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Paul, I use the Astronomik 6nm filters and don't recall seeing this issue, but it's been a few months since I did any narrowband (QLD summer weather!). I'll see what I can dig up.
Regarding the lack of AR glass on the sensor, I've only seen that anomaly twice in several years of imaging with a 1600, most notably on Alnitak when imaging the Horsehead....but it doesn't show up with halos like you're seeing (although I use a refractor, so no diffraction spikes), but more of a square matrix effect. And certainly not on stars anywhere near the magnitude you're describing (but again note I use a small-ish refractor).
Incidentally, I find the cost of the Astrodons shipped into Australia prohibitive compared to the competition, but every pocket is different...
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