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In the Baader line, 4 filters provide some violet filtration. Note: the higher the cutoff wavelength, the stronger is the minus violet filtration and the shorter the achromat that can be used.
[Aside: divide the f/ratio by the aperture in inches. If 3 or more, no filter is needed. If 2.5 to 3, a gentle filter (low cutoff) will work. If 2-2.5, use a strong minus-violet filter. And, if under 2, use the strongest minus violet filter and keep magnifications low, where CA is least objectionable]
Baader Moon & Sky Glow--~420nm (also cuts some other wavelengths)
Baader Semi-Apo--420nm (very little spectral change)
Baader Fringe Killer--430nm (blues not affected)
Baader Contrast Booster--485nm (this one kills most nebulae somewhat, so it's primarily a Moon/double star filter) (yields a "warmer", more yellow tone because all violet and some blues are gone)
Hope that helps.
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