Last week, I mentioned an editing technique called frequency separation. This is an amazing Photoshop technique that allows you to edit the tones and colors in an image separate from the details. This means that you can smooth out skin tones, or other tones without losing texture and detail in the image. Frequency separation is usually used on portraits, but here I will show you the process on my milky-way image. The process for doing this is somewhat complex, but once memorized it becomes invaluable. If you want to try this but don't have Photoshop, you can download a free trial here (this is an affiliate link so if you eventually make a purchase I will get a cut)
To start with, you want to duplicate your background layer twice. You can do this by dragging the layer down onto the "new layer" icon at the bottom of the layers panel, or by pressing Command/Control J.
Then, you want to hide the top layer, "layer 1 copy", by clicking on the little eye next to it. Then select the middle layer, "layer 1". Proceed to apply a gaussian blur (in the filter menu) to the image just until you the details go mushy when you are zoomed into 100%. This is your tone and color layer.
Then, select the top layer (make sure you make it visible again) and go to Image>Apply Image.... That will bring up this dialogue box. Here is the part that will take a little bit of memorization and attention to detail. First, change the layer from merged to the middle layer that you blurred (in this case, layer 1). Then change the blending mode from "Multiply" to "Subtract" and set "Scale" to 2 and "Offset" to 128. At this point, the image should look all gray, with some lines and other details in it.
The last step is to change the gray (detail) layer's blending mode from "normal" to "Linear Light." The image should look normal again now.
Now that all of the detail oriented stuff is over, you can start to edit. There are a few different things you can do. You can selectively blur the tone layer to smooth out colors, paint in new colors, or clone textures around in the image. In this image, I ended up re-painting the colors with my brush tool. To do this, just create a new layer between the color and texture layers and then use your paint brush with a low flow on that layer to paint in new colors. The great thing, though, is that the textures in the image are preserved while details are left alone, for example, I painted purple in the top left corner.
You can see that if I move the layer above the detail layer that the brush stokes cover everything evenly and it no longer look natural:
The next way to edit an image is to selectively blur the color and tone layer even more in some places. Make sure you are on the right layer (the one you originally blurred), and then grab your lasso tool and pull the feather up so you get a clean edge. I edited the top left corner again, but I over did it to show you the blurring. This technique is best for smoothing skin in portraits, but you want to make smaller selections on the face where there are similar tones, and you also want to avoid details.
Here is the area again after being blurred with the gaussian blur filter:
The last thing you could do to your image using frequency separation is to edit the detail layer on your image. Use you clone stamp tool on the detail layer to do this, but make sure that you only have "current layer" selected for the tool's sampling. Just to give you an idea of how this works, I cloned the area in the red oval over to the top left corner. Notice how the colors are the same even though the trees and star details are now cloned over.
Thats it! Now all you need is some practice. Frequency may look daunting, but after a few uses the amazing benefits of having the colors and details separate will amaze you. Please let me know if you are confused or run into any problems because this technique is somewhat complicated.