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JA,
Life for me is never easy at times.....
I'm actually trying to construct an ERF filter to use with the MiniSHG.
I have a good Baader 7nm H alpha filter with blocks all other wavelengths and is ideal for H alpha imaging.
For CaK, I need something with a "good" transmission at 390nm and effectively blocks everything else including the NIR.
(I have various narrowband filters which can do this job but they are assembled into my "DIY CaK" filter)
I've been using a Baader 0.9 ND filter plus a UV-IR cut filter for all other wavelengths down to 400nm and then just removing the UV-IR filter.
(Having said that I'm about to trial an old "York Optical" IR blocker plus the 0.9ND for CaK)
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