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I see now that ZWO has a 7 position filter wheel and OPT is offering a complete filter set combo with camera and filter wheel. THe NB filters are stated by OPT to be 7nm bandpass. My question is newbie, ie, what are they referring to and is it only NB? I see 7nm, 5nm, and even 3nm filters being offered by different manufacturers with correspondingly different prices. David |
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I've been curious about this as well and did a little bit of digging around. Hopefully someone with more knowledge and experience can offer a better explanation, but it seems that the price of the filter generally correlates with the transmission of the filter. Baader's 7nm H-alpha filter, priced at $200 in 1.25", has about 93% transmission, meaning 93% of the Ha photons pass the filter and hit the sensor. $130 more is Astrodon's 5nm H-alpha filter, which Astrodon guarantees to have 95% or higher transmission. So with the Astrodon filter you're getting a narrower bandwidth (7nm vs 5nm) and equal or higher transmission. As for ZWO, they only claim to have 80% transmission for their 7nm Ha filter. That seems like it would be a step down in SNR compared to Baader's filter with the same bandwidth. I imagine there are also many other factors that determine quality. Some links: http://www.astrodon.com/uploads/3/4/9/0/34905502/astrodonnarrowbandfaq.pdf http://carlostapia.es/curvas_filtros/baader_Ha_7nm.html |
