Hi all, since many asked and as promised, below is the (pretty simple) technique I have been using since several years to blend in Ha and OIII narrow band data to my images.
1) I get all (L) RGB Ha and OIII fits images aligned and registered.
2) In Pixinsight (but any other software of your choice is fine) I assemble the RGB base from the individual R,G,B fits. I than fully process the RGB (color balance, DBE, non-linear stretching etc) paying attention the image is pleasant, with well saturated stars but not white clipped. As I am happy with the result I save the RGB image as 16 bits TIFF file.
3) If a Luminance image is available I process it in PI as well. Again I keep the contrast not too strong, if necessary I will increase it later on. Luminance is then also saved as 16 bits TIFF file.
4) I process the Ha and, if available, OIII fits. These narrow band channels must be processed carefully avoiding to clip the high lights (where the signal is stronger). Experience here dictates the right balance between contrast and detail visibility, so you will have to find your own way here. NB images are than saved in TIFF.
At this point most of the process in PI is done.
5) In PS I open the images, I copy and paste the luminance (if available) over the RGB base and change the blending mode to "luminosity". I then work on curves, levels saturation etc. until I am satisfied with the final result. Target is to have the LRGB completely processed as it is going to be published.
6) I than copy and paste the Ha image over the LRGB sandwich and change the blending mode to "lighten". On the layer menu I colorize this layer (menu "adjustment> hue and saturation" than tick the box "colorize"), on the slider I select the hue associate to H-alpha (by moving the slider of hue to 330/345, red with a hint of magenta to simulate the Hb contribution (*)), set the saturation to +100 and brightness to -50. I convert then the property of this hue and saturation Layer to "clipping mask" so it effects only the Ha layer and not the (L)RGB sandwich below. (*) the right hue setting depends also on the color the HII region has in the RGB base, some nebulas have a strong emission of Hb, other don’t, so play a bit here to find the best value..
7) At this point I further play with curves and levels on the Ha layer (preferably using adjustment layers set as well as clipping mask) to get the result that best fit my expectations. In some extreme cases (but usually this is not necessary), to prevent the signal of the stars of the Ha layer to affects the (L)RGB base, I work on the curves of the Ha reducing the highest value from 255 to about 200, pinning meanwhile the central point of the curve. In presence of very bright stars in the field, with the clone stamp I remove the stars’ image on the Ha layer if they affect the color balance of the star coming from the LRGB image below.
8) If I have an OIII image I proceed in the same way adding a further “OIII” layer but setting the hue slider to a value around 195 (tail color)
9) As I am happy with the result I flatten the image and voilà, I have my beautiful HaOIIILRGB super composition
The print screen attached I believe is self explaining enough showing how a stack looks like before merging the layers down.
The use of this technique to add Ha and OIII allows me to ensure the colors of the stars, dust and reflection nebulas are completely preserved through the whole process and there is no color cast introduced by the narrow band images, unlike what it happens when you add the Ha to the R channel only (or the OIII to GB channels). Furthermore, where the Ha signal is lower than the (L) RGB signal, it means that there is no recorded Ha emission in the area and therefore the color remains neutral (for example dusty areas that actually in FRONT of the HII emission nebula, the color of the dust remains “natural” (as it comes from the RGB balance), in nice contrast with the background HII emission. If you add the Ha to the R channel you also add a pedestal that redden the hue of the background areas. I believe this is one of the features many appreciated on my Ha/OIII RGB images and as you see it is easily achieved
Last, this technique permits easily fine-tune the color and contribution of Ha and OIII without touching at all the (L)RGB base, hence giving a very big degree of freedom during the processing.
I hope this help you with your processing
Regards
Marco