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Grey filter with 13% passage. Who would like to observe the moon with large telescopes, usually needs a relativley dark moon filter, so that also a sufficient absorption is present with very large openings also. This filter is recommended thereby starting from a diameter of 200mm. Who thus with 250mm or 300mm observed is particularly recommended this filter. Observers, in addition, who can to observe take the somewhat brighter filter ND 25 with 200mm.
The optical quality is very good. The glass is plan-parallel and also at the edge has one still another perfect moon picture and must not with distortions be annoyed.
Our expert comment:
The neutral density value is usually logarithmic. For most manufacturers, the transmission corresponds to 1/10 ^ ND, i.e. it falls with increasing ND.
However, Lumicon used ND as a pure percentage. ND13 is hence equivalent to a 13% light transmission and ND25 to a 25% light transmission. An ND25 filter is therefore lighter than an ND13.
(Marcus Schenk)
