Sellmeier, so something's wrong.

OK, in any event, the Sellmeier equation looks new to me and that
is what I need to learn. I looked it up in Wikipedia, and that
equation looked pretty straightforward. Hopefully there is no typo
in the Wikipedia formula.

I am attempting to apply the Norman test (Burt A. Norman, S&T Nov
1957, PP38-40) to plate glass for a Ritchey-Chrétien (RC) secondary.
In this test, light passes twice through the secondary; the back
surface is optically flat and silvered. For this test to work, it
is required that the eccentricity equals the ratio of refractive
indices of glass to air in order to get a null test. While the
hyperbola for any specific RC design has a fixed conic constant
(i.e., fixed eccentricity), the wavelength for the test is flexible
to the limit the glass must be able to transmit that wavelength.

The plan is to take advantage of the fact that the refractive
index of glass changes as a function of wavelength much more
than air. I'm thinking this offers a lot of flexibility in
which amplification I can use and which type of glass. I reason
that it should even be possible (practical???) to use plate
glass. The present design is a 315 mm diameter F/4.8 primary
with an amplification of 4.8. I am polishing the primary;
secondary has only seen rough grinding thus far.

Dominic-Luc Webb