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help with Ha RGB imaging

red dwalf

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hi all,

i managed to get some Ha data the other night, first time since buying the filter months ago, now doing some RGB sub to go with it,

now i`m not sure how to construct the data in Photoshop, do i construct the LRGB subs as normal then overlay the Ha data as another lumiance channel afterwards or do i use the Ha subs as i would the luminance subs and drop the luminance subs altogether ?

many thanks in advance.

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right, so best to blend the Ha and luminance together 60/40 ish, then overlay that onto the constructed RGB subs,

cheers Steve

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  • 2 weeks later...
Guest ollypenrice

A layer applied as Luminance in Ps defines the illumination of the layer below. If the layer below is RGB you are illuminating all three colour channels as if they were the deep red of Ha. But they're not, so this cannot be very truthful. Blue and green are certainly not deep red.


I regard the application of Ha as luminance as a last resort and one to be carried out judiciously. FIrst I apply it where it belongs, in red. After all, that's what it is. If you paste Ha onto red and set the blend mode to Lighten the Ha will brighten the red where the Ha is brighter but not where it is fainter. This has the big advantage of not affecting the stars because the stars are fainter in Ha than in red. (Note how small Ha stars are.)


For galaxies this is the beginning and end of the Ha story. Apply the Ha as above and you're done.


For image-dominating emmisson nebulae the Ha holds so much tantalizing detail that you are overwhelmingly tempted to apply it as luminance as well. The danger is that you'll get a pink image full of blue star haloes. Yuk. But if you've added the Ha to red as above, the image will, ideally, be too red at this stage. Adding a little Ha as Lum will dilute that, but I rarely exceed 15% opacity.



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