by Chris Peterson » Wed May 19, 2021 2:42 pm
Ann wrote: ↑Wed May 19, 2021 2:16 pm
Chris Peterson wrote: ↑Wed May 19, 2021 1:20 pm
VictorBorun wrote: ↑Wed May 19, 2021 3:37 am
the order of RGB colors correctly maps the order of wavelenghts of the
narrow-band filters:
B 438 nm
OIII 502 nm
V 555 nm
I 814 nm
H-alpha 656 nm
NII 658 nm
Note that the B, V, and I filters are all broadband. An image made with just these three would approximate "true" colors, with some distortion due to the red channel being pushed to longer wavelengths than we can see, and having poor overlap with the green channel. The addition of the narrowband filters adds more color distortion, although probably not too severe since, as you point out, these narrow bands are mapped into the output color channels where we would expect to find them.
Technically, the use of the IR filter means that this image is described as "false-color".
But probably not so far from "true color" to make Ann too unhappy!
Hmm. I haven't commented on this APOD, except to express my delight over the cute kitties that were hidden in one of the links, and that Orin "unearthed" for me!
The reason why I haven't commented on the Necklace Nebula is precisely because I don't trust the colors. Yes, I can imagine that the green stuff in the center of the nebula is green OIII, and the pink "flares" emerging from the "necklace" itself might be Hα, possibly mixed with something to make it look diluted.
But what is all the blue stuff? And why is it blue? The way I understand it, planetary nebulas are not typically blue at all. And I hate it when non-blue objects are shown as blue, because it makes me so disappointed to find out that they are really non-blue. For example, I once read about a certain kind of white dwarfs (helium-rich white dwarfs, I think), that turned blue when they grew colder. I thought that was really neat.
Yes, but then I read about what really happened to these white dwarfs! At a certain temperature, when the white dwarfs had grown so cold that they emitted most of their light in the infrared part of the spectrum, their emission shifted from invisible infrared to visible red! So these stars grew
bluer by growing
redder!!!
(Yes, I know, I know - the wavelengths of visible red light are shorter than the wavelengths of infrared light. So instead of emitting photons of increasingly large wavelengths, these cooling helium-rich dwarfs suddenly "jumped backwards" and emitted light of a shorter wavelength, namely red! Therefore these optically redder white dwarfs could be described as growing bluer! I understand the reasoning, but I hated it!)
So, in short, Chris, does the Necklace Nebula really emit a lot of truly blue light, or should I ignore this nebula the way I ignore those "blue but red" helium white dwarfs?
Ann
This white dwarf is only a few thousand years old. It has to be so hot that most of its output in in the UV. So in the context of the filters used, it is blue. The apparent blue of the nebula is, perhaps, a mix of reflected light from the central stars and emissions from oxygen (which the wideband blue filter will capture). The choice of color mapping is consistent with an approximation of true colors, but there is more to it than that, since we know nothing about the weighting of each channel in the final processing.
[quote=Ann post_id=313389 time=1621433789 user_id=129702]
[quote="Chris Peterson" post_id=313386 time=1621430428 user_id=117706]
[quote=VictorBorun post_id=313379 time=1621395476 user_id=145500]
the order of RGB colors correctly maps the order of wavelenghts of the [url="https://esahubble.org/images/potw2117a/"]narrow-band filters[/url]:
[b][color=#0000FF]B 438 nm[/color][/b]
[b][color=#00FFFF]OIII 502 nm[/color][/b]
[b][color=#00FF00]V 555 nm[/color][/b]
[b][color=#FF0000]I 814 nm
H-alpha 656 nm
NII 658 nm[/color][/b]
[/quote]
Note that the B, V, and I filters are all broadband. An image made with just these three would approximate "true" colors, with some distortion due to the red channel being pushed to longer wavelengths than we can see, and having poor overlap with the green channel. The addition of the narrowband filters adds more color distortion, although probably not too severe since, as you point out, these narrow bands are mapped into the output color channels where we would expect to find them.
Technically, the use of the IR filter means that this image is described as "false-color". [b][color=#0040FF]But probably not so far from "true color" to make Ann too unhappy![/color][/b]
[/quote]
Hmm. I haven't commented on this APOD, except to express my delight over the cute kitties that were hidden in one of the links, and that Orin "unearthed" for me!
The reason why I haven't commented on the Necklace Nebula is precisely because I don't trust the colors. Yes, I can imagine that the green stuff in the center of the nebula is green OIII, and the pink "flares" emerging from the "necklace" itself might be Hα, possibly mixed with something to make it look diluted.
But what is all the blue stuff? And why is it blue? The way I understand it, planetary nebulas are not typically blue at all. And I hate it when non-blue objects are shown as blue, because it makes me so disappointed to find out that they are really non-blue. For example, I once read about a certain kind of white dwarfs (helium-rich white dwarfs, I think), that turned blue when they grew colder. I thought that was really neat.
Yes, but then I read about what really happened to these white dwarfs! At a certain temperature, when the white dwarfs had grown so cold that they emitted most of their light in the infrared part of the spectrum, their emission shifted from invisible infrared to visible red! So these stars grew [b][color=#0040FF]bluer[/color][/b] by growing [b][color=#FF0000]redder[/color][/b]!!!
(Yes, I know, I know - the wavelengths of visible red light are shorter than the wavelengths of infrared light. So instead of emitting photons of increasingly large wavelengths, these cooling helium-rich dwarfs suddenly "jumped backwards" and emitted light of a shorter wavelength, namely red! Therefore these optically redder white dwarfs could be described as growing bluer! I understand the reasoning, but I hated it!)
So, in short, Chris, does the Necklace Nebula really emit a lot of truly blue light, or should I ignore this nebula the way I ignore those "blue but red" helium white dwarfs?
Ann
[/quote]
This white dwarf is only a few thousand years old. It has to be so hot that most of its output in in the UV. So in the context of the filters used, it is blue. The apparent blue of the nebula is, perhaps, a mix of reflected light from the central stars and emissions from oxygen (which the wideband blue filter will capture). The choice of color mapping is consistent with an approximation of true colors, but there is more to it than that, since we know nothing about the weighting of each channel in the final processing.