caliu wrote: ↑Sun Dec 22, 2024 12:09 pm
Well, excuse me, but this is one of the most unreal and badly processed, the red channel cannot have the shadows so illuminated because it paints the shadows of the green and blue channels, and although it is a narrow band image, you have three channels, red, green and blue, what is not possible is that the red channel paints everything in red, no matter how much data it has collected in that color. The shadows of the red channel, can not start with such intensity that tinges all the shadows of the three channels of red. you will tell me that there are many hours of Ha, but that can not mean that the blue and green also have to be dominated by red. for my understanding of astrophotography, this is a crude painting.
I'm not absolutely sure what you complaint is, but I want to point out that there is no such such as a "true-color narrowband image".
The idea of narrowband imagery is to assign certain "false" colors to certain wavelengths, specifically to wavelengths which are emitted by certain elements in an ionized state, in order to show the presence of particular elements in a nebula and their degree of ionization of these elements.
Consider the most famous narrowband image ever, the one called The Pillars of Creation:
A very quick search turned up no information on what filters were used for this image, but I feel convinced that the filters were OIII at 500.7 nm, which is about this color,
███, Hα at 656.3 nm, which is this color,
███, and SII at 672.4 nm, which to the human eye is the same color as Hα, or this color,
███.
In narrowband photography, the OIII channel was probably assigned this color,
███, and the red Hα was probably assigned this color,
███. SII, the longest wavelength channel, was allowed to stay red.
The narrowband "Pillars of Creation" picture is absolutely not a "true-color" image, but it does give us interesting information about the elements that are present in this part of the Eagle Nebula.
To produce a "true-color" image, you would have to photograph the nebula through red, green and blue filters, and preferably give all three filters the same exposure times. You would be likely to end up with an image that looks something like this:
As you can see, a "true-color" image of the Eagle Nebula is very red, because the red light of Hα is so much brighter than the cyan-green light of OIII to an objective recorder like a camera. As for SII, it looks the same as Hα to our eyes.
So what's wrong with the narrowband picture of the Heart Nebula that was the APOD on December 17?
APOD 17 December 2024.png
Near to the Heart Nebula
Image Credit & Copyright: Jeff Horne & Drew Evans
Nothing is wrong with it. We can easily understand that the blue-looking heart of the Heart Nebula is rich in cyan-green OIII, and this OIII has been assigned a blue color. Most of the rest of the picture is red, from a combination of Hα and SII, which are both red wavelengths. I don't mind the palette of the APOD at all, because the Heart Nebula really
is quite red, and I think that the blue heart looks pretty. Moreover, thanks to the OIII, we can see the supernova remnant in the lower left corner of the APOD, and we can also easily spot three planetary nebulas at right and upper right.
Let's look at a few RGB pictures of the Heart Nebula:
And to summarize: There is nothing wrong with the palette of the APOD of December 17, 2024. This APOD is a narrowband image, and the purpose of narrowband images is never to produce "true-color" images. Nevertheless, each astrophotographer is free to assign the sort of colors to his narrowband filter images that creates an image that
approaches, but is not equivalent to, an RGB image.
Ann
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Near to the Heart Nebula
