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Re: [APML] OIII line color
Paul,
Good information. Although we did not have narrowband
filters for every emission line, Thor Olson and I collaborated
on an approach similar to what you discussed. The filters
had some overlap but did provide a good separation of the
major emission lines in the veil nebula. These images were
then mapped into a "HOT" color space to obtain a more accurate
representation of the nebula colors. You can see the
result here:
http://www.af9y.com/veilrgbhot.htm
One of the problems with this approach is obtaining correct
star colors since they are broadband sources and are not
properly represented by the line spectrum mapping. We used
a hybrid combining approach in an attempt to overcome this
issue.
Mike Cook
AF9Y
Date sent: Thu, 18 Jul 2002 02:32:39 -0500
To: astro-photo@seds.org
From: "Paul M. Rybski" <rybskip@idcnet.com>
Subject: Re: [APML] OIII line color
Send reply to: astro-photo@seds.org
> 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
> > > >
> > > >
> > > >
> > > >
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> > >
> > >
> > > Astronomical photography: http://www.astropix.com
> > >
> > >
> > >
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> > >
> >
> >
> >
> >Mike Cook, AF9Y
> >http://www.af9y.com
> >mwcook@cris.com
> >
> >
> >
> >
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>
>
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Mike Cook, AF9Y
http://www.af9y.com
mwcook@cris.com
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