APOD: M27: The Dumbbell Nebula (2016 Nov 02)

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APOD: M27: The Dumbbell Nebula (2016 Nov 02)

Post by APOD Robot » Wed Nov 02, 2016 4:12 am

Image M27: The Dumbbell Nebula

Explanation: The first hint of what will become of our Sun was discovered inadvertently in 1764. At that time, Charles Messier was compiling a list of diffuse objects not to be confused with comets. The 27th object on Messier's list, now known as M27 or the Dumbbell Nebula, is a planetary nebula, the type of nebula our Sun will produce when nuclear fusion stops in its core. M27 is one of the brightest planetary nebulae on the sky, and can be seen toward the constellation of the Fox (Vulpecula) with binoculars. It takes light about 1000 years to reach us from M27, shown above in colors emitted by hydrogen and oxygen. Understanding the physics and significance of M27 was well beyond 18th century science. Even today, many things remain mysterious about bipolar planetary nebula like M27, including the physical mechanism that expels a low-mass star's gaseous outer-envelope, leaving an X-ray hot white dwarf.

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Ann
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Re: APOD: M27: The Dumbbell Nebula (2016 Nov 02)

Post by Ann » Wed Nov 02, 2016 5:04 am

I am surprised at the way the colors have been mapped in today's APOD.
OIII is a bluish green color at a wavelength of about 501 nm, whereas Hα is red color at about 656 nm. Almost all planetary nebulas that show both OIII and Hα emission are oxygen-rich near the central star and Hα-rich further out. Therefore, I expect most planetary nebulas to be green (or blue) near the center and red further out.

An example of what I would expect the Dumbbell Nebula to look like can be seen in the picture at left by Astro Cruise.

In today's APOD, however, the entire inner part of the Dumbbell Nebula is very red, whereas the outer parts are blue. I take it that green OIII has been mapped as red, and red Hα has been mapped as blue.

Is there a reason for that?

Ann
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Re: APOD: M27: The Dumbbell Nebula (2016 Nov 02)

Post by Boomer12k » Wed Nov 02, 2016 7:07 am

Ditto, Ann's post....

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Betelgeuse93

Re: APOD: M27: The Dumbbell Nebula (2016 Nov 02)

Post by Betelgeuse93 » Wed Nov 02, 2016 9:33 am

Me too, no scientific accuracy at all...

Image

http://astronomis.at/Astronomis/Nebulae ... l_soft.jpg

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Simon

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Re: APOD: M27: The Dumbbell Nebula (2016 Nov 02)

Post by Chris Peterson » Wed Nov 02, 2016 2:16 pm

Betelgeuse93 wrote:Me too, no scientific accuracy at all...
Any lack of "scientific accuracy" has nothing to do with the choice of color mapping, and everything to do with the editorial decision to accept an image with no technical information attached.
Chris

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Re: APOD: M27: The Dumbbell Nebula (2016 Nov 02)

Post by Visual_Astronomer » Wed Nov 02, 2016 4:23 pm

Jumping on the "me-too" wagon here, but this is the strangest picture I've seen of M27. On the positive side, it does reveal curious details not seen in the more typical photographs.

heehaw

Re: APOD: M27: The Dumbbell Nebula (2016 Nov 02)

Post by heehaw » Wed Nov 02, 2016 5:27 pm

Visual_Astronomer wrote:... this is the strangest picture I've seen of M27. On the positive side, it does reveal curious details not seen in the more typical photographs.
I love accurate pictures that show familiar objects in (literally) a strange light: it reminds me every time that we are only, ever, seeing the tip of the iceberg.

jhayes_tucson

Re: APOD: M27: The Dumbbell Nebula (2016 Nov 02)

Post by jhayes_tucson » Wed Nov 02, 2016 6:05 pm

Ann (and others,)
Thanks for your questions. Narrowband imaging is quite different from RGB imaging. First, by looking at the object with narrowband filters peaked at different wavelengths, it is possible to pick up detail in the object that might be barely visible with broadband filters. Remember that a red filter lets through a wide range of spectral lines from all of the different elements in the object. If you restrict the view to within about 5 nm around the Ha line (for example,) you greatly increase the contrast of features that exist in the hydrogen clouds only. Second, narrowband imaging always maps the result into a false color palette. The normal Hubble palette (HSO) maps OIII into blue (which is roughly correct,) Ha into green (which isn't even close,) and SII into red (pretty good.) Since Ha is normally the dominant element, its emission almost always swamps everything else so an additional step has to be taken to normalize all of the signals to the weakest signal (usually sulfer.) Even in the best processed Hubble images, the final colors don't do a good job of showing the actual distribution of gasses in the object. If you want to understand gas distribution, you need to look at the raw images for each channel. What the HSO palette helps to do is to paint a stunning scene of detail that is not otherwise seen with broadband imaging--with maybe just a little bit of information about how the elements are distributed. The colors are mostly an artistic choice.

When you use only two filters (as in this image,) you have to figure out how to map the result into something that is visually revealing. M27 has very strong signals in both OIII and Ha and they overlap quite a bit. If you simply combine the images without proper pre-processing, you end up with a screaming violet image that shows little detail. The most important part of processing a bicolor image is to come up with a synthetic green channel that best splits the contributions from each element. I spent over four months and made over a dozen attempts to process this data before I realized that the trick is to spatially separate the color components using something called local histogram equalization. This approach can essentially separate the high spatial frequency components from the lower spatial frequency components for each element, allowing the underlying colors to come through. As with any narrowband image, the goal isn't to exactly map the location of the contributing elements; it's to bring out the underlying detail using a pleasing false color palette that approximates how the image detail maps back to the elements themselves. Clearly, the result is something striking and at the same time, a bit unfamiliar. That's because you are seeing features in the object that aren't always so immediately obvious. In this case they are cosmic shockwaves in interstellar hydrogen and oxygen.

I hope that explanation helps to better understand what you are seeing in this image.

Best regards,
John


PS For anyone who is interested, this image was taken with a Celestron C14 Edge on an AP1600 mount. The camera is a FLI ML16803 and focus was held in real time (with the shutter open) using Optec FocusLock and an IFI ONAG guider. The total exposure was 6.3 hours @-25C. Processing with PixInsight and a bit of PhotoShop. Location: "Fly by Night" Roll Out Observatory, Bend Oregon

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Re: APOD: M27: The Dumbbell Nebula (2016 Nov 02)

Post by geckzilla » Wed Nov 02, 2016 7:39 pm

Thanks for the detail, John. And try not to let some folks get you down on the colors. I've learned that there are a lot of strong opinions on the matter and it mostly comes down to bias in human perception—that is, people want colors delivered to them as their eyes might perceive it, even if it makes no sense or causes a tremendous limitation in the conveyance of data.
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Ann
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Re: APOD: M27: The Dumbbell Nebula (2016 Nov 02)

Post by Ann » Wed Nov 02, 2016 8:20 pm

Thanks for your explanation, John.

I'm the extreme color nerd at this site, and it irks me a lot when color is used in a strange way and no explanation is given for it. But now you have explained the reason for you choice of hue.

You wrote:
As with any narrowband image, the goal isn't to exactly map the location of the contributing elements; it's to bring out the underlying detail using a pleasing false color palette that approximates how the image detail maps back to the elements themselves.
Yes, I feared that the overly red hue was chosen because it was considered pleasing. As a lover of all blue things, I'm not too happy when things are made to look redder than they really are because the color red is considered pleasing.

Nevertheless, I do realize that a lot of people really love it when their astronomical objects are made to look red. And I have to accept that astrophotographers may well share this love of the color red, or else they may want to color their astronomical pictures so that they appeal to as many people as possible. I have to respect both these possibilities.

You wrote:
Clearly, the result is something striking and at the same time, a bit unfamiliar. That's because you are seeing features in the object that aren't always so immediately obvious. In this case they are cosmic shockwaves in interstellar hydrogen and oxygen.
I can see the shockwaves, and I really appreciate them. Visual_Astronomer and heehaw like them too. I agree, your picture makes us see the Dumbbell nebula in a new light, and that is a very good thing, regardless of what I think of its color.

Ann
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jhayes_tucson

Re: APOD: M27: The Dumbbell Nebula (2016 Nov 02)

Post by jhayes_tucson » Wed Nov 02, 2016 10:26 pm

Ann,
I can appreciate that the colors might not appeal to everyone and you aren't the only one to express some distaste for this palette. Until you try to process this kind of data it's hard to realize how hard it can be. I personally like to assign the hydrogen signal to the red channel and ionized oxygen to the blue channel. That fundamental choice flows forward into the result. The "red" signal in this image gets mixed up with the other channels in a way that it becomes a pale pink/violet color while the blue gets mixed with the green channel to produce the pale blue/grey/purple variations. You can mess with the balance a bit to make the blues more blue but that pulls the "red" deeper into the purple. The balancing act is a challenge and I doubt that I could ever make everyone happy no matter how it gets mixed. I just want you to understand that the data itself does a lot to drive the result once you pick the channel assignments.

John

Ironwood

Re: APOD: M27: The Dumbbell Nebula (2016 Nov 02)

Post by Ironwood » Thu Nov 03, 2016 1:04 am

The awesome link "remains mysterious" in the text of this APOD helped me picture how scientists and astronomers try to understand these strange phenomena. Well done!

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Ann
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Re: APOD: M27: The Dumbbell Nebula (2016 Nov 02)

Post by Ann » Thu Nov 03, 2016 5:23 am

jhayes_tucson wrote:Ann,
Until you try to process this kind of data it's hard to realize how hard it can be. ...

The balancing act is a challenge and I doubt that I could ever make everyone happy no matter how it gets mixed. I just want you to understand that the data itself does a lot to drive the result once you pick the channel assignments.

John
I know nothing about processing data, but you said you spent hours and hours trying to extract as much information as you possibly could out of the data you had acquired, and then you had to make the picture look good, too.

I know I can only begin to guess at how hard this must be. But I love looking at the fruit of other people's hard labor, their great astrophotos, because they really do reveal things about the universe that I absolutely didn't know before. Your picture is one of those that reveal new things to me. So thank you so much for your hard work, John!

Ann
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