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I use an Astronomic 2" CLS-CCD filter in front of all other filters.
I have to due to sodium & mercury lamp light pollution.
That effects the colours as well.
It obviously blocking some of the yellow/orange colour.
I find that an Ha filter gives a slightly better FWHM on stars so
it is giving sharper images.
It helps to combine it with the Red channel & even a bit of the Luminance
channel to make a much nicer picture.
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