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Kevin Franklin
1,638 followers -
Husband, father, cardiologist, amateur astrophotographer, and student of Kung Fu.
Husband, father, cardiologist, amateur astrophotographer, and student of Kung Fu.

1,638 followers
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I get asked frequently enough in my own posts, along with comments in other threads, about colour in astrophotography. What is real, true, natural colour, and how do our images compare to what the human eye would see if we could go there?

Here's the long answer.

To begin with, all colour is just perception. Electromagnetic radiation (including visible light) has no inherent colour as we think of it, but simply exists as a continuous spectrum of energy/wavelength/frequency, from gamma rays at shorter wavelengths, to low-frequency radio waves.

Because of the portion of the light spectrum that penetrates the atmosphere, and by virtue of evolution by natural selection, Earth's creatures percieve light in a narrow range of the spectrum, between ultraviolet an infrared wavelengths. In the case of humans, we have developed retinas with three types of cone cells, the photoreceptor cells which contain proteins that preferentially absorb different ranges of wavelengths; short (blue), medium (green) and long (red). These receptors aren't particularly sensitive, and require high light levels for a signal to be triggered. The other class of photoreceptor cells, rod cells, are more sensitive across a broader spectrum, and therefore cannot differentiate colours. This is why we have poor colour vision in low light conditions. The signals generated by the triggered retinal cells travel to the occipital lobe of the cerebrum where they are combined, processed and perceived as "colour" across the gamut of the visible spectrum.

This is, of course, analogous to the way a colour CCD chip collects photons through an array of red-, green- and blue-coloured filters, the output of which is then synthesised by a camera or computer into a visible colour image. One key difference is that unlike the retina, a CCD can be shuttered to different (and in astrophotography, often very long) exposures, enabling the visibility of objects, and colours, that simply cannot be discerned by the human eye. Anyone who has looked down the eyepiece to see the faint nebulosity of even the brightest deep-sky objects can attest to the limitations of the eye in comparison to long-exposure photographs of the same object. The blending of the red, green and blue channels can be automated, for example by setting the "white balance" in a DSLR camera, or manually, as is often done in astrophotography, by rendering the background as "neutral" or setting reference stars to "white", all with the objective of achieving as natural a colour balance as possible.

So when we think of colour, we must remember that it is not an absolute phenomenon, that our perception of it is a construct, generated from raw monochromatic data which are processed in a way as to render colour information, and that this holds true whether one is talking about biological vision or an CCD camera, and is completely subjective to the observer.


And the short answer?

All colour is an illusion, so it doesn't matter if it isn't real in the photos, and human vision is far too limited to see the depth of colours we can display in modern astrophotography anyway.

And don't get me started on narrowband and other forms of false-colour imaging... that's totally fake.
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Just a relative size comparison here, Saturn taken an hour or so before Cassini's demise, Uranus and Neptune shortly after, in slightly better seeing.
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After watching the Cassini probe diving into Saturn the other night - sorry, no meteor visible through my meagre telescope - I turned my sights on our more distant solar system cousins, Uranus and Neptune. Apart from the colours, neither offers much for the Earth-based observer, and both have had a fly-by visit from just one spacecraft (Voyager 2). I wonder whether a Cassini-style orbiter mission should be on the menu.

15th September 2017, Meade LX90 8" SCT with 2X Barlow lens using QHY 5L-II CMOS camera through Orion LRGB filters. Processing in Autostakkert, Registax and Photoshop.
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I'm a bit late to the party, I know, but I haven't had access to a computer to process my photos since the solar eclipse a few weeks ago. Clouds foiled my plans, and I completely missed totality at my location, with this the best shot I was able to get before the clouds rolled in.

Nikon D5000 at 300mm focal length with Kenko ND100000 filter. 1/1000s at ISO 100, f/5.6. Processing in Photoshop CS6.
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Nice pre-dinner drink.

No, I did not finish it on my own...
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Umm... isn't that a Christian?
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And the people you run into! I found +Ciro Villa​ outside the Space Shop and stopped for a chat and a selfie. It was great to catch up live in person 👍
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If you've never been here, come if you can.
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Best Father's Day ever!
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Unfortunately, this was the best shot I got during totality yesterday. The view from the rooftop of the +Acme Feed & Seed​ in downtown Nashville, TN at 1:29pm. The eclipse was hidden by a thick cloud that came in just two minutes before totality, then took half an hour to move on.
Samsung Galaxy S7, panorama courtesy of the Google Photos Assistant.
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