Soul Nebula in Cassiopeia: Light Wavelengths

This image is a companion to one I posted on 11/25. It is the same subject and telescope, but taken with a filter that passes certain wavelengths of light and blocks others (see in comment). The difference is interesting, even if you don't have the patience to read through the following

Why the filter? Short answer: it makes the nebula brighter.
Are the colors realistic? Short answer: No.

I'm often asked if the colors in my astro images are "realistic" - if those are the colors we would see with our eyes. That's a more complicated question than it seems at first. This is my humble attempt at explaining a little about the fascinating topic of light wavelengths as applied to astro images.

Light is just radiation; different colors are different wavelenthgs of radiation. Human eyes see ligth in a certain range of wavelenghts - the familiar colors of the rainbow. The rainbow also extends in both directions past what we can see: past violet is ultraviolet, for instance, and past red is infrared. Stars emit radiation at many wavelengths including ultraviolet, infrared and beyond.

Modern digital camera sensors are truly amazing, and are sensitive to the entire visual spectrum as well as infrared and ultraviolet. Typical cameras like the one in your phone include filters that block infrared and ultraviolet so they don't mess up your pictures.

Cameras designed for astro imaging don't have these filters, so they can capture wavelengths that our eyes can't see. Why? A typical hydrogen emission nebula (the "red" ones) emit radiation in the far red to near infrared range, corresponding to the exitation of hydrogen atoms. For astro imaging we want to gather up that invisible "light" as well.

Almost all of my astro images are taken through a filter of some sort. The most common filter I use is for filtering out light pollution (Baader Moon & Sky Glow). It allows most light through but blocks certain wavelengths corresponding to typical light pollution sources, such as street lights. Street lights are yellow-ish, so images taken through this filter have less yellow than our eyes typically see. So I've lost some "realism," but the trade-off is my images are much clearer without the effects of light pollution.

This image is taken with a different filter - a "dual bandpass" filter (Optolong L-eNhance) that isolates the H-Alpha, H-Beta, and Oxygen III nebula emission lines (wavelengths). The image is therefore less realistic still, but there's distinctly more "brightness" in the hydrogen nebula itself, which is what we're primarily interested in. This also allows me to take images when there's a bright moon out (filtering out "moon wavelengths"), which would otherwise be impossible.

Related question: Why are the colors in Hubble and Webb images so "whacky?" Short answer: They are made up of monochrome images taken through very narrow band filters, then mapped to colors we can see. Are the colors realistic? No, but that's okay because we wouldn't be able to see that stuff otherwise.

Another related question: Why is the Webb telescope "infrared?" 
Long answer: The universie is expanding in all directions, and things farther away are moving faster - we have Edwin Hubble to thank for that observation. As things move away from us the light is shifted towards the red (Doppler effect). The light from things furthest away from us in the universe is red-shifted entirely out of the visual spectrum into the infrared. Those most distant objects are some of the things that Webb is designed to see. Additionally, infrared is absorbed by our atmosphere and very little of it makes it's way to the ground - that's why we need infrared telescopes in space.