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Friday, November 20, 2009

Colors in Astro Images

I made two animations to show difference between different Color schemes used in my images.
At an image of the Elephant's Trunk Nebula abowe, differen color variants are animated together.
First is pure Hydrogen alpha channel, second is a RGB-broadband image, next is H-aRGB mix. After that, there is pure Narrowband compositions from three channels, H-a, S-II and O-III. First of them is a "Natural Color" composition mixed so, that resulting colors are close to visible spectrum to human eye. Last image is a "Mapped Color" composition, where all three channels are mapped to selected color. In this case the mixture is S-II=Red, H-a=Green and O-III=Blue, this combination is generally known as a Hubble-palette (HST), after a Palette used in images by Space Telescope Hubble.
What is the truth then?
Deep space targets are generally so dim, that human eye is not cabable to see any colors from them.
Closest to real colors is the RGB image, if colors are balanced correctly.
Usually a pure white Star is used to measure a correct color balance between Red, Green and Blue data.
Next closest is the H-aRGB mix, where H-alpha filtered image is used to boost contrast and details in image.
It tend to distor the Red balance in image though.
Third, in reality order, is a Narrowband mixture, where different channels are mapped to visible colors as closely as possible by the wavelenghts.
I'm using a mixture by Richard Crisp.
H-a is mapped to Red with about 20% S-II, O-III is Green and the Blue is mixed from O-III and about 15% H-alpha to compensate missing H-beta.
The Hubble palette is least realistic color scheme. It's made famous by wonderful pictures from the Hubble Telescope. This palette is developed to maximize visual difference between different ionized Elements in Nebula. Even though it's used in sientific purposes, it's beautiful as well. I use HST-palette for esthetic reasons only, with that tool I'm able to reveal some of the hidden beauty.
An other good reason to use it is, that it's a de facto standard and makes possible to compare images from different narrowband imagers.
An other animated example of color palettes, the Pelican Nebula.
There is many "Schools" in astro imaging, some are more purist, than others. I have some principles in my work too. As an astro imager I don't add anything to images. Only manipulation done is Stretching, curves and levels. As an artist I balance different components in an image differently, so I can show the hidden beauty by a way I like to.

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