[ATM] refractive index of air

[Jim Burrows]

At 2007-08-08 14:10 +0200, Dominic-Luc Webb wrote:

>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.

I've got an aversion to null tests (backed up by 
the Hubble disaster).  You could run this test as 
a Foucault (if it doesn't end up with too small 
an f/) by slapping a mask on the front and 
calculating where the surface must be to give the 
ray path that you observe for each mask 
hole.  The main advantage is that you can plot 
the mirror deviations from the required secondary surface to match the primary.

Figuring an RC secondary to match a primary is a 
real hassle (I'm still fiddling on mine) - 
essentially you have to get both R and b to where 
they belong.  If the test says the edge is high, 
then, unlike good ol' parabolas, you can't just 
whack the edge with a small tool - that screws up 
the R and probably the system RMS increases.

         -- Jim Burrows
         -- http://home.earthlink.net/~burrjaw
         -- mailto:burrjaw at earthlink.net
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