Stacking for night sky images
You might hear the term “stacking” in photography. One can stack for focusing or perspective or star trails. This, however, is stacking for starry night skies, including the Milky Way.
The idea here is to take several photos, one right after another, to reduce digital noise that generally occurs when pushing the camera’s sensitivity higher and higher. Then we throw the images into an app such as Starry Landscape Stacker (Mac) or Sequator (PC). The app will perform its mathematics magic to make life easy for you. It will identify the stars on each of your images, align them, and then stack them.
Above: Both images are zoomed in at 200% after they have been prepped for maximum ugliness in Lightroom (more on that later). The first image is a noisier single exposure image. The second image is the image that has been stacked in Starry Landscape Stacker. You can see that it is a noticeably cleaner image.
You may stack them in Photoshop, but these programs simplify the process and are either inexpensive or free. I’ll use Starry Landscape Stacker as the example since I have a Mac. However, the process is about the same for Sequator. Let’s check it out!
Out in the field
Yes, we will start with what you do with your camera first. After all, it’s photography, isn’t it? You will photograph the Milky Way with your tripod-mounted camera, taking photos one right after the other. Click! Click! Click! Click!
I like to take at least 15 photos, usually doing between 20-25. The more photos you take, the greater the noise reduction. To a point, anyway. 25 is good. Three, not so much. In this example, I did 20.
Prepping the sky images
At home, prep your sky photos. Unfortunately, you sort of need to make them look like rubbish. You may prep them in Lightroom or some other photo editor. Starry Landscape Stacker has an easy-to-follow tutorial, which you should watch before using. The basics are below.
Do the following:
-Use Custom WB to keep the photos consistent
-Increase Brightness
-Decrease Contrast
-Increase Blacks (look at left side of histogram)
-Remove Lens Vignetting and Chromatic Aberration
-Mild Color Noise Reduction
Avoid the following:
-Auto White Balance for each photo
-Adding Contrast, Clarity, and DeHaze.
-Adding color through Vibrance, Saturation, and HSL
-Brightening through Highlights and Whites
-Applying lens distortion corrections
You can perform all these later.
After prepping all identically, export these hideous looking images as 16-bit TIFF files. They will then be ready for stacking!
Stacking the sky images
In Starry Landscape Stacker, select your ugly looking TIFF files.
An image will appear. The stars will trail and it will be covered in red dots. How fun!
Fun with red dots
Your sky should be covered with red dots, each one allegedly representing a star.
You’re going to add more stars. Add some more quickly around the edges of the sky and along the foreground and anywhere else there are stars. Above, I’ve also added some more in the arch of the rock formation. When you are finished, click “Find Sky”.
Blue sky mask
Clicking “Find Sky” will result in a blue mask in what Starry Landscape Stacker thinks is the night sky. If the mask is wrong, paint in more of the sky or erase it from the foreground. You can zoom in to see the actual pixels and control the size of the brush, similar to other photo editing programs.
When satisfied, click “Align and Save”. You will see the program aligning the images with one particular image, which will show in a task bar. This typically does not take very long.
The stacked image
After processing, your app will align all the stars and produce an image that should look like your single Milky Way photos, only with a little less noise. That wasn’t so hard, was it?
Go ahead and save your final output image as a 16-bit TIFF file. Starry Landscape Stacker will give you several different options depending on which algorithm you prefer. Toggle back and forth between several of the different options to see what is most appealing.
Looking for distortion and anomalies
I don’t always save the image using the same algorithms. Sometimes, I prefer a different one from what I’ve used before. Look very closely, especially along the horizon line, to see what appeals to you. If the program or your mask creates anomalies or distortions, it will typically be just above the horizon line. If you’re not sure, you can always save several of the choices and closely examine them later.
If it’s an issue with your mask, the program allows you to go back and work on the mask some more, saving what you had previously.
Saving the stacked image
When satisfied, go ahead and click “Align and Composite”. This will also save the image with and without a mask. I don’t usually don’t end up using the mask because I create my own masks for blending in low-ISO foregrounds (and this will probably be another article in the not-so-distant future).
Further post-processing
Above is the completely processed photo.
In post-processing, you can bring out quite a bit of the stars through careful use of contrast, color correction, de-hazing, clarity, and detail. Remember, the other photo had all this turned down to help Starry Landscape Tracker do its magic. Now it’s time to turn them back up to make the image look better. Don’t overdo it.
I also sharpened the image. And I altered the color of the sky from its bland color to more of a blue because it looked more aesthetically pleasing against the rock formation than a warmer night sky for this particular photo. This is admittedly not accurate. The sky was not blue that evening. But for this photo, it felt right. And since I had taken several photos of the arch this evening, it also helped to differentiate it somewhat.
I ended up cropping it to a square for personal aesthetics, so this isn’t perhaps the best example since the stars are not quite as sharp as other photos I took during the same evening. But nonetheless, it gives you an idea of what you can do with stacking software. As mentioned above, this is blended with a low-ISO foreground. I also light painted this for additional drama.
This is another photo with more of a black sky that was taken just before the photo we were discussing. The same process was used all the way through as well.
I hope this helped describe the process of stacking and encourages you to go out and try one of these programs for yourself.
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