APOD: Wisps Surrounding the Horsehead Nebula (2019 Apr 03)

[APOD Robot]

Wisps Surrounding the Horsehead Nebula

Explanation: The famous Horsehead Nebula in Orion is not alone. A deep exposure shows that the dark familiar shaped indentation, visible just below center, is part of a vast complex of absorbing dust and glowing gas. To bring out details of the Horsehead's pasture, an amateur astronomer used a backyard telescope in Austria to accumulate and artistically combine 7.5 hours of images in the light of Hydrogen (red), Oxygen (green), and Sulfur (blue). The resulting spectacular picture details an intricate tapestry of gaseous wisps and dust-laden filaments that were created and sculpted over eons by stellar winds and ancient supernovas. The Flame Nebula is visible just to the left of the Horsehead, while the bright star on the upper left is Alnilam, the central star in Orion's Belt. The Horsehead Nebula lies 1,500 light years distant towards the constellation of Orion.

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[orin stepanek]

I love the photo's color scheme; nicely done! What is the large ball to the left of the Horse's head? It must be huge!

[Indigo_Sunrise]

Gorgeous image!

Why does it appear like there is a 'bubble' around Alnilam..? Is it possibly a 'side effect' of processing? Or perhaps it's noticeable in other images of that star?
*offtolookforcomparisons*

[NHcycler]

I am constantly in awe of the talent, knowledge, and creativity shown by the “amateurs!” Bravo Mario Zauner!

[mjsakers]

Gorgeous image!

Why does it appear like there is a 'bubble' around Alnilam..? Is it possibly a 'side effect' of processing? Or perhaps it's noticeable in other images of that star?
*offtolookforcomparisons*

Alnilam is the bright star closer to the upper left corner of the photo. Also, Alnitak, the other "belt" star in this photo is the one directly above the Flame Nebula, not showing as bright probably due to the "processing"

[neufer]

I love the photo's color scheme; nicely done! What is the large ball to the left of the Horse's head? It must be huge!

Assuming that you aren't simply talking about the Flame Nebula referenced in the Explanation:

• HELIUM-STRONG STAR HD 37776
  
  https://iopscience.iop.org/article/10.1 ... X/726/1/24
  https://iopscience.iop.org/article/10.1 ... /1/24/meta

[orin stepanek]

[Boomer12k]

Awesomely colored image...looks like it was painted almost...amazing detail...

:---[===] *

[neufer]

---

• Yeah...that's what I assumed with my answer above.
  
  HD 37776
  
  IC 432
  
  IC 435

[Chris Peterson]

Gorgeous image! 8-)

Why does it appear like there is a 'bubble' around Alnilam..? Is it possibly a 'side effect' of processing? Or perhaps it's noticeable in other images of that star?
*offtolookforcomparisons*

An artifact, I think. The star is thousands of times brighter than the nebulosity around it, and in the long exposures required for the latter would have resulted in a massive bloated blob. The bubble effect is probably what's left from trying to edit that out.

[Chris Peterson]

I love the photo's color scheme; nicely done! What is the large ball to the left of the Horse's head? It must be huge! :shock:

IC 432. A reflection nebula.

[orin stepanek]

Thanks guys; i appreciate your answers! I just didn't recall seeing that on previous photos of the area!

[Ann]

Thanks guys; i appreciate your answers! I just didn't recall seeing that on previous photos of the area!

Click to view full size image

Horsehead and Orion Nebula region.
Photo: Terry Hancock.

Today's APOD looks different in many respects because its color scheme is not a close approximation of what the human eye would see, if our eyes were many times more sensitive than they are. The picture of the Horsehead region and the Orion nebula by Terry Hancock is an RGB image, and therefore it corresponds a lot more closely to what the color-sensitive cones in our retinas would see, if, as I said, our eyes were many times more sensitive.

Note the tiny black protrusion in Terry Hancock's image, which is the Horsehead nebula. To the left of the Horsehead is a smallish orange "cloud" crossed by a strong black "band". That is the Flame nebula. Above the Flame nebula is Alnitak, and to the lower left of Alnitak is IC 435, which is a combined blue reflection and red emission nebula. Its color in Terry Hancock's image is therefore slightly purplish.

Ann

[Nitpicker]

But it is one of the best approximations to what our eyes might see, if they were more sensitive and had narrowband filters. If ifs and buts were sweets and nuts, we'd all have a merry solstice.

[Ann]

Let's compare the appearance of IC 435 in Hα, OIII and SII (the APOD) with an RGB image.

Click to view full size image

Pale orange IC 435 in Hα, OIII and SII.

Click to view full size image

Red and blue IC 435 (at upper left) in RGB.

I recommend this much larger RGB image by Paddy Gilliland FRAS where you can see IC 435 well. Note that IC 435 (at upper left) looks like a red patch of Hα light overlaid by streaks of blue reflection nebulosity.

Note that the shape of IC 435 in the APOD. Note that it corresponds well to the patch of red nebulosity in the RGB images. That is because the filters used for the APOD strongly detects red Hα light, but it is rather bad at detecting blue broadband light. All it can see of blue light is the blue-green OIII line at 501 nm. That is why IC 435 looks orange in the APOD, as a result of a lot of red Hα and a little bit of green OIII.

Note that the ionizing star of IC 435, HD 37776, shows up quite poorly in the APOD. Since HD 37776 is a hot star dominated by ultraviolet and blue light, the red and green filters of the APOD does not detect it very well. (Though I assume that the processing might have something to do with it, too.)

The reason why I wrote this post is to explain why IC 435 looks rather different in today's APOD than it does in most pictures of the Horsehead nebula region.

Ann

[Nitpicker]

The red/orange/brown/yellow shades in the APOD are showing the dominance of the red channel (Ha ~656nm [red]) and green channel (OIII ~500nm [cyan-green]), with a relative lack of signal in the blue channel (SII ~672nm [the reddest wavelengths recorded]). The yellow shades are where the the signal from the oxygen reaches parity with the hydrogen.

It is true that no blue wavelengths were recorded for this APOD, but that's not why there is less blue in it. It is less blue because there is less signal from the sulfur. SII and Ha are very close in wavelength, both very red. The APOD has separated them, where a "true colour" broadband image cannot.

What I am less sure about is the reasoning behind the different exposure times for each channel. If I attempt to account for the 2x2 binning in the G and B channels, it looks like the effective ratios from each channel are about R=10%, G=60%, B=30%. I am sure there must be a reason to do this, but I don't know how one would decide on a basis.

[Chris Peterson]

What I am less sure about is the reasoning behind the different exposure times for each channel. If I attempt to account for the 2x2 binning in the G and B channels, it looks like the effective ratios from each channel are about R=10%, G=60%, B=30%. I am sure there must be a reason to do this, but I don't know how one would decide on a basis.

Keep in mind that the exposure time used to collect the data does not necessarily correspond to the strength of the signal in final image. I normally set my exposure time based on the S/N level I desire in that data channel, and then weight the channels separately in the final image.

[Ann]

The red/orange/brown/yellow shades in the APOD are showing the dominance of the red channel (Ha ~656nm [red]) and green channel (OIII ~500nm [cyan-green]), with a relative lack of signal in the blue channel (SII ~672nm [the reddest wavelengths recorded]). The yellow shades are where the the signal from the oxygen reaches parity with the hydrogen.

It is true that no blue wavelengths were recorded for this APOD, but that's not why there is less blue in it. It is less blue because there is less signal from the sulfur. SII and Ha are very close in wavelength, both very red. The APOD has separated them, where a "true colour" broadband image cannot.

Click to view full size image

The Horsehead region in different wavelengths.
Photo: ESA Science.

The Horsehead region contains a lot of intrinsically shortwave light (~450 nm and shorter), but the filters used for the APOD were not able to detect much of this blue light. That is not wrong in itself, of course. But it does mean that the portrait of the Horsehead region will look different than it does in an RGB image.

The pictures at right show the Horsehead region in different wavelengths. Each picture is "true", but they do look different.

Personally I would love to see astrophotographers, both amateurs and professionals, explain not only what filters they used, but also why they chose those particular filters. What did they want their picture to show us? And if they "color-mapped" their filter exposures in an unusual way, was there a reason for that? Maybe just aesthetics?

I would love to know. Because different filters and different mappings do affect what we see in the picture.

Ann

[Chris Peterson]

Personally I would love to see astrophotographers, both amateurs and professionals, explain not only what filters they used, but also why they chose those particular filters.

As a rule, images made in the visible spectrum are either involve broadband RGB filters, in which case the intent is "true color" (although processing can significantly impact the final result, since there's really no absolute "true" when it comes to color), or they are using narrowband filters, in which case the intent is generally pretty obvious simply from the choice of filters.

[Ann]

Personally I would love to see astrophotographers, both amateurs and professionals, explain not only what filters they used, but also why they chose those particular filters.

As a rule, images made in the visible spectrum are either involve broadband RGB filters, in which case the intent is "true color" (although processing can significantly impact the final result, since there's really no absolute "true" when it comes to color), or they are using narrowband filters, in which case the intent is generally pretty obvious simply from the choice of filters.

What I don't understand about today's APOD is the blue mapping of the reddest channel. But maybe that was for aesthetic reasons, or in order to map Hα as red and OIII as green.

Ann

[Nitpicker]

What I am less sure about is the reasoning behind the different exposure times for each channel. If I attempt to account for the 2x2 binning in the G and B channels, it looks like the effective ratios from each channel are about R=10%, G=60%, B=30%. I am sure there must be a reason to do this, but I don't know how one would decide on a basis.

Keep in mind that the exposure time used to collect the data does not necessarily correspond to the strength of the signal in final image. I normally set my exposure time based on the S/N level I desire in that data channel, and then weight the channels separately in the final image.

Thanks Chris. That does make sense. So, on what basis should one decide to weight the channels? Is this APOD representative of the relative strengths of signal recorded in each channel? Given that the SII is seemingly so overpowered by the other two channels, I am wondering whether showing it amplified might be more informative. As it is, I can't really tell where the sulfur is.

[Nitpicker]

What I don't understand about today's APOD is the blue mapping of the reddest channel. But maybe that was for aesthetic reasons, or in order to map Hα as red and OIII as green.

Ann

The APOD is just "RGB=HOS", rather than the "RGB=SHO" that is perhaps more commonly seen in narrowband images, and which is sometimes referred to as the Hubble palette for narrowband images, I think.

Some might argue that with the predominance of hydrogen in the universe, it makes more sense to map Ha, and not SII, to the red channel. And the colour of OII wavelengths is arguably closer to green than blue. So, that leaves SII to be mapped to the blue channel. Simple.

And with the blue channel being overpowered by the others in this APOD, the combination is probably reasonably close to the true colours of the wavelengths passing through the filters. (But I can well imagine that lovers of blue might not like it as much.)

[Chris Peterson]

What I am less sure about is the reasoning behind the different exposure times for each channel. If I attempt to account for the 2x2 binning in the G and B channels, it looks like the effective ratios from each channel are about R=10%, G=60%, B=30%. I am sure there must be a reason to do this, but I don't know how one would decide on a basis.

Keep in mind that the exposure time used to collect the data does not necessarily correspond to the strength of the signal in final image. I normally set my exposure time based on the S/N level I desire in that data channel, and then weight the channels separately in the final image.

Thanks Chris. That does make sense. So, on what basis should one decide to weight the channels? Is this APOD representative of the relative strengths of signal recorded in each channel? Given that the SII is seemingly so overpowered by the other two channels, I am wondering whether showing it amplified might be more informative. As it is, I can't really tell where the sulfur is.

I think there are good reasons to maintain photometric accuracy (that is, where the intensity of each output channel corresponds to the actual intensity of the signal mapped to that channel, regardless of the exposure time used), and also for deliberately changing the photometric ratios between the channels (e.g. boosting the SII signal). If it were me, I'd probably try different things with the data and see what showed the most interesting or richest structure and detail.

[Ann]

Let's compare the appearance of IC 435 in Hα, OIII and SII (the APOD) with an RGB image.

Ann

My bad. The nebula is, of course, IC 432. Forget everything I said about IC 435.

Ann

[Nitpicker]

My bad. The nebula is, of course, IC 432. Forget everything I said about IC 435.

Ann

Don't be too hard on yourself, Ann, they are just funny coloured puffs of almost nothing, after all. I was also unsure which was which when I was writing my earlier posts, so I only referred to the image colours in general.

But both puffs are said to be reflection nebulae, and I suppose the interesting thing about them, in this context, is that their reflected starlight is largely filtered out from the narrowband APOD, revealing the elemental nature of the puffs themselves.