How to post-process astrophotographs — Part 2

How to post-process astrophotographs — Part 2

This post was updated on December 3rd, 2018

Table of Contents

Introduction

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In the first article of this series we started post-processing a night sky photo in Adobe® Lightroom. As I mentioned, I would be satisfied with the results from that blog post up until today, if I hadn’t read an article in Outdoor Photographer, The Milky Way Dilemma by Glenn Randall, from December 2017. So in this part of the series we are going to continue the post-processing. I will now show you what results are possible within Adobe® Lightroom.

Why does Astrophotography need special post-processing

I heard it first when I visited Iceland in late 2017: the human eye cannot see color at night. Sounds weird at first, especially when you think of all the red brake lights you have actually seen at night. Well, it turns out the human eye cannot see reflected color at night. So if an object seems red at daytime because it is absorbing all other colors of the light and is only reflecting the reds, you will not see it in red at night. But if an object emits colored light, you will be able to see the color. Apparently, at night we use the rods in our eyes to see, which are color blind. At daytime, however, we use the cones instead. So when you look at your astrophotographs, you might wonder why the sky has a tint of green and not the dark blue you expected. This is because your camera is able to capture the color of the night sky. So, the question is, do you want the sky to look like “you see it” or do you want to show “its true colors”? If the latter, then you have done all the post-processing you need to do. But if you rather want to show it as most people would believe it is, follow me for some more post-processing.

Post-processing in Adobe® Lightroom

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This approach works for a certain type of images, more on that later. If you are following this guide step by step on your own photo, now would be a very good time to create a Virtual Copy of your image to save your results. This, so you can test the later Adobe® Photoshop adjustments without having to start from scratch.
In order to get the night-time colors we are used to seeing, we need to adjust the Tone Curve. You find the Tone Curve Panel just below the Basic Panel in Adobe® Lightroom.

Set it to Point Curve: Linear and click on the little square in the lower right corner. (Both marked red in the next figure).

Your Tone Curve Panel should now look like this instead:

Adjusting the Red channel in Adobe® Lightroom

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In the Tone Curve Panel select Red from the choice hiding behind RGB directly below the Tone Curve. Now click on the graph about one-quarter up from the lower left corner and one-quarter to the right, drag that point down a bit. Ensure that you are staying in line and are not dragging diagonal.
Then click one-quarter down from the lower right corner and one-quarter to the left and drag that point up a bit. Your Tone Curve should now look like a very stretched S as in the following figure.

The photo looks now like this:

Admittedly this looks more like some alien world, but there are two more Curves to adjust, before judgement time.

Adjusting the Green Channel in Adobe® Lightroom

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To make adjustments to the Green Channel you have to shift from the Red to the Green Channel in your Tone Curve Panel. I don’t think this came as a surprise, did it? You will see a linear graph again, which we are going to tweak a bit. Once more drag point about one-quarter up from the lower left corner and one-quarter to the right a little down. And for the point about one-quarter down from the upper right corner and one-quarter to the left, you will have to drag that up a bit. Just so that your curve now forms a stretched S. It should look somewhat like in the next figure.

In case of my image it now looks like this:

Our work has paid off so far, the alien-world look is gone. There are still a few more improvements that we can make by adjusting the Blue Channel, which is the next step.

Adjusting the Blue channel in Adobe® Lightroom

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Once more we have to change the channel on which we are working, this time we are going to choose the Blue Channel. The initially displayed graph is a linear one again. Our adjustments though differ a little from the previous two channels. This time we will drag the point, located one-quarter to the right and one-quarter up from the lower left corner, a little up. We will also drag the point, located one-quarter to the left and one-quarter down from the upper right corner, a little down. So the Blue Curve essentially mirrors the Red and Green Curve. After these changes the curve should look somewhat like this:

Our final image in Lightroom looks now like this:

This is not a bad result, and, in my opinion, a clear improvement to what we started with. But, it still has some flaws, which you will spot easily on a calibrated screen. The snow has a slight purple tint and the hut’s door a clear red tint. Both caused by the Tone Channel changes that had an impact on the entire image. I could, of course, brush over those areas and use some color or white balance adjustments to correct the colors again, but there is an easier way, using Adobe® Photoshop. If you have an image which only has silhouettes in the foreground of a beautiful night-sky, you might be able to use Adobe® Lightroom only. Next week I will start with the virtual copy again, processing it in Adobe® Photoshop to create an image without color cast in the foreground.

 

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 kind of compensation for this article.

All photos taken by Lille Ulven Photography.

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