According to the Astronomik transmission curves, the major artificial light pollution emission lines for mercury at 435.8, 546.1, 577.0, and 578.1 nm, and for sodium at 589.0, 589.6, 615.4, and 616.1 nm.
It looks like the Astrodons cut out everything from 570 to 620 nm (i.e. a substantial amount of light pollution), although the B and G filters will still pick up the other wavelengths.
The IDAS LP spectrum is very similar to the Astrodon/Astronomik L spectrum, but it blocks the major light pollution wavelengths. Assuming reflections aren't a problem, I would imagine that you'd see a huge benefit with the IDAS in front of the Astronomik RGB filters.
Your idea of replacing the L filter with the IDAS seems sound in theory to me, but you'd still get some light pollution in the B and G filters.
Anyhow, I would have thought that a 48 mm threaded filter would be easy to add into your optical train? Many nosepieces and connectors are threaded for 48 mm filters.
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Originally Posted by gregbradley
The Baader filters claim to be optimised to reduce some effects of light pollution. In my experience light pollution seems to effect the green channel the most so check out the specs of the green filter most heavily.
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I'm using Baader filters with my SBIG STF-8300M. There's a gap between R and G at 580 nm, but all other Hg/Na wavelengths are passed through. It's certainly better than imaging with a DSLR (red channel is nowhere near as bad), but the green channel seems most severely affected as you pointed out.
Hmmm I might have to try putting an LPS filter in my nosepiece. (I sold my Lumicon a little prematurely... oops.)