APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

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APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by APOD Robot » Mon Mar 08, 2021 5:05 am

Image Three Tails of Comet NEOWISE

Explanation: What created the unusual red tail in Comet NEOWISE? Sodium. A spectacular sight back in the summer of 2020, Comet NEOWISE, at times, displayed something more than just a surprisingly striated white dust tail and a pleasingly patchy blue ion tail. Some color sensitive <a rhef="https://www.syfy.com/syfywire/comet-neo ... piral-arms" >images showed</a> an unusual red tail, and analysis showed much of this third tail's color was emitted by sodium. Gas rich in sodium atoms might have been liberated from Comet NEOWISE's warming nucleus in early July by bright sunlight, electrically charged by ultraviolet sunlight, and then pushed out by the solar wind. The featured image was captured in mid-July from Brittany, France and shows the real colors. Sodium comet tails have been seen before but are rare -- this one disappeared by late July. Comet C/2020 F3 (NEOWISE) has since faded, lost all of its bright tails, and now approaches the orbit of Jupiter as it heads back to the outer Solar System, to return only in about 7,000 years.

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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by Ann » Mon Mar 08, 2021 7:54 am

Nice! I always love to see a blue tail on a comet, but I don't mind seeing a comet with a blue and a red tail, either! 😀

It's almost like seeing a red emission nebula with blue stars in it, after all! :D

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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by Knight of Clear Skies » Mon Mar 08, 2021 7:54 am

Nicely explained in the mouse-over image, clear and concise. I'll have to take another look at my image to see if I have any hints of the carbon and sodium.

Nicolas Lefaudeux has some other excellent images of NEOWISE on his site to complement the APOD. This animation showing a spiral jet is the standout for me.
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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by Holger Nielsen » Mon Mar 08, 2021 8:49 am

I don't get it: "Red sodium light"? Neutral sodium is famous for the doublet D Fraunhofer lines at 589.00 and 589.59 nm. But those have a distinct yellow color. The link given also identifies the sodium tail light with a wavelength of 5890 "angstroms". I know of no bright red sodium line, so is the colour really red or is this a result of image processing?

The image is otherwise very impressive and so is the animation dug out by Knight of Clear Skies.

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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by stevewiggins » Mon Mar 08, 2021 8:53 am

Comet West (1976) had a broad red portion to it's tail. Also the nucleus was clearly visible throughout the day.

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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by Ann » Mon Mar 08, 2021 9:06 am

Holger Nielsen wrote:
Mon Mar 08, 2021 8:49 am
I don't get it: "Red sodium light"? Neutral sodium is famous for the doublet D Fraunhofer lines at 589.00 and 589.59 nm. But those have a distinct yellow color. The link given also identifies the sodium tail light with a wavelength of 5890 "angstroms". I know of no bright red sodium line, so is the colour really red or is this a result of image processing?

The image is otherwise very impressive and so is the animation dug out by Knight of Clear Skies.
Very good observation, Holger.

Please note that practically all stars that are called "red" in "astro-speak" are in fact orange-yellow to the eye.

I think astronomers call yellow objects red by habit.

As for the red color of the sodium tail in the APOD, I believe that the picture has been processed in order to make the tail look redder than it actually was.

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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by JohnD » Mon Mar 08, 2021 10:48 am

Thank you, Holger Neilsen, you got ahead of me!

The abstract of the paper linked to, https://ui.adsabs.harvard.edu/abs/2020D ... Y/abstract by Ye, Zang, Brewer et al, says , "Strong sodium and potassium lines are also present. We also detect H2O+ lines in the visible-red wavelength (5800 to 7400 Ang), though they are much weaker than the sodium line, suggesting that the red straight tail reported by numerous observers in mid-July was likely dominated by sodium atoms rather than H2O+ ions." and that they did not detect any Lithium, which might produce red light. Red light, from sodium?
Thank you Ann too, but that picture shows clearly red, not yellow, whatever astronomers call it!

And what is "H2O+"? A water molecule that is short of one electron, clearly, so has a positive charge. But that electron is what ties one of the hydrogens to the oxygen, so why isn't it HO+. with a free proton flying off?

And the 'mouse-over' describes the red tail as being due to "Neutral" sodium atoms. So, not charged, and not capable of photo emission, which I thought occured as electrons moved back into their orbitals, after beingn removed ?

John, even more confused than usual.

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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by Eclectic Man » Mon Mar 08, 2021 11:58 am

Just a request to the APOD administrators. This is a wonderful image, but most screens are in 'landscape' mode, and this image is in 'portrait' mode for no readily apparent reason. Please consider that when there is no real 'up' or 'down', to present the image in 'landscape' mode so that we can view as much of the image as possible at once.

Thank you.

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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by nlefaudeux » Mon Mar 08, 2021 12:42 pm

Hi all, some elements of answer:
JohnD wrote:
Mon Mar 08, 2021 10:48 am
And the 'mouse-over' describes the red tail as being due to "Neutral" sodium atoms. So, not charged, and not capable of photo emission, which I thought occured as electrons moved back into their orbitals, after beingn removed ?
the sodium tail is indeed considered to be made of neutral sodium atoms, see here, or google "neutral sodium tail".
Holger Nielsen wrote:
Mon Mar 08, 2021 8:49 am
I don't get it: "Red sodium light"? Neutral sodium is famous for the doublet D Fraunhofer lines at 589.00 and 589.59 nm. But those have a distinct yellow color. The link given also identifies the sodium tail light with a wavelength of 5890 "angstroms". I know of no bright red sodium line, so is the colour really red or is this a result of image processing?
about the color of the sodium tail, it was indeed red on the dslr images (see also here and here).

I think there are 2 main reasons why the sodium tail appeared in DSLR images (while sodium wavelength is yellow to the eye):
- First cause: color rendition/white balance of DSLR. Monochromatic color rendering is known to be not so good when the wavelength is between the RGB filter of DSLR (which is indeed the case of sodium that is between green and red filter of DSLR color filter array). Indeed, sodium light taken with “daylight” white balance (similar to the one i used for having the dust tail white) indeed appears much more red than yellow. See for instance this image of sodium streetlight where the light is much orange/red (look at the street more than the lightbulb itself which is saturated and thus appears yellow). I think this is the main reason.
- Second possible cause: there could be other components in the sodium tail. For instance, some images taken with light pollution filters (which block most of the sodium light) still show some red beneath the ion tail (like here), but not as prominent as other unfiltered dslr images. therefore, there could be other components than sodium in the tail. for instance H2O+ and NH2 are candidates as they are in 600-700nm range (red), which could make this tail further red. We need more comets like NEOWISE to know exactly what is going on !

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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by Knight of Clear Skies » Mon Mar 08, 2021 1:55 pm

Thanks for clarification @nlefaudeux and congratulations on a superb image.
Eclectic Man wrote:
Mon Mar 08, 2021 11:58 am
Just a request to the APOD administrators. This is a wonderful image, but most screens are in 'landscape' mode, and this image is in 'portrait' mode for no readily apparent reason. Please consider that when there is no real 'up' or 'down', to present the image in 'landscape' mode so that we can view as much of the image as possible at once.

Thank you.
Personally, I don't agree with this. Artistically speaking some images are far more pleasing in portrait orientation, so it should be left to the author of the image. The constellation Orion is a good example, I'd always present as a portrait. It's Orion the hunter rather than Orion the recumbent. (If I ever view Orion from the Southern hemisphere I'll be sure to stand on my head.)

If you wish to see the image in landscape you could download and rotate the image.
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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by Chris Peterson » Mon Mar 08, 2021 2:01 pm

Ann wrote:
Mon Mar 08, 2021 9:06 am
Please note that practically all stars that are called "red" in "astro-speak" are in fact orange-yellow to the eye.

I think astronomers call yellow objects red by habit.
No they don't!

Star colors are described the way they are because of the structure of their blackbody curves, which have peaks that don't necessarily correspond to colors as human perceive them.

I've never heard of any astronomer calling an emission line a color any different from the way most people would perceive it. For instance, the primary sodium emission line is always described as yellow, never red.
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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by orin stepanek » Mon Mar 08, 2021 2:42 pm

Orin

Smile today; tomorrow's another day!

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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by Ann » Mon Mar 08, 2021 2:57 pm

Chris Peterson wrote:
Mon Mar 08, 2021 2:01 pm
Ann wrote:
Mon Mar 08, 2021 9:06 am
Please note that practically all stars that are called "red" in "astro-speak" are in fact orange-yellow to the eye.

I think astronomers call yellow objects red by habit.
No they don't!

Star colors are described the way they are because of the structure of their blackbody curves, which have peaks that don't necessarily correspond to colors as human perceive them.

I've never heard of any astronomer calling an emission line a color any different from the way most people would perceive it. For instance, the primary sodium emission line is always described as yellow, never red.
That's most certainly true, Chris. Astronomers don't call the dominant circa 589 nm sodium line red, of course!


No, but they call small low-mass main sequence stars red ("red dwarfs"). And they call stars that have tripled or quadrupled their size because they have ceased their hydrogen fusion in their cores red ("red giants").

Even if they look yellow to the eye.

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Follow the ionized water?

Post by neufer » Mon Mar 08, 2021 6:25 pm

Chris Peterson wrote:
Mon Mar 08, 2021 2:01 pm
Ann wrote:
Mon Mar 08, 2021 9:06 am

Please note that practically all stars that are called "red" in "astro-speak" are in fact orange-yellow to the eye.

I think astronomers call yellow objects red by habit.
No they don't! Star colors are described the way they are because of the structure of their blackbody curves, which have peaks that don't necessarily correspond to colors as human perceive them.

I've never heard of any astronomer calling an emission line a color any different from the way most people would perceive it. For instance, the primary sodium emission line is always described as yellow, never red.
https://www.syfy.com/syfywire/comet-neowise-goes-from-red-to-green-and-has-spiral-arms wrote:
  • Phil Plait @BadAstronomer
    Jul 21, 2020, 9:00 AM EDT (Updated)
<<The [comet C/2020 F3 (NEOWISE)] straight tails are the ion tails. These are from ices that turned to gas from sunlight, then are ionized (have one or more electrons stripped from them) by ultraviolet light from the Sun. These gases glow at characteristic colors. Blue is generally from carbon monoxide (CO+), and green from diatomic carbon, two carbon atoms attached together (C2+).

But in Voltmer's photo there's also a red ion tail, and that's been the subject of much discussion online. I wasn't sure what it might be at first; sodium was my first guess but that glows yellow. Ionized water (H2O+) glows red, but the only way to know would be to look at spectra, where the light form the comet is dissected by color.

Happily my colleague Karl Battams pointed me toward a message board that had spectra! Weirdly, there's very little going on in the red part of the spectrum. There is some glow from water, but not much. However, for several days there was a strong glow from sodium (this emits at two colors very close together at 589.0 and 589.6 nanometers if you're curious). It was strong in early July, but faded considerably over the course of about a week.

Hmmmm, the red ion tail was strong early on, but faded, and is now not seen. That fits with sodium, but why does yellow sodium look red in photos?

Voltmer proposes an idea in his post with his photo: In color digital cameras, yellow and blue add together to form a gray color, and the water emission pushes this slightly red. So the bright sodium and carbon monoxide colors amplified the glow from water, but when the sodium faded the feeble light from water got drowned out, leaving just the blue. That idea makes sense to me, and does explain what we see.>>
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Re: Follow the ionized water?

Post by nlefaudeux » Mon Mar 08, 2021 8:22 pm

neufer wrote:
Mon Mar 08, 2021 6:25 pm
  • Phil Plait @BadAstronomer
    Jul 21, 2020, 9:00 AM EDT (Updated)
<<The [comet C/2020 F3 (NEOWISE)] straight tails are the ion tails. These are from ices that turned to gas from sunlight, then are ionized (have one or more electrons stripped from them) by ultraviolet light from the Sun. These gases glow at characteristic colors. Blue is generally from carbon monoxide (CO+), and green from diatomic carbon, two carbon atoms attached together (C2+).

But in Voltmer's photo there's also a red ion tail, and that's been the subject of much discussion online. I wasn't sure what it might be at first; sodium was my first guess but that glows yellow. Ionized water (H2O+) glows red, but the only way to know would be to look at spectra, where the light form the comet is dissected by color.

Happily my colleague Karl Battams pointed me toward a message board that had spectra! Weirdly, there's very little going on in the red part of the spectrum. There is some glow from water, but not much. However, for several days there was a strong glow from sodium (this emits at two colors very close together at 589.0 and 589.6 nanometers if you're curious). It was strong in early July, but faded considerably over the course of about a week.

Hmmmm, the red ion tail was strong early on, but faded, and is now not seen. That fits with sodium, but why does yellow sodium look red in photos?

Voltmer proposes an idea in his post with his photo: In color digital cameras, yellow and blue add together to form a gray color, and the water emission pushes this slightly red. So the bright sodium and carbon monoxide colors amplified the glow from water, but when the sodium faded the feeble light from water got drowned out, leaving just the blue. That idea makes sense to me, and does explain what we see.>>

red color of the sodium is most likely due to the bad DSLR color response, with no need to invoke weird effects.

see here, how monochromatic light color is rendered according to the transmission spectrum of the filters.

Image

the 590nm yellow color is not rendered correctly if it comes from a monochromatic source. narrow spectrum color rendition in DSLR is just not reliable for wavelengths in between the filters, ie yellow and cyan. for yellow color to be rendered correctly, it must come from a broad spectrum source.

other aspects can play a role, but to me, the main cause is definitely be color rendition of dslr.

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Re: Follow the ionized water?

Post by Chris Peterson » Mon Mar 08, 2021 11:54 pm

nlefaudeux wrote:
Mon Mar 08, 2021 8:22 pm
red color of the sodium is most likely due to the bad DSLR color response, with no need to invoke weird effects.

see here, how monochromatic light color is rendered according to the transmission spectrum of the filters.

Image

the 590nm yellow color is not rendered correctly if it comes from a monochromatic source. narrow spectrum color rendition in DSLR is just not reliable for wavelengths in between the filters, ie yellow and cyan. for yellow color to be rendered correctly, it must come from a broad spectrum source.

other aspects can play a role, but to me, the main cause is definitely be color rendition of dslr.

Nicolas
Maybe. I'm certainly critical of the color response of single-shot color sensors when it comes to emission lines. But I'll note that I've seen sodium emissions captured many times in meteor trails with such cameras, and they always appear somewhere in the yellow-orange range, never red, and never anything like what I see in today's APOD.
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Re: Follow the ionized water?

Post by VictorBorun » Tue Mar 09, 2021 12:19 am

nlefaudeux wrote:
Mon Mar 08, 2021 8:22 pm
the 590nm yellow color is not rendered correctly if it comes from a monochromatic source. narrow spectrum color rendition in DSLR is just not reliable for wavelengths in between the filters, ie yellow and cyan. for yellow color to be rendered correctly, it must come from a broad spectrum source.
other aspects can play a role, but to me, the main cause is definitely be color rendition of dslr.
Nicolas
I think that to redshift the expected amber (=yellow and some orange) look of Na 590 nm line in posted snapshot with Nicon D800 there must be another reason.The camera should treat Na line fairly yellowy:)
So there must be some huge anti-green, or pink correction.
We can't blame deep blue of CO+ ion tail [/img2] for an anti-green bias.
So it must be large and bright dust tail. Maybe it was greenish and triggered color correction?

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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by Eclectic Man » Tue Mar 09, 2021 10:47 am

Knight of Clear Skies wrote:
Mon Mar 08, 2021 1:55 pm
Eclectic Man wrote:
Mon Mar 08, 2021 11:58 am
Just a request to the APOD administrators. This is a wonderful image, but most screens are in 'landscape' mode, and this image is in 'portrait' mode for no readily apparent reason. Please consider that when there is no real 'up' or 'down', to present the image in 'landscape' mode so that we can view as much of the image as possible at once.

Thank you.
Personally, I don't agree with this. Artistically speaking some images are far more pleasing in portrait orientation, so it should be left to the author of the image. The constellation Orion is a good example, I'd always present as a portrait. It's Orion the hunter rather than Orion the recumbent. (If I ever view Orion from the Southern hemisphere I'll be sure to stand on my head.)

If you wish to see the image in landscape you could download and rotate the image.
The 'way up' people view the stars depends on the latitude of the observer. As you accept, Orion is only 'that way up' in the northern hemisphere, indeed the name Orion is Greek, but that does not mean other cultures do not have their own names for constellations / asterisms. I doubt that the people of Australia, Tierra del Fuego etc. stand on their heads to view the night sky.

I merely suggest that APOD administrators consider, taking into account the image makers' opinions, that where there is no definite 'way up' for an image they should consider presenting them in landscape mode, so that people can view as much of the image as possible on monitors, most of which are in landscape mode and cannot easily be changed to portrait mode. After all, as you suggest, if you wish to see the image smaller and in portrait mode you could download and rotate the image.

(Frankly if I ever got an image that was being considered for APOD I'd be happy with whatever way up they wanted to present it.)

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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by JohnD » Wed Mar 10, 2021 11:31 am

Thanks to those above who answered my Q (I think - the discussion is a bit esoteric!)
Would it be reasonable to take from that, that the red colour is artificial and that sodium does glow yellow, to th eye at least?

Neufer, quoted the BA as syaing that "ionised water glows red", but in my ignorance I still must ask, how can a whole water molecule be ionised and not fly apart?
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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by Chris Peterson » Wed Mar 10, 2021 2:05 pm

JohnD wrote:
Wed Mar 10, 2021 11:31 am
Thanks to those above who answered my Q (I think - the discussion is a bit esoteric!)
Would it be reasonable to take from that, that the red colour is artificial and that sodium does glow yellow, to th eye at least?
This appears too red to be explained as a camera artifact of yellow sodium light. It would be good to know the processing details. It would be good to see the raw image or images before processing.
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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by neufer » Wed Mar 10, 2021 2:47 pm

JohnD wrote:
Wed Mar 10, 2021 11:31 am

Neufer, quoted the BA as syaing that "ionised water glows red", but in my ignorance I still must ask, how can a whole water molecule be ionised and not fly apart?
  • If one can get away with adding a single proton to water then
    one should also be able to subtract a single electron as well.
https://en.wikipedia.org/wiki/Hydronium wrote:

<<In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation H3O+, the type of oxonium ion produced by protonation of water. It is the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton (a positive hydrogen ion, H+) to the surrounding water molecules (H2O).

Hydronium is an abundant molecular ion in the interstellar medium and is found in diffuse and dense molecular clouds as well as the plasma tails of comets. Interstellar sources of hydronium observations include the regions of Sagittarius B2, Orion OMC-1, Orion BN–IRc2, Orion KL, and the comet Hale–Bopp.

Interstellar hydronium is formed by a chain of reactions started by the ionization of H2 into H+2 by cosmic radiation. H3O+ can produce either OH or H2O through dissociative recombination reactions, which occur very quickly even at the low (≥10 K) temperatures of dense clouds. This leads to hydronium playing a very important role in interstellar ion-neutral chemistry.

Astronomers are especially interested in determining the abundance of water in various interstellar climates due to its key role in the cooling of dense molecular gases through radiative processes. However, H2O does not have many favorable transitions for ground-based observations. Although observations of HDO (the deuterated version of water) could potentially be used for estimating H2O abundances, the ratio of HDO to H2O is not known very accurately.

Hydronium, on the other hand, has several transitions that make it a superior candidate for detection and identification in a variety of situations. This information has been used in conjunction with laboratory measurements of the branching ratios of the various H3O+ dissociative recombination reactions to provide what are believed to be relatively accurate OH and H2O abundances without requiring direct observation of these species.>>
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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by JohnD » Wed Mar 10, 2021 5:40 pm

Well, thank you, neufer! Thgis traditional Britsioh English had never heard of hydoxonium, let alone hydronium, so maybe I ma be forgiven for not finding it when I searched for illumination.
May I be permitted a further, related Q? If neutral H2O is bound by shared electrons, how is the third H held in place? Quantum indecision about which proton the electron is shared with?

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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by neufer » Wed Mar 10, 2021 8:00 pm

JohnD wrote:
Wed Mar 10, 2021 5:40 pm

May I be permitted a further, related Q? If neutral H2O is bound by shared electrons, how is the third H held in place? Quantum indecision about which proton the electron is shared with?
  • Three strong coequal covalent bonds trumps ionic repulsion.
https://en.wikipedia.org/wiki/Hydronium wrote:
<<Since O+ and N have the same number of electrons, H3O+ is isoelectronic with ammonia. As shown in the images above, H3O+ has a trigonal pyramidal molecular geometry with the oxygen atom at its apex. The H−O−H bond angle is approximately 113°, and the center of mass is very close to the oxygen atom. Because the base of the pyramid is made up of three identical hydrogen atoms, the H3O+ molecule's symmetric top configuration is such that it belongs to the C3v point group.>>
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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by nlefaudeux » Thu Mar 11, 2021 9:34 am

Chris Peterson wrote:
Wed Mar 10, 2021 2:05 pm
JohnD wrote:
Wed Mar 10, 2021 11:31 am
Thanks to those above who answered my Q (I think - the discussion is a bit esoteric!)
Would it be reasonable to take from that, that the red colour is artificial and that sodium does glow yellow, to th eye at least?
This appears too red to be explained as a camera artifact of yellow sodium light. It would be good to know the processing details. It would be good to see the raw image or images before processing.
here are one raw file for you to investigate, and a quickly processed version of the raw file

raw file:
https://drive.google.com/file/d/1-vT5fM ... sp=sharing

quickly processed raw file (quick flat fielding, removal of background blue sky, and white balance on the dust tail, nothing more).
https://drive.google.com/file/d/1pOM82_ ... sp=sharing

the only way to know for sure would be to take an image of the sodium light with a a7s2 and to compare the ratio of red and green on the sodium tail vs sodium lighting, on the raw file, before any white balance. one must make sure with some spectrum that it is indeed low pressure sodium lighting, and not high pressure sodium, which has a much wider spectrum.

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Re: APOD: Three Tails of Comet NEOWISE (2021 Mar 08)

Post by Chris Peterson » Thu Mar 11, 2021 12:22 pm

nlefaudeux wrote:
Thu Mar 11, 2021 9:34 am
Chris Peterson wrote:
Wed Mar 10, 2021 2:05 pm
JohnD wrote:
Wed Mar 10, 2021 11:31 am
Thanks to those above who answered my Q (I think - the discussion is a bit esoteric!)
Would it be reasonable to take from that, that the red colour is artificial and that sodium does glow yellow, to th eye at least?
This appears too red to be explained as a camera artifact of yellow sodium light. It would be good to know the processing details. It would be good to see the raw image or images before processing.
here are one raw file for you to investigate, and a quickly processed version of the raw file

raw file:
https://drive.google.com/file/d/1-vT5fM ... sp=sharing

quickly processed raw file (quick flat fielding, removal of background blue sky, and white balance on the dust tail, nothing more).
https://drive.google.com/file/d/1pOM82_ ... sp=sharing

the only way to know for sure would be to take an image of the sodium light with a a7s2 and to compare the ratio of red and green on the sodium tail vs sodium lighting, on the raw file, before any white balance. one must make sure with some spectrum that it is indeed low pressure sodium lighting, and not high pressure sodium, which has a much wider spectrum.

Nicolas
Thanks! I'll take a look at those a little later. Definitely something interesting going on here, either with the camera response or with the specifics of the association of the red tail with sodium.
Chris

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Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com