Those filters block enough additional light to be detrimental when it
comes to visual observation of many, if not most stars and galaxies. I
was more curious about a filter for visual use that selectively
blocked the natural air glow emission lines (5577, 5893 and 6300)
while allowing most of what remains to pass through largely
undiminished; basically a *dark sky* air glow filter designed for
*visual* use in areas either completely or mostly free of man-made
glows.

"W. H. Greer" <sendnom...@tome.net> wrote in messagenews:9v6is2pgpi319fi4um31nuumg8qrmivmd9@4ax.com...

On Mon, 05 Feb 2007 13:41:12 -0600, David Knisely <KA0...@navix.net
wrote:

Natural light pollution comes in two varieties: Emission and continuum.
The common emission lines from airglow are the O[I] lines at 5577,
6300 and 6364 Angstroms, as well as the faint Sodium "D" lines at 5890
and 5896 Angstroms. To block these effectively requires just a standard
"broadband" LPR filter like the Lumicon Deep-sky, Orion Skyglow, or the
IDAS LPS filters.

Those filters block enough additional light to be detrimental when it
comes to visual observation of many, if not most stars and galaxies. I
was more curious about a filter for visual use that selectively
blocked the natural air glow emission lines (5577, 5893 and 6300)
while allowing most of what remains to pass through largely
undiminished; basically a *dark sky* air glow filter designed for
*visual* use in areas either completely or mostly free of man-made
glows.
Such a filter, used visually under a dark sky, should provide natural
looking views of stars, galaxies, etc. against a somewhat darkened sky
background.

I am not sure if such a filter will work. At least as far as the sodium D line
is concerned, the mechanism of light production in high pressure sodium (HPS)
lamps around this line is extremely complicated.

The actual sodium D lines ARE blocked already. Basically what happens is HPS
lamps exhibit self-reversal because the D lines are resonance lines, so what
you get is lots of light AROUND the actual D lines themselves and no light
exactly AT D. Here's a rough description of the phenomenon for the Sodium D
line and for the Mercury blue line (which is much less noticeable):

http://ioannis.virtualcomposer2000.com/spectroscope/absorption.html

The light at the "sides" of the D lines is called "the wings" of the D line,
and it's pretty much the light which is actually responsible for HPS lighting
and light pollution. You can see a picture of the entire spectrum of an HPS
(second to last pic) on this page:

http://ioannis.virtualcomposer2000.com/spectroscope/Na.html

As a result of the D lines having "wings", a substancial area of the spectrum
has to be killed if the filter is to make any sense. Unfortunately, the area
that needs killing includes useful red, yellow and green light which
contributes to faint visibility.

To make things worse, all the rest of the lines of Sodium exhibit thermal
broadening, which forces the lines to emit an entire band around them (lines
to the right on the pic), so these cannot be blocked very efficiently either,
unless a whole chunk is killed along with the central wavelength.

The situation is far better with low pressure sodium (LPS) and high pressure
mercury (HPM) lamps, because these lamps do not exhibit self-reversal. Well,
they do, but it's not noticeable. A filter which blocks the EXACT frequency on
these lamps works beautifuly. With HPS lamps the situation is a mess.

All this mess was essentially unavoidable, because one of the main reasons for
the extreme popularity of the HPS lamps is PRECISELY their ability to reverse
the D line and broaden the rest of the Sodium lines, so that engineers could
solve the annoying problem of the monochromaticity of the LPS lamps.

Ok, but doesn't most light pollution glow come from HPS lamps? If this is the
case, then the radiation should be pressure-broadened.

I am not sure if such a filter will work. At least as far as the sodium D
line
is concerned, the mechanism of light production in high pressure sodium
(HPS)
lamps around this line is extremely complicated.

I think he was talking about natural sodium D line emission in the
stratosphere where it is not significantly pressure broadened.
Exciting this emission in the sodium rich layer with a dye laser is
one trick used to create fake guide stars.

http://www.llnl.gov/urp/science/lgs_www/lgs.html

And filters against the sodium D-lines like Nonad and the Baader
neodymium do work very well photographically provided that your local
streetlamps are low pressure sodium. It is not for nothing that
professional observatories mandate low pressure sodium lamps in full
cut off fixtures in their vicinity.

Regards,
Martin Brown
--

Huh? Why would one need to use a filter to block the sodium D line on *dark*
skies?

Ioannis wrote:
Ok, but doesn't most light pollution glow come from HPS lamps? If this is
the
case, then the radiation should be pressure-broadened.

Bill's proposed filter use was for *dark* skies, not light-polluted
ones.

At least in the U.S., you are correct as far as city skies are
concerned.

--
Brian Tung <brian@isi.edu
--

Ioannis wrote:
Huh? Why would one need to use a filter to block the sodium D line on
*dark*
skies?

Better contrast. You may find it interesting to know that planetary
nebulae improve noticeably with narrowband filters under dark skies as
well as light-polluted skies.

--
Brian Tung <brian@isi.edu
--

"Brian Tung" <brian@isi.edu> wrote in message
news:eqdcrf$fcl$1@praesepe.isi.edu...
Ioannis wrote:
Huh? Why would one need to use a filter to block the sodium D
line on
*dark*
skies?
Better contrast. You may find it interesting to know that
planetary nebulae improve noticeably with narrowband filters under
dark skies as well as light-polluted skies.

I am well aware that narrow band filters improve contrast under most
conditions. What I fail to understand is how SPECIFICALLY *blocking
the D line* benefits the contrast under *dark* skies.

Unless there's ALWAYS some residue D line HPS/LPS radiation
regardless of location.

Natural light pollution comes in two varieties: Emission and
continuum. The common emission lines from airglow are the O[I] lines
at 5577, 6300 and 6364 Angstroms, as well as the faint Sodium "D"
lines at 5890 and 5896 Angstroms.

I am well aware that narrow band filters improve contrast under most
conditions. What I fail to understand is how SPECIFICALLY *blocking the D
line* benefits the contrast under *dark* skies.

Unless there's ALWAYS some residue D line HPS/LPS radiation regardless of
location.

A few messages back in this thread David Knisely wrote:

Natural light pollution comes in two varieties: Emission and
continuum. The common emission lines from airglow are the O[I] lines
at 5577, 6300 and 6364 Angstroms, as well as the faint Sodium "D"
lines at 5890 and 5896 Angstroms.

That's *natural* airglow. Perhaps this answers your question?

--
Greg Crinklaw
--

There is some natural Sodium D-line emission in the airglow light from
the Earth's atmosphere. Indeed, ground-based astronomers have used
intense laser light to excite the sodium in the upper atmosphere to
create an artificial star bright enough for adaptive optics systems to
use as a reference. The strongest airglow lines are the 5577 Angstrom
and the slightly weaker 6302/6364 Angstrom lines of Oxygen (O[I]),
followed by the Sodium D line (about half the strength of the O[I]
lines), and some very weak bands in the deep red from the Meinel
rotation-vibration of the OH radical. There is also a weak "pseudo
continuum" level of emission due to Nitrogen Dioxide from around 5000 to
6500 Angstroms. Clear skies to you.

David W. Knisely KA0CZC@navix.net
--