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Nebulae emit light at a variety of wavelengths, but most are outside the visible band or they are in the deep red, where our eyes have very little sensitivity at night.
But they also tend to emit light at wavelengths we can see at night, and see quite well:
486nm Hydrogen Beta--blue
496nm Oxygen III (doubly ionized oxygen)--blue green
501nm Oxygen III--blue green.
So here is how the filter works:
Our eye sees fainter features and nebulae when there is more contrast with the backgound.
By filtering out almost all light other than the 3 wavelengths I mention, the sky gets 90% darker but the nebula is barely touched in terms of its brightness.
As a result, we see much more nebula and the background in the eyepiece goes to jet inky black.
Of course, stars are also dimmed a lot since they emit a lot of light at wavelengths now filtered out.
If you are seeing color, it is a sign your eyes have not yet dark adapted, because our scotopic, night, vision is color-blind and can only see shades of gray.
It is possible for a nebula (M42 comes to mind) to be so bright in a telescope that it starts turning your color vision back on. We refer to this as mesopic because a limited color response is possible.
Night vision is most sensitive at night around 500nm (blue green), while daytime (photopic) vision is most sensitive at 550nm (green).
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