Re: ATM H-Alpha filters

[Anthony Stillman <atmer@flash.net>]

>How does one use birefringence to make an H alpha filter?

Birefringent materials have two indices of refraction which are
polarization dependent.  Properly oriented (perpendicular to the optical
axis) light passing through will, dependent on polarization, either travel
faster or slower.  Positive birefringence describes materials (like quartz)
where the extraordinary ray (e) propagates more slowly (has a higher index)
than the ordinary ray (o) (lower index).  Negative birefringence describes
materials (like calcite) with the opposite behavior.  Both of these indices
are also wavelength dependent.

A polarized wavefront oriented 45 degrees to both the ordinary (normal to
the optical axis, the other normal) and the extraordinary axis (parallel to
the optical axis) can be decomposed into (viewed as) two polarized
wavefronts, one parallel to the ordinary axis, the other parallel to the
extraordinary axis. These two polarized wavefronts propagate at different
velocities in the birefringent material.  Consequently their superposition
(composition) may be seen as a single rotating polarization.

The wavelength dependence of the indices means that the polarization of
some colors rotates more than others.  A polarizer positioned where the
wavefront exits the material will pass mostly only the color (frequency)
with the properly oriented polarization.  And the set of integer multiple
of that frequency which are also properly oriented as they rotated an extra
turn

The frequencies fully blocked or fully passed are dependent on the path
length traveled through the material.  The spacing between these
frequencies is also dependent on the path length traveled through the
material.

A stack of birefringent blocks, each a different thickness and separated by
polarizers, like a pile of fine and coarse toothed combs stacked to pass
light through only a single space and obscure it with one comb tooth or
another everywhere else, will pass only a single color of light.

This is called a lossy birefringent chain network.  Or the Lyot solar filter.

There are several optical filters which employ the same basic principals.
Some are termed "lossless" as they contain no polarizers.  The Solc filter.
Others do not use birefringent materials at all.

Anthony


acti labores iucundi
Accomplished labors are pleasant.