How to post-process astrophotographs — Part 3

How to post-process astrophotographs — Part 3

This post was updated on December 3rd, 2018

Table of Contents

Introduction

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In last week’s blog article on how to post-process astrophotographs %mdash; Part 2 in Adobe® Lightroom, I showed you how to create a pleasing astrophotograph. But it came with a catch. The foreground caught a color cast by the changes I made to the Tone Curve. In this week’s third and last part of post-processing astrophotographs I am going to show you how to make the Tone Curve adjustments without changing the foreground in Adobe® Photoshop.

Post-processing in Adobe® Photoshop

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We are starting our work from the results that we archived with the basic Adobe® Lightroom adjustments in this image:

Use Edit -> Edit In -> Adobe® Photoshop to open the image in Adobe® Photoshop. You might get asked if you want to change the color space of your image before it opens in Adobe® Photoshop. This happens, when your standard color space in Adobe® Photoshop is different from your image’s color space.

Since my photo uses the biggest color space PhotoPro RGB I will not convert to a smaller color space here.
Now that the image is finally opened in Adobe® Photoshop it looks like this:

As you can see in the Layer Panel there is one single, locked, layer called Background.

If you cannot see the Layer Panel press F7 or go to Window -> Layer to activate it. Since the layer is locked we cannot do much work on it, but since I want my changes to be non-destructive anyway, we will create a new layer in the next step.

Creating a new Layer in Adobe® Photoshop

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For the next steps I do not want to work on the original layer. This because I do want to have one layer that preserves the original, should the case occur where I have to go back and start over again. So I am now creating a duplicate of my original layer. Since I am not certain yet if I want to make changes to the sky and the ground, I am actually creating two new layers right away. To create a duplicate layer I am opening the context menu by CTRL clicking on a Mac / clicking with the right mouse button (Windows) on the name of the orignal layer and am choosing Duplicate Layer. The short-key for this action is CMD+J (Mac) / Ctrl+J (Windows). If you are using the menu to create a layer copy a pop-up window open like this:

Within the pop-up you can give the new layer a name and you could copy it into another document as well. In this case I am going to name my first layer Layer 1 and the second one, created in the same way, Sky. The Sky Layer is the one we are going to work on in the next few steps. But first let’s have a brief look at how the Layer Panel now looks like:

Selecting and masking the sky

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For the next steps we are going to work with the Sky layer, so make sure that you have it as the active layer in the Layer Panel. Due to the good contrast between the mountain range and the sky, using the Quick Selection Tool works well to select the foreground.
Selecting the sky was not that easy with the Quick Selection Tool. Whenever I touched one of the stars the selection would suddenly incorporate the entire image. So selecting the foreground and then inverting the selection is a better choice.
When you choose the Quick Selection Tool you will see a toolbar like this:

In the second field you can choose if you want to select a new area losing what you might have selected before. This is good for the initial selection, but you want to make sure to use either the adding to selection or the subtract from selection settings when you later want to change your selection. I chose a relatively small hard brush to make my selection. This because I don’t want to accidentally add too much sky. Nor do I want to keep too much distance from the mountain range to not select the sky. With a smaller brush I can stick close to the top of the mountain without getting too much of the sky selected.
Clicking and dragging my mouse across the mountain eventually leads to this first selection:

Improving the initial selection

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In order to improve my selection, I am now changing the zoom level to 100%, before I am changing the mode of my Quick Selection Tool to Add to Selection. Now I am again clicking on areas that should be included in the selection and drag the mouse along. Occasionally it is more of a clicking along than a dragging along, depending on the size of the area that I need to add to my selection. Some areas are hard to select with the Quick Selection Tool, so I am changing to use the Polygonal Lasso Tool for these.
The toolbar for the Polygonal Lasso Tool looks a little different, but essentially has the same functionality:

For example to add that bit of snow in the center of the photo to my selection, I am using the Polygonal Lasso Tool. The contrast here is not strong enough, to be detected sufficiently by the Quick Selection Tool, but the lines are straight enough to be easily followed with the Polygonal Lasso Tool.

My final selection looks like this:

Inverting the selection and creating a Layer Mask

Pressing Shift+Cmd+I (Mac) or Shift+Ctrl+I (Windows) inverts this selection and I now have the sky as active selection. My next step is to create a mask, which is revealing the sky and hiding the foreground from my Sky Layer. To do this I only have to click on the Create Layer Mask button in the Layer Toolbar (circled in red in the next figure).

As you can see, the area of the mask that is white, corresponds with the sky in my image, while the black area in the mask corresponds with the foreground. If for some reason I needed to add or remove areas from the mask now, I could just paint over them with a brush in white or black. In my case my selection looks good, so I don’t need to tweak it more.

Adjusting the Tone Curve in Adobe® Photoshop

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So far I have only made some preparations to non-destructively change the Tone Curve of the photo. So now I am going to add an Adjustment Layer to make the necessary changes to the Tone Curve. I am chosing the Curves Tool from the Adjustment Panel right above the Layer Panel (circled in red in the next figure).

This creates immediately a new layer in my Layer Panel called Curves 1.


It also opens the Curve’s Properties Window.

Changing the Red Tone Curve in Adobe® Photoshop

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To make changes to the red Tone Curve I am changing the RGB from the Curve Property Window to Red. What a surprise. As before in Lightroom I am now locating the point one-quarter to the right and one-quarter up from the lower left corner in the linear curve. I am clicking on it and drag it down a bit. The second point to adjust in this Tone Curve is located one-quarter to the left and one-quarter down from the upper right corner, and this one I am dragging up a bit. My Red Tone Curve now forms a stretched S-shape as you can see in the next figure.

Once more the photo looks a little like from an alien world, but the next steps will fix that.

Changing the Green Tone Curve in Adobe® Photoshop

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To make changes to the green Tone Curve I am changing the Red from the Curve Property Window to Green. As before in Lightroom I am now locating the point one-quarter to the right and one-quarter up from the lower left corner in the linear curve. I am clicking on it and drag it down a bit. The second point to adjust in this Tone Curve is located one-quarter to the left and one-quarter down from the upper right corner, and this one I am dragging up a bit. My Green Tone Curve now forms a stretched S-shape as you can see in the next figure.

Are you already spotting the color casts in the snow, that I was complaining about in the Adobe® Lightroom section? Don’t worry, I will just finish up with the blue curve and then it will literary be a click to remove them.

Changing the Blue Tone Curve in Adobe® Photoshop

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To make changes to the blue Tone Curve I am changing the Green from the Curve Property Window to Blue. As before in Lightroom I am now locating the point one-quarter to the right and one-quarter up from the lower left corner in the linear curve. I am clicking on it and drag it up a bit. The second point to adjust in this Tone Curve is located one-quarter to the left and one-quarter down from the upper right corner, and this one I am dragging down a bit. My Blue Tone Curve now forms a stretched mirrored S-shape as you can see in the next figure.

The final adjustment in Adobe® Photoshop

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Already when I had made the Tone Curve adjustments in Adobe® Lightroom, I realized that they need to be made to the sky only. Right now, we have made all the right preparations, but the Curve 1 Layer is still affecting the entire image and not just the visible part of the layer directly underneath. To change that I simply have to convert the Curve 1 Layer into a Clipping Mask. I create a Clipping Mask by selecting that Curve 1 Layer and pressing Alt+Cmd+G (Mac) / Alt+Ctrl+G (Windows).
My Layer Panel now looks like this:

The difference in the Layer Panel between a common layer and a Clipping Mask is the arrow pointing from the Clipping Mask down to the layer it is affecting. Also the Clipping Mask Layer is a little indented compared to the underlying layer.

The final result looks like this:

Some more tips

  1. If you are post-processing a photographs with a reddish foreground or maybe a silhouette in the foreground, you might be able to use Adobe® Lightroom only.
  2. If your foreground is a seascape or a lake with a reflection, try inverting the mask so that the Tone Curve affects the seascape and not the sky.
  3. If your foreground includes snow, you will most likely get the best results if the Tone Curve is not affecting the snow.
  4. Try to make only subtle changes to the Tone Curve for a more natural looking result.
  5. Have fun!

Now compare this image to the one we started out with and let me know in the comments, which one you prefer.

 

All trademarks and copyrighted items mentioned are the property of their respective owners.

I am in no way affiliated with any of the products used in this post-processing process. I do not receive any compensation for this article.

All photos taken by Lille Ulven Photography.

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