So reflection nebulae are scattering (changing the angle) of the incoming starlight proportional to the inverse 4th power of wavelength, this is the Rayleigh scattering mentioned above. Reflection nebula are blue because the light you are seeing has been scattered through a large angle and only blue light scatters to large angles. Same reason the sky is blue and sunset is red. A narrowband filter is designed to have high transmission at one or these days several emission lines, notably Ha, O3, S2, Hb. It has low transmission away from these so it attenuated very well LP (normally the sodium line). Because it has a narrow band pass it also attenuated quite well a broadband source. Imagine a filter that has a square transmission spectrum, say 0% out of band and 100 % over a 10nm region. Imagine imaging a broadband source, which emits uniformily over all visible wavelengths. The broadband photons from the source that lie inside the peak transmission of the filter will make it through, but that’s photons over a 10nm window, vs without the filter the entire visible spectrum, 300nm wide. So the change in amount of light let through is reduced by a factor of 30.
This is is what is happening with your Pleiades image. Some light will get through as it’s a broadband source, but it will be attenuated heavily by the band pass of the narrowband filter.
The great thing about facts is that they continue to be true whether or not you believe them. You came here asking why your Pleiades image was below par, you received an answer about 4 times. You posted asking for help so perhaps take on board what has been said. Finally, if you disagree with something, your statement of ‘you’re a 100% wrong’ is fairly useless without explain why you think we are wrong. But of course you are welcome to use a narrowband filter to image the Pleiades if that’s your thing and forever wonder why your images don’t turn out as you expect.
Edited by Narrowbandpaul, 09 March 2019 - 05:16 PM.