Starship Asterisk*

Thor's Helmet

Explanation: This helmet-shaped cosmic cloud with wing-like appendages is popularly called Thor's Helmet. Heroically sized even for a Norse god, Thor's Helmet is about 30 light-years across. In fact, the helmet is actually more like an interstellar bubble, blown as a fast wind from the bright, massive star near the bubble's center sweeps through a surrounding molecular cloud. Known as a Wolf-Rayet star, the central star is an extremely hot giant thought to be in a brief, pre-supernova stage of evolution. Cataloged as NGC 2359, the nebula is located about 15,000 light-years away in the constellation Canis Major. The sharp image, made using broadband and narrowband filters, captures striking details of the nebula's filamentary structures. It shows off a blue-green color from strong emission due to oxygen atoms in the glowing gas.

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I'd be a bit thore too, if i had that star radiateing at me like that.

Beyond wrote:I'd be a bit thore too, if i had that star radiateing at me like that.

LOL!!!!

:---[===] *

Ooooo....OXYGEN....so we could breath there, right?????


Always a great image!!!

:---[===] *

What a splendid image for a THOR'SDAY!

I'm teaching myself a few basics about astrophotography at present, so would like to check my understanding of the filters used in creating this image - and would be grateful if the astrophotographers of the forum would correct any inaccuracies.

http://www.cxielo.ch/gallery/f/ngc2359
Image Data
...
Filters: Astrodon Gen 2, 3nm/5nm Narrowband
Exposures: Ha:OIII:RGB (660:720:180:180:180min)

1. H-alpha (Hα) is a red spectral line created by hydrogen with a wavelength of 656.28 nm, which occurs when a hydrogen electron falls from its third to second lowest energy level.

So I assume that the Hα filter was centred on 656nm, with 1.5nm on either side?

2. Doubly ionized oxygen, [O III] emits light in the green part of the spectrum primarily at the wavelength 500.7 nanometres (nm) and secondarily at 495.9 nm.

The information given above says that the filter for OIII had a band of 5nm. How would this work when the two wavelengths involved are just about 5nm apart? Where would the filter be centred?

3. RGB is light in the normal human visible range.

So, am I right when I surmise that today's Apod shows us an image of what this nebula would "really" look like, in the unlikely event of human's being near enough to see it, but with the colours emitted by Hα and OIII enhanced?

Margarita

PS. I found it interesting to compare today's image with this one which seems to have been taken using LRGB filters. It was an Apod in August 2008

Mmm. I somehow missed the link to the Robert Frost poem about Canis Major . Nice poem - new to me. I need to explore more of Frost - he isn't a poet that I am well- acquainted with.
Margarita

PS. I've just found that there is a recording of Frost reading The Road Less Taken (the only one of his poems that I knew already) on YouTube. I won't embed it here, but poetry-lovers might like the link.

http://youtube.com/watch?v=ie2Mspukx14

MargaritaMc wrote:What a splendid image for a THOR'SDAY!

I'm teaching myself a few basics about astrophotography at present, so would like to check my understanding of the filters used in creating this image - and would be grateful if the astrophotographers of the forum would correct any inaccuracies.

http://www.cxielo.ch/gallery/f/ngc2359
Image Data
...
Filters: Astrodon Gen 2, 3nm/5nm Narrowband
Exposures: Ha:OIII:RGB (660:720:180:180:180min)

1. H-alpha (Hα) is a red spectral line created by hydrogen with a wavelength of 656.28 nm, which occurs when a hydrogen electron falls from its third to second lowest energy level.

So I assume that the Hα filter was centred on 656nm, with 1.5nm on either side?

2. Doubly ionized oxygen, [O III] emits light in the green part of the spectrum primarily at the wavelength 500.7 nanometres (nm) and secondarily at 495.9 nm.

The information given above says that the filter for OIII had a band of 5nm. How would this work when the two wavelengths involved are just about 5nm apart? Where would the filter be centred?

3. RGB is light in the normal human visible range.

So, am I right when I surmise that today's Apod shows us an image of what this nebula would "really" look like, in the unlikely event of human's being near enough to see it, but with the colours emitted by Hα and OIII enhanced?

Margarita

PS. I found it interesting to compare today's image with this one which seems to have been taken using LRGB filters. It was an Apod in August 2008

Ha and OIII filters allow the narrowband of Ha and OIII light that isn't visible with normal RGB filters. There are many faint nebulae that simply aren't visible with just RGB and can only be seen with Ha and OIII. Some astrophotographers process Ha and OIII in either false colour or in a way that approximates "true" colour. If the Ha and OIII in a nebula is really bright, it can be seen in just RGB.

Also if we could be right next to these nebulae, no way could we ever see the detail and colour in a photograph. The reason for this is that the human eye is too weak to perceive the light with intensity assuming that it's due to them being "calibrated" to the G2V light of our Sun. I'm not too sure on the technical aspects of eyes and vision, so I think someone else will provide a much better answer!!!

You are mostly but not completely right, I think, starsurfer.

Certainly almost any nebula in space is too faint for us to spot the red color of Ha. I don't think there is any nebula at all in the nearby universe that is bright enough to allow us to see its Ha light. The reason for this is that the red Ha line falls in a part of the visual spectrum where the sensitivity of the human retina is really quite low.

However, a good broadband red filter will certainly detect some red Ha light, there is no question of that. David Malin, who photographed the sky through broadband red, green and blue filters, had no difficulties photographing a lot of Ha. But you will no doubt detect more Ha and more structure with a good narrowband Ha filter than with a broadband red one.

There is also no reason why good broadband green and blue filters should not detect quite a lot of OIII light. Moreover, you may spot the green color of OIII light with your own eye with the aid of a telescope, if you look at some of the more colorful planetary nebulae. The planetary known as the Blue Snowball, which to some people looks like a green snowball, is an example.

As for Thor's Helmet, I myself doubt that its surface brightness is high enough for humans to spot color in it, no matter what kind of telescope we use to look at it. However, I note that David Malin has taken a picture of it, which can be found in his book, A View of the Universe. In David Malin's picture, the blue-green "bubble" is rather whitish, though with a definite hint of blue or blue-green. The blue parts of the "horns" of Thor's Helmet are barely visible in Malin's image. Instead the horns look red or pink.

If I compare Malin's image with todays APOD, I'd say that today's APOD is much more colorful. I'd say its colors of Martin Rusterholz's image have been enhanced, but not distorted.

If our ability to detect color in faint extended nebulae were many times better than it is, then Thor's Helmet might actually look to us the way it does in Martin Rusterholz's image.

Ann

It's true that broadband filters will detect any of the emission lines that narrow band filters do. However, there is a caveat to that, in that you would reach a sky limited exposure long before picking up enough of those emissions. Narrow band filters allow you take much longer exposures, in fact, it's almost essential, because you are only collecting light from that particular emission line. The narrower band pass the better, because you aren't picking up as much spurious light from other sources. Here is a link to some, if not all of the useable emission lines.
http://zuserver2.star.ucl.ac.uk/~msw/lines.html

I'm not too sure on the technical aspects of eyes and vision...

The more I look at it, I think it looks like a bear's head with it's paws holding on to and licking a honey comb. I like the name Thor's Helmet a lot better and it does look like it. I'd hate to take a Rorschach test. I'd flunk with flying colors.

Wow! I finally saw Ursa Minor

stephen63 wrote:It's true that broadband filters will detect any of the emission lines that narrow band filters do. However, there is a caveat to that, in that you would reach a sky limited exposure long before picking up enough of those emissions. Narrow band filters allow you take much longer exposures, in fact, it's almost essential, because you are only collecting light from that particular emission line. The narrower band pass the better, because you aren't picking up as much spurious light from other sources. Here is a link to some, if not all of the useable emission lines.
http://zuserver2.star.ucl.ac.uk/~msw/lines.html

Thank you for your help, Stephen. What are the units of measurement used in the link you gave What I've been learning is in nanometres but this site has units of an order of magnitude smaller. (For example, Hα is given as 6562.80 rather than 656). I've come across picometres but they are 10^-15, which is several orders of magnitude smaller than nanometres.

Margarita

MargaritaMc wrote:Mmm. I somehow missed the link to the Robert Frost poem about Canis Major . Nice poem - new to me. I need to explore more of Frost - he isn't a poet that I am well- acquainted with.
Margarita

PS. I've just found that there is a recording of Frost reading The Road Less Taken (the only one of his poems that I knew already) on YouTube. I won't embed it here, but poetry-lovers might like the link.

http://youtube.com/watch?v=ie2Mspukx14

Thanks for the link, Margarita. Frost is an iconic American poet, but there's significant tension between his public image and the reality of his life story. While Frost fashioned a persona as the quintessential New Hampshire Yankee farmer, he was born in San Francisco and spent his first twelve years here before moving to Massachussetts after his father died. Although he did own a farm, he earned his living as a college teacher and author. His poetry was first published while he was living in England, and the incident that inspired "The Road Less Taken" was a walk in Gloustershire with his close friend the English poet Edward Thomas, who would shortly thereafter die in the first world war. When Frost returned to the US, the success of his first book in England opened doors in the New York publishing world.

Anthony Barreiro wrote:

MargaritaMc wrote:Mmm. I somehow missed the link to the Robert Frost poem about Canis Major . Nice poem - new to me. I need to explore more of Frost - he isn't a poet that I am well- acquainted with.
Margarita

PS. I've just found that there is a recording of Frost reading The Road Less Taken (the only one of his poems that I knew already) on YouTube. I won't embed it here, but poetry-lovers might like the link.

http://youtube.com/watch?v=ie2Mspukx14

Thanks for the link, Margarita. Frost is an iconic American poet, but there's significant tension between his public image and the reality of his life story. While Frost fashioned a persona as the quintessential New Hampshire Yankee farmer, he was born in San Francisco and spent his first twelve years here before moving to Massachussetts after his father died. Although he did own a farm, he earned his living as a college teacher and author. His poetry was first published while he was living in England, and the incident that inspired "The Road Less Taken" was a walk in Gloustershire with his close friend the English poet Edward Thomas, who would shortly thereafter die in the first world war. When Frost returned to the US, the success of his first book in England opened doors in the New York publishing world.

Frost is one of many poets that I have heard of and, during a working life with an over-busy career, haven't had - or haven't created - time to explore. One of the delights of being retired is teaching myself to take the time simply to read poetry.

Mmm, I could sense Gloucestershire in Frost's poem. Lovely county. Yes, it is easy to feel the connection with Thomas.
And Frost originated in San Francisco? It is the only part of the States that I know, having spent a month's honeymoon there in 1994!

Margarita

MargaritaMc wrote: Thank you for your help, Stephen. What are the units of measurement used in the link you gave What I've been learning is in nanometres but this site has units of an order of magnitude smaller. (For example, Hα is given as 6562.80 rather than 656). I've come across picometres but they are 10^-15, which is several orders of magnitude smaller than nanometres.
Margarita

Shift the decimal place one to the right and you now have the wavelength in angstroms. 656nm=6560 angstroms!

MargaritaMc wrote:

stephen63 wrote:It's true that broadband filters will detect any of the emission lines that narrow band filters do. However, there is a caveat to that, in that you would reach a sky limited exposure long before picking up enough of those emissions. Narrow band filters allow you take much longer exposures, in fact, it's almost essential, because you are only collecting light from that particular emission line. The narrower band pass the better, because you aren't picking up as much spurious light from other sources. Here is a link to some, if not all of the useable emission lines.
http://zuserver2.star.ucl.ac.uk/~msw/lines.html

Thank you for your help, Stephen. What are the units of measurement used in the link you gave What I've been learning is in nanometres but this site has units of an order of magnitude smaller. (For example, Hα is given as 6562.80 rather than 656). I've come across picometres but they are 10^-15, which is several orders of magnitude smaller than nanometres.

Margarita

For historical and practical reasons, astronomers are quite fond of quoting wavelengths in Angstroms (10 A = 1 nm).

On an earlier comment - there are a few nebulae bright enough to see the reddish tint of H-alpha, at least as a modification of the overall color. Some planetaries look rather more purple when seen in a broad or multiple-and filter than using a filter for [O III] alone. I know some observers who say they see it in the Orion nebula, but I haven't seen that myself. (Age seems to change color response to some of these emission lines - [O III] looks more blue-green now than the emerald green I remember 30 years ago).

stephen63 wrote:

MargaritaMc wrote: Thankyou for your help, Stephen. What are the units of measurement used in the link you gave What I've been learning is in nanometres but this site has units of an order of magnitude smaller. (For example, Hα is given as 6562.80 rather than 656). I've come across picometres but they are 10^-15, which is several orders of magnitude smaller than nanometres.
Margarita

Shift the decimal place one to the right and you now have the wavelength in angstroms. 656nm=6560 angstroms!

Aah! Light dawns! So THAT is what angstrom is! I assumed that it was a separate system altogether - like metric and imperial.

Margarita

And thank you also, NGC3314.

With two photographers here, can I give a quote from my own initial post and ask for your input?

Doubly ionized oxygen, [O III] emits light in the green part of the spectrum primarily at the wavelength 500.7 nanometres (nm) and secondarily at 495.9 nm.

The information given above says that the filter for [OIII] had a band of 5nm. How would this work when the two wavelengths involved are just about 5nm apart? Where would the filter be centred?

MargaritaMc wrote:And thank you also, NGC3314.

With two photographers here, can I give a quote from my own initial post and ask for your input?

Doubly ionized oxygen, [O III] emits light in the green part of the spectrum primarily at the wavelength 500.7 nanometres (nm) and secondarily at 495.9 nm.

The information given above says that the filter for [OIII] had a band of 5nm. How would this work when the two wavelengths involved are just about 5nm apart? Where would the filter be centred?

Usually centered in the 5007-A line. The 4959 and 5007-A lines have numbers of photons in the ratio 1:3 (fixed by atomic physics), so all the information is there in the brighter one and the contrast against continuous sources (like stars) is best for a narrow filter. There are other pairs of spectral lines with similar connections, but [O III] is the best-known. There are some specialized applications where one would observe the 4959 line - one I've seen is observing a galaxy whose redshift puts the 5007 line outside the available set of filters but allows observation of the 4959 line at the expense of longer exposure times. Back in the days of photographic plates with their nonlinear intensity response, this line ratio was used to help calibrate intensities in spectra, since the ratio had to be 3:1 at each point in a nebula.

(Nitpick: the ratio is 3:1 in photons, which means that value divided by the wavelength ratio = 2.97 in energy per unit time. End of pedantry)

NGC3314 wrote:

MargaritaMc wrote:And thank you also, NGC3314.

With two photographers here, can I give a quote from my own initial post and ask for your input?

Doubly ionized oxygen, [O III] emits light in the green part of the spectrum primarily at the wavelength 500.7 nanometres (nm) and secondarily at 495.9 nm.

The information given above says that the filter for [OIII] had a band of 5nm. How would this work when the two wavelengths involved are just about 5nm apart? Where would the filter be centred?

Usually centered in the 5007-A line. The 4959 and 5007-A lines have numbers of photons in the ratio 1:3 (fixed by atomic physics), so all the information is there in the brighter one and the contrast against continuous sources (like stars) is best for a narrow filter. There are other pairs of spectral lines with similar connections, but [O III] is the best-known. There are some specialized applications where one would observe the 4959 line - one I've seen is observing a galaxy whose redshift puts the 5007 line outside the available set of filters but allows observation of the 4959 line at the expense of longer exposure times. Back in the days of photographic plates with their nonlinear intensity response, this line ratio was used to help calibrate intensities in spectra, since the ratio had to be 3:1 at each point in a nebula.

(Nitpick: the ratio is 3:1 in photons, which means that value divided by the wavelength ratio = 2.97 in energy per unit time. End of pedantry)

Yeah. What he said.

50 mm sq. unmounted NII 3 nm
(NII3_50S)
$1095
At this point, I'll stick with just the Ha

THANK YOU, THANK YOU, NGC3314

Here is a photo of your namesake as a thank you gift!
Margarita

I noticed in the list of images brought up through the CXIELO Observatory link that there is a galaxy (NGC 3344) that is known as the Sliced Onion Galaxy. What a great name.

DavidLeodis wrote:I noticed in the list of images brought up through the CXIELO Observatory link that there is a galaxy (NGC 3344) that is known as the Sliced Onion Galaxy. What a great name.

Yes, it is a great name for a galaxy. Just sort of brings tears to your eyes, doesn't it

Beyond wrote:

DavidLeodis wrote:I noticed in the list of images brought up through the CXIELO Observatory link that there is a galaxy (NGC 3344) that is known as the Sliced Onion Galaxy. What a great name.

Yes, it is a great name for a galaxy. Just sort of brings tears to your eyes, doesn't it

Nyuk Nyuk Nyuk