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This workflow is courtesy of Craig Stocks at Utah Desert Remote Observatory
To be clear, I didn't invent this way of using Photoshop for astro images. I found this workflow on the YouTube channel of UTDR, presented by Craig Stocks. It's a fantastic way to process images that I haven't seen anywhere else. I've used Pixinsight to complete these steps in the past, but Craig's method is much easier. When it comes to color, sharpening, and blending Photoshop can't be beat.
Adding Halpha and Continuum Subtraction
Now it's time to add the Halpha. Remember, the group is in Screen blending mode, so it will only add Halpha where it's brighter than the pixels underneath.
Even though the Halpha filter cuts out most other light, it still allows some red light that comes from other sources (the continuum). You can see that in the witch's face in the Halpha image. I fix that by subtracting the RGB_red layer from the Halpha, leaving (more or less) only the red actually contributed by Halpha. Now, the entire image won't be overwhelmed by red.
I set the Halpha group to impact only the red channel by double-clicking on the group and unchecking G and B in the resulting Layer Style dialog box. This could also be done by using a Hue/Saturation layer in colorize mode.
Halpha before subtraction.
Halpha after subtraction.
Leave only the R checked under Advanced Blending.
Halpha added without continuum subtraction.
Halpha added with continuum subtraction.
Global Adjustments
Now it's time to add adjustments that affect the image as a whole, not just individual layers or layer groups.
To do that I create a new layer group between the Stars and Halpha groups. Here I brighten the image using levels and tweak colors using Hue/Saturation. The Curves layer is in there, but I don't actually use it for this image.
Layer group with global adjustments.
The solid fill layer adds a layer of dark gray. By putting this layer into Lighten blending mode, we bring any darker pixels below up to the solid fill layer's brightness. We don't want anything to be completely black.
This is one of the best tips from Craig's videos.
The image after global adjustments. The changes aren't major, but they are important.
Adding the stars
We're ready to finish this image by putting back the stars. Since the Stars group is in Screen blending mode, and the background is black (or 0), only the stars will show up, and the black background will be replaced by the layers below.
I use the Minimum filter to remove some of the multitude of tiny, distracting stars, and a Levels mid-tone adjustment to shrink the remaining stars.
When the stars are screened back in, they will probably be washed out, so I boost the saturation and then tweak the colors as necessary.
Stars layer with Minimum filter and Levels and Hue/Sat. adjustments.
And we're finally done!
A single 160s image taken through a Luminance filter.
Final image with every Photoshop layer turned back on.
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