Now to combine the Ha with the RGB. How?

[The Admiral]

Something you experienced imagers do all the time, but I'm new to processing narrow-band images, so a bit of guidance would be much appreciated. I fully expect, though, that there'll be as many ways as there are imagers!

I've just acquired an RGB image of the Horsehead and Flame nebulæ and environs, using a OSC, and I've also got my data for Ha using a dual band filter (L-Extreme).

Note that I do not have Pixinsight or Photoshop. I do have Affinity Photo, but I'm pretty new to that and wouldn't call it familiar. I stack and process using AstroPixel Processor. The most logical way would seem to treat the Ha as an L layer, but what would be the most effective way of combining these two datasets?

Ian

[Elp]

Not familiar with affinity but the most straightforward way to do it is, open both, on the ha version select all, copy, open the RGB one and paste on top, it should paste as a new layer. Set it's blend mode to luminosity.

There's other ways but the above is the easiest in PS or GIMP.

Note the lextreme data will also include o3 signal. To specifically select the ha you'd have to go into the channels panel and select the red layer only and copy and paste as per above, but you might loose colour accuracy due to only selecting the red.

[The Admiral]

OK, thanks. APP has a routine for extracting just the Ha, which is what I have done.

Ian

[ollypenrice]

Sorry, but I think it is a bad idea to use NB data as a luminance layer. Maybe twelve years ago everyone used to do this and the forums were full of cutesy pink nebulae dotted with blue-haloed stars!  The thing is that Ha is a deep red and OIII lies on the blue-green border. That, therefore, is where they should go in the final image. If you use NB as luminance you will illuminate the whole image with the light from just those colours, so anything like blue reflection nebulosity will not be illuminated and the NB will, in effect, go incorrectly into all channels.

What you need is a program with layers. In Ps you could put the NB layer over the OSC and try blend mode Lighten or blend mode Screen. Perhaps you can do this in Affinity? I think it must be possible in GIMP. I would do this with both images fully stretched, by the way. The point of this method is that the NB signal will be applied where it is brighter than the OSC, meaning it will go into the right places.

Olly

[The Admiral]

Thanks Olly, perhaps I should get to grips with Affinity. I have extracted the Ha from the dual band output, which of course is grey scale. Presumably that will have to be assigned as red. The L-Extreme has given me ugly halos around Alnitak and another star, so I wouldn't want them polluting the broadband image. Would I be correct in that a starless Ha image would allow me to achieve that and give better star colours? APP can remove stars, apparently, though how it would deal with the halos I'd need to investigate.

Ian

[Elp]

Amendment from my above: luminosity can be used if overlaying narrowband data, but with the combination of a layer mask so it only adds selectively. I did it with my Veil image using HA and O3 as lum.