How to Reduce Digital Noise in Astrophotography Using Exposure Stacking

Astrophotography has become increasingly popular in recent years, with good reason. There’s something about the night sky, stars, and The Milky Way that are fascinating to us. They remind us of how small we are and how huge the universe we live in really is. Photographing them can make for some pretty spectacular images.

Digital Noise in Astrophotography

As camera technology has advanced, photographing the night sky has become possible for photographers of all levels and budgets. Low-light performance continues to improve, allowing us to photograph the stars at higher and higher ISOs. However, digital noise continues to be one of the biggest challenges for astrophotographers.

There are a number of different approaches to dealing with digital noise in your astrophotography, from your camera settings to the way you process them in post-production.

Digital noise is caused by a couple of things. Firstly, the camera sensor heats up as it exposes an image, causing an increase in noise. Secondly, an increase in sensor sensitivity, or ISO, can lead to more digital noise in your images. As both high ISO values and long exposures are going to lead to more digital noise, you’re going to need a strategy to deal with it in your astrophotography.

Exposure Stacking

There is a technique called exposure stacking that is very effective in reducing the digital noise in your photos. You take multiple exposures with the same settings, stack them into layers inside Photoshop, align the stack, then Photoshop will create an image based on the median of all the stacked exposures. The final image will show the parts of your exposures that are consistent through each layer, like the stars. Because digital noise is random, and changes from one exposure to the next, it will not be visible in the final stacked image.

If you’re still following me, great. It sounds complicated, but I’m going to walk you through exposure stacking step-by-step and you’ll see it’s really not that difficult. It can take a little time to get right, but it’s totally worth it when you see the difference it can make in your night sky photos.

Capturing the Stars In-Camera

There are plenty of other articles that will teach you in detail how to take great astrophotography, so I won’t go into it here. However, there are a few considerations that are required to get the exposures correct in order to be able to use the exposure stacking technique later.

1. You need multiple exposures with the same camera settings. You can take as many shots as you want, but I would suggest using a minimum of 10. Try to capture them as close together as possible to minimize movement of the stars between each exposure. The more time that lapses from the first exposure to the last, the more work will be required to stack them properly.

2. Turn off Long Exposure Noise Reduction. This is probably called something like “Long Exposure NR” in your camera. It will cause each exposure to take twice as long when it’s turned on, meaning there will be twice as much movement of the stars between exposures. It also means you’ll be double-processing your images, causing a reduction in image quality.

3. Make sure the stars in your photos are pinpoint. They need to be sharp and have as little streaking as possible. You can work out the maximum exposure time to create pinpoint stars based on the focal length of your lens using this tool.

Import and Develop in Lightroom

Again, there is a wealth of information about how to process astrophotography in Adobe Lightroom. All I do in Lightroom is check each exposure to eliminate any images that are unusable due to camera movement, do a basic edit, then open my selected images to Photoshop as layers.

Use “Open as Layers in Photoshop” to do exposure stacking. Go to: File > Edit In > Open as Layers in Photoshop.

The main things to remember here are that you make sure to sync your edits with all the exposures that you’ll be using and to avoid over-processing the images in Lightroom. Avoid sharpening and noise reduction at this stage of the process. Also take it easy on contrast, clarity, and dehaze. You can perform more creative edits on the final stacked image.

Aligning and Stacking Exposures in Photoshop

Ensuring your images are all aligned correctly is vital when doing exposure stacking. If they are not, you will end up with blurry stars. There are a couple of ways to align exposures. Try the auto-alignment method first and if it doesn’t do a good job you’ll need to use the manual method.

Auto Alignment

1. Select all layers.
2. Select Edit > Auto-Align Layers…
3. Select Auto. Click OK.

Manual Alignment

1. 1. Make only the bottom two layers visible.
   2. Select the second layer and change its blend mode to Difference. You’ll see the image go mostly black with white specks. The white areas represent the parts of the two visible images that are not aligned correctly.

3. Click Edit > Free Transform.

4. Click View and make sure Snap is unchecked.
5. Zoom in on a corner, hold down command/control and move the corner box around until you see the white parts of the image line up and turn black. It will take some trial and error.

6. 6. Repeat with each corner of the image. You may need to go back to readjust a corner that you’ve already moved. It won’t be perfect, but try to get it as close as possible.
   7. Press return to exit Free Transform mode, then change the blend mode back to Normal.
   8. Make the layer you’ve just adjusted invisible and the next one up visible.
   9. Repeat with every layer, aligning each one with the base layer until they’re all aligned as well as possible.

Stacking Layers

1. Make sure all layers are visible and selected.
2. Right-click on one of the layers and click Convert To Smart Object.

3. Click Layer > Smart Objects > Stack Mode > Median.

Finish up

When Photoshop has finished working its magic, you should end up with an image that’s much cleaner with significantly less noise than you started with. Your stars probably won’t look quite as sharp when zoomed into 100%, especially if the alignment wasn’t quite right, but you’ll be the only person who looks that closely. Don’t forget to crop the edges that have moved during the alignment process.

Now you can apply any other creative edits you might like to your image. You can either do this while still in Photoshop or save the image and apply the adjustments back in Lightroom.

This may seem like a complicated process, but once you’ve done it once or twice you’ll get much quicker. If you’re anything like me, you’ll find the effort is worth it for the lovely, clean, noise-free astrophotography images it gives you.

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