Chris Peterson wrote: ↑Wed May 19, 2021 1:20 pm
VictorBorun wrote: ↑Wed May 19, 2021 3:37 am
Deathfleer wrote: ↑Tue May 18, 2021 8:21 pm
I believe that the featured image is not artificially colored but color enhanced.
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?