[SML] sodium vapor

[Judith Kupferman]

That's really fascinating, because they are really bright! The light
pollution is awful for us real people, but must be easy to filter
because it's only 2 wavelengths.

" low pressure sodium vapor streetlights are used in the San Diego
area as an accommodation to the nearby Mt. Palomar observatory.  The
resulting light pollution is relatively easy for the observatory to
filter away.  If other light sources were used, the resulting light
pollution would render the 200" telescope almost useless as a research
tool."