Messier 76 - "Spirit of 76" - Red - White - and Blue

On Tue, 5 Dec 2017 15:16:47 +0000, Martin Brown
wrote:

On 05/12/2017 05:00, RichA wrote:
On Saturday, 18 November 2017 12:27:59 UTC-5, Razzmatazz wrote:
Just before winter sets in the outer arms of the Milky Way pass overhead. Embedded in them are regions of star formation and star death. M76 is a Planetary Nebula where the central star has shed its outer atmosphere shedding its hydrogen (the bright red areas) and oxygen (blue areas) into a shell, which is expanding into the surrounding space.

https://www.astromart.com/common/ima...7.jpg&caption=

Which colour for double-ionized oxygen is real? Most shots I've seen, it's been green, but it's blue here.

OIII to my eyes is just on the dark green side of cyan if bright enough.
With a bit of cunning you can create a plausible looking pseudo colour
image from just OIII 501nm and Ha 656nm narrowband by treating stars and
nebulosity differently in the post processing.

The problem is, color isn't a physical thing, but physiological. We
don't perceive color in terms of wavelength, but in terms of a
function of both wavelength and intensity.

Take a 501 nm source and adjust its intensity and people will see
colors ranging from greenish-black to greenish-white. That is, people
will identify several hundred colors by simply changing the intensity.

That's why it's pretty pointless to worry about "true" or "accurate"
color in most astronomical images (certainly those containing narrow
band emission sources). How the output of different filter channels is
mapped to the broad red, green, and blue channels of our output
devices really just depends on the intent of the imager: an aesthetic
result, or colors which enhance features of interest.