Re: (IAAC) Orion Natrium Filter

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

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