Image 1: Close-up of a region in the Veil Nebula
Merging
At this stage we should have two cleaned up images, one of the starfield only and the other of the nebulosity. We may also want to add some narrowband data on top.
For H-Alpha regions in other galaxies I prefer to process that data separately and then add it on top of the RGB image. Similarly I may do this for other narrowband data (the Oxygen O-iii greenish blue line comes to mind). For H-A regions I remove the stars from the data and throw that part away. I enhance the nebulosity and convert the monochrome to colour and make it red. Ideally it should be the red of the H-A line (656 nm). I reduce the background noise so I am seeing a set of small patches of red. Now this H-A data is ready to go.
The RGB stars, RGB Nebula and (if you have them) H-Alpha images are overlaid in GIMP. The RGB Nebula layer at the base is “Normal”, the star image is included as “lighten only” and the H-Alpha is “addition”. Check carefully that the three are aligned properly because some of the software insists of flipping the image whether you want this or not. More than once I’ve noticed that the stars had been flipped relative to the nebulosity! Compare the result with the first complete image or one on the web to be sure! The combined image is made into one TIFF file (flatten image command in GIMP). Save this TIFF Image.
This file is now loaded into photoshop and colour balanced (“levels”, making the background properly black) and cropped. You may want to fine-tune things in Astra Image, but keep all copies because you are just as likely to make a mess as a masterpiece.
This process will have made a nice and clean astrophotography image with fine sharp colourful stars and a well-defined nebula.
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