APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by starsurfer » Fri May 22, 2015 3:55 pm

A "natural" colour image by CHART32 can be seen here.

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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Chris Peterson » Fri May 22, 2015 4:13 pm

starsurfer wrote:A "natural" colour image by CHART32 can be seen here.
Not sure I'd call it that. It's essentially a narrowband image of the nebula combined with RGB to yield reasonable star colors. The Ha and O[III] give a suggestion of "natural" color, but overall I think it's still fairly artificial.
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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Ann » Fri May 22, 2015 5:02 pm

The colors in the image processed by Johannes Schedler look so much better and closer to "probable visual 'enhanced' color reality" than the visual plus IR color image in the APOD, in my opinion.

But I will not join the discussion about whether they are "natural" or not.

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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by geckzilla » Fri May 22, 2015 8:29 pm

You'd have to define natural, anyway. There seems to be several lines of what "natural" means to people. One of those is "what I want my eyes to see" (more bias) / "what I think my eyes would see" (less bias) and another is "the wavelength approximated by an RGB display" and yet another is a kind of combination of those two where a few compromises are made in order to create a more aesthetically pleasing image. Whether or not those compromises are acceptably "natural" or not is another argument. To me, Schedler's image is clearly color balanced (the OIII got moved to the blue channel) rather than representative of reality.
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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by geckzilla » Sat May 23, 2015 12:29 am

I can create an image that uses all appropriate wavelengths and nearly the same filters as the CHART32 image. It's missing SII but that's yet another red, so NII is a good enough replacement. OIII is added as this color: XXXX and Ha and NII are both pure red additions. There is no way to get this nebula to be blue if you are sticking to reality because this nebula is simply not blue. It's dominated by red, orange, and yellow. You'll notice that even after all the narrowband filters are added, the central star is still blue. It's really the only blue thing about the nebula.

http://www.geckzilla.com/astro/NGC2440_RGB.jpg
http://www.geckzilla.com/astro/NGC2440_RGB+OIII.jpg
http://www.geckzilla.com/astro/NGC2440_RGB+OIII+Ha.jpg
http://www.geckzilla.com/astro/NGC2440_ ... Ha+NII.jpg
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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by geckzilla » Sat May 23, 2015 1:35 am

If we arbitrarily decide, well, I don't like that greenish yellowish brownish color, then we can scoot the OIII away from 502nm to occupy a much bluer color (XXXX), then increase its brightness greatly so that it rivals the dominance of the red light like Ha and NII, and finally force the colors to separate even farther with a saturation adjustment... it becomes this.
http://www.geckzilla.com/astro/NGC2440_arbitrary.jpg
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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Ann » Sat May 23, 2015 4:40 am

geckzilla wrote:You'd have to define natural, anyway. There seems to be several lines of what "natural" means to people. One of those is "what I want my eyes to see" (more bias) / "what I think my eyes would see" (less bias) and another is "the wavelength approximated by an RGB display" and yet another is a kind of combination of those two where a few compromises are made in order to create a more aesthetically pleasing image. Whether or not those compromises are acceptably "natural" or not is another argument. To me, Schedler's image is clearly color balanced (the OIII got moved to the blue channel) rather than representative of reality.
I'll settle for "the wavelength approximated by an RGB display".

There are many, to me, "colorfully weird" often HST planetary nebula images out there. I can understand, I really can, that they may have been imaged through a specific set of filters for scientific purposes. Another explanation might be, sometimes, that the nebula was imaged through the set of filters that was most economic for the Hubble telescope itself. Again I must respect that.

But I would have found it so helpful if the Hubble release of a strangely colored nebula was accompanied by a lower-quality RGB image of the same nebula.

Finally I have to agree with what you said about the color balancing of Schedler's image. I like it the way it looks, but I would have respected it if the OIII channel had been shown as green.

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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by Ann » Sat May 23, 2015 4:49 am

geckzilla wrote:I can create an image that uses all appropriate wavelengths and nearly the same filters as the CHART32 image. It's missing SII but that's yet another red, so NII is a good enough replacement. OIII is added as this color: XXXX and Ha and NII are both pure red additions. There is no way to get this nebula to be blue if you are sticking to reality because this nebula is simply not blue. It's dominated by red, orange, and yellow. You'll notice that even after all the narrowband filters are added, the central star is still blue. It's really the only blue thing about the nebula.

http://www.geckzilla.com/astro/NGC2440_RGB.jpg
http://www.geckzilla.com/astro/NGC2440_RGB+OIII.jpg
http://www.geckzilla.com/astro/NGC2440_RGB+OIII+Ha.jpg
http://www.geckzilla.com/astro/NGC2440_ ... Ha+NII.jpg
Thanks, Geck! I'm very pleased that the central star looks blue in all your images. As for the nebula, I'm not really asking for a blue emission nebula. Do "RGB blue" emission nebulas really exist?

I prefer the RGB image because I think of it as the most "natural". I note, too, that it is the "bluest", because there are hints of blue in the innermost part of the nebula. The last image is nice too, in a "Christmas color" way. How did you color balance the narrowband filters in that one?

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Re: APOD: NGC 2440: Pearl of a New White Dwarf (2015 May 17)

Post by geckzilla » Sat May 23, 2015 8:10 am

I have no idea if there is any kind of emission nebula out there that's blue. I think you'll have to settle for the reflection nebulas like the Pleiades. The last three are just turning on the layers. I made the OIII brighter because it would be overwhelmed by the two red ones otherwise. It's not like I measured the flux or luminosity (what's the right word?) of each emission and adjusted their brightness accordingly. I've never had that kind of discipline in processing—not that you thought that I did, but it's the only other way I can think to do it.
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