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Mick,
Not scientifically of course as Kiwis are not overly edumicated but the 4Nm will allow less of the "other" light in, JUST the Ha, whereas the 13Nm will allow a wider selection of light, Ha as well as "normal". Stars will be less visible in the 4, and more in the 13.
I use the Baader 7Nm.
Gary
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