Hi Raymo, I think I know why you have blue in the centre of your Lagoon. A lot of this is probably (as a saying from where I come from) "
teaching a grannie to suck eggs", due apologies if so ... but it helps for both me and hopfully others to put out what I think I understand, especially if I get it wrong
. I think there are two reasons:
1: In an image such as the one you presented, you should not centre all the histograms over each other. Over 50% of your image is dominated by the red Lagoon Nebula, and a histogram of
this image should have a significantly larger than average red/pink component. Images of smaller DSOs that have a lot of neutral dark grey/black sky background are much better approximated as being a neutral colour balance - red, green and blue curves should roughly overlay each other. You should only overlay the histogram 'cones' in cases where most of your image
ought to be neutral. A simple example would be a picture of a large red circle surrounded by a dark grey rectangle - the red histogram is skewed to the right of the others by the presence of so many red pixels.
In your Lagoon picture, you are suppressing the red in your image by centring the histograms into a single cone, and so the Lagoon loses some of its red colour ... but it keeps the blue:
2: Hidden behind the H-alpha in the Lagoon is a solid component of blue-green H-beta, especially where there is strong emission and less interstellar dust in the way. Your image demonstrates this.
The ratio of H-a to H-b is used in astrophysics to estimate interstellar reddening by intervening dust is Hα/Hβ/Hγ= 2.86:1:0.47, an example is NGC3576 and NGC3603 in Carina (
Mike Sidonio's great image of it). NGC3603 is much further away, and much more reddened by dust as the H-beta and gamma are scattered along the way. A
pure hydrogen emission colour is pinkish, even dangerously closer to the dreaded 'magenta' that imagers try and avoid
[but it's not exactly magenta either, it's more delicate than that]. A hydrogen emission nebula is only pure red if there's a lot of intervening dust, otherwise it has the blue-green and blue components that make it more pinkish than pure red.
Put the two together - for a big cloud of dominantly reddish hydrogen emission, if you force the red, blue and green components to have the same weighting, a lot more of the blue will come out so long as there's not too much intervening dust. It's not the same as the varying camera/DSLR detector sensitivity to H-a and H-b, but a similar resulting effect). Yet another problem is a reddish sky background caused by light pollution, which, when roughly neutralised to grey, can take out some of the red from a nebula. Photographing red nebulae is not so easy!
It doesn't mean it's "wrong" (colour interpretation, what people like, what different equipment can do, can all be pretty subjective, so go with what you like
), but I think this is why many Lagoon images are reddish and this one of yours has more blue.
And before anyone talks of pots, kettles and black, I definitely get the colour balance thoroughly skewed in most of my imaging attempts!!
