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You still aren't reading what I am saying! The red filter I have does *pass* H-alpha, but is *not* an H-alpha interference filter! It is a simple dye glass color filter with some of the characteristics of the Kodak Wratten #23a light red filter, and is intended *only* for viewing lunar and planetary detail (such as the dark albedo markings on Mars). Its transmission at H-alpha is not very high (probably around 40% to maybe 60% at best), so while it does show the brighter H-alpha features, it cuts out enough light to hurt the view of the fainter ones. However, the red passband of a filter like the DGM Optics NPB is close to 90%, so there is little attenuation of H-alpha. This makes the somewhat fainter red features more easily visible. These fainter details were not seen in the simple red filter because the red filter does not have the transmission that the dual-band filters do.

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Thus you mean that you did not see all the red spots in the red filter only because it transmits only (probably) 40-60% at Ha. But they must be certainly visible in the Ha filter, because they are seen in the OIII+Ha filter. Am I interpreting correctly?

BTW which features among those listed in the picture (A-M) are seen red in the OIII+Ha? Are there others?


PS since in post #2050151 you defined your filter to be "equivalent" to W23A ("I have looked at M42 using my "Mars" filter (a Wratten #23a equivalent red filter), ..."), please could you be clearer on this point (eg brand and model)? I have a W23A and I can repeat your tests (In Fact I already did it a month and a half ago, but not in ideal conditions I do not recall if I mentioned them here).

Edited by Mauro Da Lio (12/16/07 05:59 AM)

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