+1 Astrodon Series II.
I currently have the 50x50mm square LRGB set with 5nm Ha and 3nm OIII and SII filters. The latter 3nm filters aren't cheap but really dig out the signal over the background ADU, ideal for getting weak SII photons. I don't do much narrowband work as dark skies tend to provide for traditional RGB imaging.
What I also like about the Astrodon's (also present in the CS filter set) is that there is no wavelength cross-talk between the red and green bandpasses, thus the red filtered data can make a great luminance for emission nebula as opposed to clear filtered data (RRGB compostion). Alan Holmes of SBIG did a write up a long time ago on the topic;
Quote:
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In the new filter set, the red and green passbands have been slightly reduced. The
attenuation of red H-alpha nebulosity that occurred in the old filter set has been corrected.
...
There is a “hole” in the new filter set between 570 and 610 nM. While this may
offend the purist, there are no significant astronomical emission lines in this hole. Stars
have very wide spectral distributions, as illustrated in Figure Two, so no loss of
information results from this hole – you do not miss “yellow” stars (which are yellowwhite).
However, sodium vapor streetlights terribly pollute this region, ...
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Ref:
http://ftp.sbig.com/pdffiles/SBIGFilterSet.pdf