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Re: [APML] OIII line color
Dear All,
Every semester in my introductory astronomy class, I introduce
emission line spectra by having the class view separately the visual
spectra of hydrogen, helium, mercury vapor, low-pressure sodium vapor
and neon through transmission-line gratings. To show them how poor a
descriptive tool are the words used to describe visual color, I have
the entire class suggest names for the colors of each emission line
displayed by each of these elements. They can easily come up with
names for the colors displayed by hydrogen and mercury vapor but find
challenging the four lines of helium between 4730 and 5015 Angstroms
and impossible the plethora of lines in the neon spectrum. In
addition, they have difficulty coming up with verbal color
descriptives for lines closely spaced in wavelength but in different
gassees. Your discussion of the colors of lines between 5007 and
4861 A brought this exercise to mind.
As already discussed, when taking broad-band filter observations
of H II regions, the ensemble color will be the mixture of colors
contributed by each line's unique intensity with every other. The
problem arises that not every H II region will exhibit atomic line
intensity ratios identical to every other one. A specific ionized
hydrogen (H II) region _may_ display a wide variation in emission
line intensities, depending on the location in the nebula displayed
and on the metal abundances peculiar to that nebula.
While the Balmer* lines will probably exhibit ratios
characteristic of an optically thin nebula at 10,000# degrees Kelvin,
the forbidden O III lines at 4959 and 5007 Angstroms (at a fixed
ratio of about 1:1.5, respectively) will vary from bright in a nebula
with solar metal abundances to nearly invisible in a more metal-rich
one. This clearly will have an influence on the blue or blue-green
hues of a color rendition of an H II region. In addition, near H
alpha, forbidden N II at 6548 and 6584 A as well as forbidden S II at
6717 and 6734 A contribute to the characteristic red color of the H
II region. Some additional lines of oxygen at 6300 A and sulfur at
6312 A contribute to the red color, while the helium line at 5876
contributes yellow. However, the helium emission is sufficiently
subdued that it hardly makes any visual impact in an H II region.
The physically proper way to create the correct color mix for
each H II region -- and within each H II region -- would require
observing through narrow-band interference filters for each
color-group of lines, then combining the data observed through them
in Photoshop. Though this may appear an excessive challenge,
something like that is done with WFPC2 on the HST. I am unaware of
any amateur attempting the same undertaking. Bruce Balick did use
this technique when he was observing planetary nebulae at Kitt Peak
and from HST. His observations can be found at
http://www.seds.org/billa/bb/pn.html
along with those by George Jacoby and others.
Paul Rybski, Physics, UW-Whitewater
___________________________________________________________________
* H alpha (6563 A), beta (4861 A), gamma (4340 A) and delta (4101 A)
all contribute to the visual color of the nebula, with H alpha
dominating in the optically thin conditions characteristic of an H II
region.
# No matter at which H II region we look, the gas temperature
returned by a number of different measures is nearly always 10,000
degrees Kelvin.
>Jerry,
>
> Although what you say about monitor space is true, the chart
>itself is translated to the best representation within the color
>or printer space. So the color shown on that chart for the OIII
>lines is MUCH more accurate than a typical filter set can produce.
>For example, there is no doubt that the lower OIII line is a lime
>green. It is not a blue green.
>
>Mike Cook
>AF9Y
>
>
>Date sent: Wed, 17 Jul 2002 02:33:07 -0400
>To: astro-photo@seds.org
>From: Jerry Lodriguss <jml@astropix.com>
>Subject: Re: [APML] OIII line color
>Send reply to: astro-photo@seds.org
>
> > Hi Mike,
> >
> > I think one problem is that the locations of the two OIII lines lie outside
> > of the Maxwell triangle that would define any monitor's color space. So,
> > while these two spots are plotted correctly on the diagram as pure spectral
> > colors on the spectral locus, they can't be correctly displayed on a
> > monitor, neither on the CIE chromaticity diagram they are plotted on, nor
> > on any RGB emissive display of a deep-sky image.
> >
> > So the green they would look like visually can't be displayed correctly on
> > any monitor, no matter what you do. You can't make an accurate color print
> > of them either.
> >
> > Sorry. <G>
> >
> > Jerry
> >
> >
> >
> > At 10:22 PM 7/16/2002 -0400, you wrote:
> > >Tony's right, we've had the color discussions several
> > >time on APML. For the new folks, here's a repeat of
> > >my posting showing the proper colors:
> > >
> > >The correct color for the two OIII lines, Hb, Ha can be
> > >shown on a chromaticity chart. Here is the chart with
> > >circles showing the color of those lines:
> > >
> > >http://www.af9y.com/chrom.jpg
> > >
> > >This example also shows the mix of Ha/Hb required to
> > >produce the sometimes seen pink/violet type color.
> > >Note that it takes a mix of 2.5 parts Ha to 1 part Hb for
> > >this color range.
> > >
> > >You can experiment with other emission
> > >line mixes using the following CIE calculator:
> > >
> > >http://www.flatnet.org/glossary/ciecalculator.htm
> > >
> > >
> > >Mike Cook
> > >AF9Y
> > >
> > >
> > > >From: "Tony Hallas" <tonyhallas@foothill.net>
> > >
> > > > Chris,
> > > >
> > > > This is an ongoing debate about the correct color for OIII... it
> > > really should be half
> > > > green, half blue... something like a green-cyan color I guess. Most
> > > color films cannot
> > > > capture this green color... it shows up better in tricolor
> > > attempts... regular color
> > > > films show this color only as a blue but there is a green component
> > > to it. The SBIG
> > > > filters capture the OIII line with more green than blue... hence the
> > > dramatic color.
> > > > Believe it or not, that green is actually toned down a bit from what
> > > came up on the
> > > > image... I took some of the yellow out. But I would rather work from
> > > this end (reduce the
> > > > green) than not have it... anyway... there are other filter sets that
> > > give different
> > > > versions of this line... maybe when I recover from the expense of the
> > > camera I'll try
> > > > another set.
> > > >
> > > > Tony
> > >
> > >
> > >
> > >Mike Cook, AF9Y
> > >http://www.af9y.com
> > >mwcook@cris.com
> > >
> > >
> > >
> > >
> > >-- APML Archives at <http://astro.umsystem.edu/apml/> ---
> > > Unsubscribe at <majordomo@seds.org>
> >
> >
> > Astronomical photography: http://www.astropix.com
> >
> >
> >
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> >
>
>
>
>Mike Cook, AF9Y
>http://www.af9y.com
>mwcook@cris.com
>
>
>
>
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