Quote: 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?
Yes, the simple red filter just doesn't have a high enough transmission to let the fainter features show up. Many of the fainter red features *do* show up in all the filters which have a red secondary passband with high transmission (my old Lumicon OIII, the DGM NPB, and the Lumicon H-Beta).
Quote: BTW which features among those listed in the picture (A-M) are seen red in the OIII+Ha? Are there others?
It is hard to describe it from memory, but the features marked A, B, C, and D, all show up with at least some faint reddish color (especially in the dual band H-Beta filter, where even M43 shows a little red). The red is not uniform along the features, but each does show at least some pinkish or reddish hues in or close to the locations you have marked. The brightest red portion is probably at and between the B and C arrows. The E-F-G feature appears as one long band in the H-Beta filter and does show some red, although again, it is not uniform along the length of the band. The location marked N is faintly red, but is also where the red starts to fade somewhat, as in the interior of the M42 "loop", there is little color visible. The features I, L, and M show little if any color in my 10 inch. Without filters, unless I use a very large aperture, I don't see the nice reddish hues I did with the filters (they are more pale pinkish or tan colorations when they are visible).
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Re: Is it possible to see the nebula with colour?
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