First we have to talk about colour.

Images we consume on our computer screens are made by presenting measures of red, green and blue at every location, or pixel, in the image. All images we consume can be made following this simple recipe.

In the astrophotography presented here the used sensor is not colour selective, it measures, almost equally well, the amount of light that falls on it regardless of colour. If it was used on its own, a black and white image would be the result. To create a colour image three colour filters are used, Red, Green and Blue. In the production of the image we view, the three resulting sensor images are mapped to Red, Green and Blue as you would expect and this produces what some call a true colour image. This is what was done in https://old-photons.blog/2021/10/12/ngc-6888/ .

There are elements, (remember that periodic table?) which are interesting to the physics of what is going on inside objects in our universe because, we can specifically look for them with matching filters.

When Hydrogen, Oxygen, and Sulphur are present in objects in our universe we can look specifically for them if they are being ionized by a nearby energy source. This ionization is somewhat a fancy word for what happens in neon lights and we should think about the characteristic stable colours.

Doubly ionized oxygen (OIII) is very popular (abundant) in nebula such as NGC6888 and produces a blueish-green colour.

Hydrogen-alpha (Ha) also is extremely abundant because it is a early stepping stone in the generation of all other elements and has a very rich deep red colour.

It appears to be a bit harder to track down information on SII. 🙂

The point of the above diagram is to show how the Hydrogen Alpha and Doubly Ionized Oxygen images are mapped into the RGB colour space and the reason for saying the outcome image depends is because we have a choice on how this is done. The resulting image is called a false colour image, but we can understand what elements are where. Hydrogen Alpha is mapped to Red and Doubly Ionized Oxygen is mapped to a equal amount of blue and green.

Below is a comparison of the Hydrogen Alpha – Doubly Ionized Oxygen (above) image and the RGB (below) image.

From the Hydrogen Alpha – Doubly Ionized Oxygen image we can clearly answer that most of the Crescent Nebula is due to the ionization of Hydrogen due to the abundance of red, and there appears to be a preceding envelope of Oxygen.

Location	Old Photons Observatory Ottawa Ontario
Date	October 2021
Conditions
Weather
Mount	Paramount MX
Optical Instrument	Celestron EDGE11 with F7 Focal reducer
Camera Instrument	SBIG STF-8300
Focus	Position = 1589 Avg HFD = 3.93 Avg FWHM = 3.19 Temperature: 11.5 Filter: Lum (slot 5)
Guider	ZWO
Foal Ratio	Imaging at f/7, Guiding at f/4.9
Focal length	Expected = 1960mm, Measured = 1954.3 mm
Exposure	4 Hours total 2h Ha (6x20min) @ 1×1 Binning 2h OIII (6x20min) @ 1×1 Binning
Image scale	Expected = 0.57 Measured = 0.57(arcsec/pix)
Image Center (J2000)	20h 12m 43.1s 38° 21′ 02.52″
Image FWHM	Measured = 2.0 arcsec (3.54 pixels)
Image Processing	Astrometric Alignment stacking in MaxIM DL Ha – mapped to R OIII mapped to G and B Digital Development 3×3 Median Kernel Filter