How to post-process astrophotographs — Part 1

How to post-process astrophotographs — Part 1

This post was updated on February 10th, 2019

Table of Contents

In the following article series, I am going to show how to post-process astrophotographs. The preparations to open the file in Adobe® Lightroom are the same as described in How To create a Monochrome Image. This article will guide you through the basic steps of post-processing in
Adobe® Lightroom. The second article will continue with the specific adjustments for astrophotography in Adobe® Lightroom CC. In the third and last article of this series, I am going to show you how to do the adjustments in Adobe® Photoshop, to create an even better photograph.
Similar results are achievable by using other post-processing programs including freeware like Gimp.

Basic post-processing in Adobe® Lightroom

Until a few weeks ago the basic adjustments made to the RAW file would have been all that I would make. But then I read an article in Outdoor Photographer by Glenn Randall on post-processing night photography, which was a game changer. I got to admit the results of a few extra steps blew my mind away. But let’s start at the beginning with the basic adjustments.

The RAW file I am working with, before making any adjustments, looks like this:

BlogAstroPost - Start How to post-process Astrophotographs a step by step tutorial Part 1/3
Figure 1: RAW file out of camera
Figure 2: Histogram of the original RAW file

Camera Calibration

I have learned over time that the Adobe® Standard camera calibration setting is not ideal. This because every camera has its calibration values, depending on the manufacturer but also on the camera model. For many camera models, Adobe® includes the model specific calibration settings as a preset. For some, like my Pentax K-3 you either have to buy them or create them by using a specific software and a color passport. I am still relying on presets, which I bought some time ago and will apply these.

Figure 3: Camera Calibration Panel

Compare the original RAW file with the image that created by this tiny change, by moving the slider to the left/right in the next photo.

Out of Camera versus Camera Calibration changed

The changes are rather obvious, the entire image seems a tad darker, than before. Compare the histogram before and after the camera calibration settings have been changed.

Figure 5: Histogram before Camera Calibration
Figure 6: Histogram after Camera Calibration

Yes, the histogram has changed a bit. It is visible by the color change of the triangle in the left upper corner of the histogram, but also compare the position of the “big waves”. The resulting image has a little more black clipping than the original, but we will fix that to some extent later. Since camera calibration changes not only the colors in the image but also the histogram it is an excellent choice to adjust this before making any changes in the Basic panel. Otherwise, you would have to go back into the Basic panel and fix everything again.

Lens Corrections

I am using the automatic lens corrections to remove potential chromatic aberration and to correct the lens-distortion. This particular photo has no chromatic aberration to start with. But removing the distortion improves the image a lot. Compare the before and after below.

Figure 7: Without Lens Calibration versus with Lens Calibration

The main effect that the lens calibration has on this photo might be hard to spot, but it has flattened the image. It also has brightened up the snow on the sides of the image. This means using lens calibration also has an effect on the histogram. This explains why it is best to use it before you make any changes to the settings in the Basic panel.

Figure 8: Histogram before Lens Correction
Figure 9: Histogram after Lens Correction

The resulting histogram looks in its clippings like the original histogram. But the shape has changed and the resulting image looks like this:

BlogAstroPost - LensCorrections
Figure 10: Photo after Lens Calibrations

Basic Panel

The scene was lit up by a waxing moon and the lights of our car, so it wasn’t a pitch black night to start with. I still have to make some adjustments to create a nice white point and a true black point in the image. If I had detected any areas of dirt from either my lens or sensor, I would now be removing them. However, there are no spots in the photo, so I don’t need to bother about them.

Black Point adjustments

A photo of the night sky is rather dark, especially when you are in an area with little to no light pollution. So there should at least be one point of true black in this image. It should, however, not be so dark that all shades are lost in a sea of black.

There are four ways of adjusting the Black slider in the Basic panel:

    1. Entering a numeric value
    2. Manually moving the slider
    3. Holding the Alt key while moving the slider
    4. Holding the Shift key and double-clicking on the word Black

Entering a numeric value sounds a little cumbersome. Unless of course, you are talking to someone who has worked on the same file and is now telling you which numbers he used. Moving the slider is an improvement to entering the values, as you can watch what changes the movement means to your image. Still, do you know when you have that one point of true black? Sure, you can watch your histogram while moving the slider. I would not recommend this way unless you have a centered histogram. It is, however, a good method for fine adjustments.
But then there are semi-automatic ways of adjusting the sliders of the Basic panel — at least for the Black, White, Shadows and Highlight sliders.

Using the Alt key to change the black settings

Holding the Alt key while moving the Black slider shows you a white image until revealing the first dashes of color, which indicate black clipping. Using this technique, you would move the slider just to the point before the first bits of color occur. I have used this method in the following figure.

BlogAstroPost - BlackPointALTKEY
Figure 11: Photo with Black Point adjusted with Alt-Key method

I am not satisfied with these results. The image seems washed out. Of course, I now have removed all black clipping from the original image. The resulting black value is at +9 as you can see in the Basic Panel view below.

Figure 12: Basic Panel settings

Looking at the histogram, you can also see that there is no more black clipping. So this technique works best on brighter photos, which should not have any black clipping in the end. But it does not really work for darker images like this one.

Using the Shift key to change the black settings

Thus I use the fourth technique of holding the Shift key and double-clicking on the word Black again. This results in a darker but much more natural looking image. It actually moves the Black slider into the minus direction.
And the resulting image:

Figure 13: Basic Panel settings

And the resulting image:

BlogAstroPost - BlackPoint
Figure 14: Photo with Black Point settings

White Point adjustments

As with the Black slider, you can use the four variants to move the White slider. In this case, as the main part of the histogram is far from clipping any white elements, both the variant using the Alt key and using the Shift key work well. They result in the same numeric values.

Figure 15: Basic Panel with White adjustment

After these changes the resulting photo looks like this:

BlogAstroPost - WhitePoint
Figure 16: Photo with White Point settings

Shadow adjustments

To adjust the shadows in the photo I am again using the Shift key double-click on the word Shadow method. I tried using the Alt key method instead, but it has the same fault that it showed with the Black slider, brightening my image too much.

Figure 17: Basic Panel with Shadow settings

My image now looks like this:

BlogAstroPost - Shadows
Figure 18: Photo with Shadow adjustment

Highlight adjustments

In this photo isn’t much to do for the highlight adjustments, but parts of the snow are a tad to bright for a night-scene. So as before I am using the Shift key double-click on the word Highlight method to auto-correct these. It does not change much in the histogram, but compresses the middle section a little more.

Figure 19: Basic Panel with Highlights

The image, after the highlight adjustment, looks like this:

BlogAstroPost - Highlights
Figure 20: Image after Highlight adjustments

Clarity adjustments

Let’s move on to the clarity adjustments. These, together with the Vibrance adjustments, usually bring the image to live for me. They have to be set manually by either entering numeric values or moving the sliders. No semi-automatic shortcut exists, and you are the only one to decide what is too much or too little when it comes to adjusting these. With clarity adjustments you cannot really go overboard though. At least not to a degree where it would ruin the image.

Figure 21: Basic Panel with Clarity settings

If you look very carefully, you can see that there is a difference in the histogram after I adjusted the clarity. I will work on that a little later.

BlogAstroPost - Clarity
Figure 22: Photo with clarity adjustments

Vibrance adjustments

The last adjustment of the basic ones is to change the Vibrance. I am not changing the Saturation as it would change all colors. Changing the vibrancy only has an effect on the duller colors. So it usually results in a more natural looking image. Of course if I wanted to convert my image into a monochrome, using the Saturation slider would be helpful. Again changing the vibrancy settings has changed my Histogram, as you can see below.

Figure 23: Histogram after Vibrancy changes
BlogAstroPost - Vibrance
Figure 24: Photo after Vibrancy changes are made

Fine-tuning of the basic adjustments

Since the changes that I have made to the Clarity and the Vibrance sliders have changed my histogram, I am now readjusting the Black, White, Shadows, Highlight settings. This will only be minor adjustments, and I am going to use the Shift key double-click method for all four.
My final settings look like this:

Figure 25: Basic Panel with fine-adjustments

Comparing the values you can see that only the Black and White settings changed a bit.

BlogAstroPost - FineAdjustmentsBasic
Figure 26: Photo with fine-tuned basic settings

Removing a potential color cast

There is a simple but effective trick to show and remove the color cast from an image. It is here that the Saturation slider is extremely helpful. If you drag the slider all the way to +100 and look at the color of supposedly white areas, you will immediately see if there is a color cast or not. If the Saturation slider changes the color of a white area into something different there is a color cast to remove.

Figure 27: Basic Panel with 100% Saturation

This is what my image looks like with the slider dragged to the max value:

BlogAstroPost - Color Cast
Figure 28: Photo with Saturation at 100%
Adjusting the White Balance

Of course, this is not the result after removing the color cast. With the help of the Saturation slider, you are only making the color cast visible. The snow in this photo now is rather yellow. This indicates that to remove the color cast, I have to drag the Color Temperature slider further into the blue. This is a full manual change, followed by a little adjustment of the Color Tint slider, which I drag a little more into the greens.

Figure 29: Basic Panel with White Balance changes

The temporary result looks like this:

BlogAstroPost - Color Cast Removing
Figure 30: Photo with less color cast, Saturation still at 100%

I am now using the little eye-dropper tool in the White Balance section of the Basic Panel. With it, I am picking a spot in the snow where all shown RGB values are roughly the same. A 1% difference won’t cause many problems. A larger difference in the RGB values can reintroduce a color cast. You have to watch the preview in the Navigator Panel before clicking on your point. This changes my white balance settings again, as shown in the image below. Now I am changing the Saturation value back to its first value of 0 by double-clicking on the word Saturation.

Figure 31: Final White Balance settings

The photo without color cast looks like this:

BlogAstroPost - Color Cast Removed
Figure 32: Photo without color cast

Welcome to an image with white snow. 🙂

Detail adjustments

Of course, this is a night-time photo, taken with a higher ISO of 3200 which includes some noise. So the remaining step in this first round of post-processing is to remove the noise from the photo.

Removing noise

I might not be able to remove the noise entirely, but should be able to significantly reduce it with the following steps. But let’s have a closer look at the photo before I lower the amount of noise.

Figure 33: Enlarged photo with noise

Depending on your camera model a similar ISO might cause less or even more noise. With the help of the Luminance slider in the Details Panel I am at least able to reduce the amount of noise. Of course, I could move the slider to the max, but then I would lose some of the edges on the mountainside and the little hut. So using the slider with some care, while looking at a 100% view to not lose significant detail, will do the trick.

Figure 34: Enlarged photo with reduced noise

The image now looks like this:

BlogAstroPost - NoiseReduction
Figure 35: Final photo for now

Compare it with the first RAW file that we started out with:

Figure 36: Out of Camera versus Noise reduced

This result is not the end

We have come a good step toward the result, and until a few weeks ago I would have accepted this as the final result too. The next part of this series is going to show you some night-sky specific settings , which will enhance the photo further. So stay tuned until the second part of this series is published on May 18, 2018.

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, it was neither offered or asked for.

All photos taken by Lille Ulven Photography.

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