Photoshop Post Processing Workflow
This article is picking up from the Siril-Post Processing and optionally the Star Removal article. The input into this therefore is a non-linear image either with stars or without them. As with many things on this site I have not come up with this on my own. In particular for the Photoshop Post Processing I borrow a LOT from Astro Ed's YouTube channel. He has an excellent video which I would suggest you go and watch.
IMPORTANT - If starting from the option to not remove the stars then one extra step is required at this point and that is to export the Fit file to a 16 bit Tiff format using Siril. That will be the format expected in Photoshop going forward.
Summary
In summary these are the steps I take from the Pre-processed Stacked Linear image (end of Pre-processing): -
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It is worth noting that I use a non-destructive Photoshop workflow - which basically means Layers, allowing a change to be easily reverted or temporarily made invisible to get a good before and after view. I would recommend creating a new layer (duplicating the "background" layer) at each stage of the process.
When all this is complete your will have in essence a complete image and worth saving, even if this is the Starless only image, in some cases the lack of stars can look really 'arty'! That said we will now recombine the Starless and stars to get back to a 'with stars' completed project.
Levels and Curves
If you have done any Astrophotography processing before you will already be familiar with the use of levels and curves to create contrast by stretching the data you have across the black to white points. In our case we already have a non-linear (stretched) image from Siril. However Photoshop can provide much more control.
When I use both levels and curves I manipulate them at the individual channel level. In the first screen shot I am moving the back point slider to the start of the data. I will also use the mid point in conjunction with the Histogram to roughly align the 3 channels data.
Similarly with the Curves, but with curves you get to push certain input levels more than others, typically by applying an 'S' curve, there is good opportunity through manipulation of each channel independently to start to push an image towards a specific colour cast. Levels and Curves can be performed iteratively, in fact I find it better to do this rather than attempting to get the look first time around.
Levels
Curves
False Luminance Layer
Increasing Contrast – Using a False Luminance & Colour Adjustments
Starting from the above section the Levels and Curves layer and copy it forward (CMD+SHIFT+ALT+E). Name it Start Neb. We now go to Channels and select just the Red channel, this is going to be used as a false Luminance. Select All and Copy (CMD A then CMD C), turn back on all the channels and go to the Layers and Paste (CMD V). This should have copied a new Layer above the Start Neb layer. Name this new Layer- Lum.
Selecting this layer change the Blending mode from Normal to Luminosity.
Level adjust this layer to bring the mid point to where the start of data appears, this will darken what might at the moment be a very light looking image. OK Levels.
Now Duplicate forward (CMD+SHIFT+ALT+E) to create a new layer and name it Lum+Levels. We will adjust the levels now for each channel by setting the High Point this time to start where the data starts. Starting with the Green and Blue I make the adjustment, for the Red channel the mid point can also be pushed a little to exaggerate the contrast a little more.
Duplicate the layer once again (CMD+SHIFT+ALT+E), the following sections will make use during the Camera Raw manipulations which are accessed with (CMD+SHIFT+A).
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Luminosity-Create Layer
Luminosity-Levels
Highlights and Contrast
Creating a Raw-Highlights and Contrast layer, go into Camera Raw using (CMD+SHIFT+A).
Under the Basics section you will find Highlights and Contrast. I drop Highlights down (-30..-20 something along those lines) and increase by a similar amount the Contrast. Use the Before and After view (square looking icon bottom right Camera Raw window).
After doing this AND after each of the various Camera Raw changes we make always go back to Levels by creating a new Layer (CMD+SHIFT+ALT+E) going to Levels and bring down the High points by dragging to the left a little to where the data starts.
Duplicate the Layer again and back to Camera Raw (CMD+SHIFT+A).
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Colour Contrast
Developing contrast by manipulation of the colour cast is one of the more optional elements of the workflow. It can generate powerful effects but, if not used carefully, can make the colour of your image look very fake and over processed. In the case of the image I was working on here I did not feel like it needed it. However, I have used it to great effect on the Rosette nebula (the first image at the top of this page). In that case I manipulated the reds and oranges to move reds to darker reds by decreasing the hue slider and increasing the saturation. Move the orange hue slider to towards yellow. By pushing Oranges towards Yellows and Reds to darker reds you are creating more contrast.
Texture
CMD+SHIFT+A – Camera Raw, looking at Texture under Basic. A matter of being careful not to end up with a slurry mess but to just tone down some of the sharpness that increases the noisy appearance. You can see in the screen shot I pulled the texture down which in my opinion reduced the grainy noise at of course the expense of clarity.
Clarity
CMD+SHIFT+A – Camera Raw, Clarity found under Texture. In the case to try and bring back some of the detail that texture took away I moved the lines up on clarity, before and after view are vital as is the use of the zoom.
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Noise Reduction
CMD+SHIFT+A – Camera Raw, Noise reduction. Major on the Noise Reduction slider and Detail. I find the contrast and the Colour Noise Reduction has very little impact on the image.
Saturation, Vibrancy, Shadows
Minor Saturation and Vibrancy boosts using the vibrancy and saturation sliders in Camera Raw and Shadow bringing the slider way down to bring back some of the shadow detail back.
