Re: [ATM] You Are a Smart Person...

[Dominic-Luc Webb <dlwebb@canit.se>]

On Sun, 26 Aug 2007, Bob May wrote:

> >From Richard S./
> You are a smart person.   Be creative.   Test under water or in
> an oil bath that reduces >the index ratio at the key surface.
> But I still think it is poor practice to test a reflecting
> >surface by refraction.
>
> Richard, the thing here is that you need to increase the
> difference in the index of refraction.  Using some liquid only
> makes things worse un5til you get to the same value as that of
> the glass where the surface becomes perfectly transparent and
> won't refract the light any.

This is where things get ugly. To keep the amplification from
being excessive (roughly <6), a high refractive is needed, let
us say above 1.54. The only liquids with high refractive index
that can approach this are quite nasty volatiles. Simple and
safe things like sugar/salt water, cedar oil, etc won't work.

I think plate glass (which I am using), Pyrex and BK7 have about
the lowest refractive index (around 1.51-1.52):

http://www.canit.se/~dlwebb/catadioptric/310schmidt/optiwhit.jpg

In considering the Norman test, I did not implement a separate
flat. The back side of the secondary is silvered 1/20 wave flat,
and I did not confine myself to visible light. This allows to
important modifications in planning:

1.
In near IR wavelengths, the refractive index of plate glass drops
below 1.51. I believe I have seen at least one liquid that has
such RI in this IR range. Hence, testing in a liquid media
could be possible.

2)
In the UV, the RI increases drammatically, favoring the option
of the Norman test for a mirror without a drammatic amplification.

I am still considering what is theoretically possible, but it is
clear that the possibilities increase if one is willing to move
outside the visible spectrum for testing purposes.

Dominic

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