Oops - come to think of it, it's more likely these "Natrium" filters BLOCK sodium than narrowly bandpass it... After all, at least in the US, much of the artificial skyglow derives from sodium-vapor (Na emission) lighting. Is it possible that "Natrium" is Orion's European name for their "Ultrablock" filter (similar to the Lumicon "UHC" filter)? Lew, awash in emission lines