The L-eXtreme filter from Optolong is a dual band that passes Ha and OIII emission lines.

The advantage of the L-eXtreme filter over the L-eNhance, which is a tri-band filter,  is that there are no transmission between the H-beta and OIII lines, as there is no nebula emission lines there. In this case, it isn't letting light pollution come through so that maximizes a nebula's signal and makes the sky background darker while imaging. The L-eXtreme filter should better isolate the emission lines only and provide better protection from light pollution.

Substrate: B270
Thickness: 1.85mm
Tpeak:  T>90%
Blocking Range: 300-1000nm
Blocking Depth: Light pollution line blocking >99%
Surface Quality: 60/40
Transmitted Wavefront RMS: λ/4
Parallelism: 30s