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I have used my Hutech IDAS LP filter with L and RGB filters (Astronimik). I came to the conclusion that I should use the LP filter by looking at the transmission curve of the Hutech and comparing it to the RGB Astronomik filters. It was clear that the IDAS was blocking certain frequencies that would have been allowed to pass if I used just the RGB filters alone.
I think there are some filters (Astrondon?) that claim to inhibit light pollution in RGB so possibly with certain RGB filters it might not be necessary to use the LP filter. And then this could be refined if one specifically knew the type of light pollution one wanted to block. In any case I have had some pretty decent results using the LP filter with LRGB though the exposure increase rather a lot!
Peter
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