I enjoy capturing images at night. The same clear skies that can make a sunset uneventful will fill with stars as the sky gets darker. One challenge with photographing night skies is managing the noise that can result from the high ISO settings necessary to keep shutter speeds low enough to prevent significant movement in the stars. The method that I choose to capture and process a scene depends a lot on the conditions and the composition.
This technique is a good starting point for shooting and processing night sky images to keep noise manageable. It is easily adapted to allow longer exposures on the ground (potentially at a different aperture and with a different focus point) to help with depth of field. This technique doesn’t require any tools besides Photoshop – there are night sky stacking programs for various platforms that may be simpler for some, but I like being able to control the blend between the sky and the ground. In this article I’ll step through how I shot and processed this image from Shark Fin Cove near Santa Cruz, California.
In the field I took 3 shots of the sky, each at ISO 6400, 20s, and f/2.8. These were taken right after each other. I then followed up with a single exposure for the ground at ISO 1600, 180s and f/2.8. You can take more shots of the sky to get more noise reduction, but alignment gets harder since stars further away from Polaris move more than those closer to the it – this can lead to more streaking.
Back home, I opened all of the files in Lightroom (you can use Adobe RAW as well). Each “sky” image was processed using the same parameters, but I processed the ground differently (different white balance, contrast, noise reduction, etc.)
I opened the ground image and one of the sky images in Photoshop as layers. I manually blended these images to form the base for processing. When you stack Milky Way images, the ground gets blurry which makes blending a challenge. I’ve learned that if I start with a pre-blended base, I can avoid having to work around the blurry foreground in the stacked images. I used some extra noise reduction on the sky layer since parts of it will be used in the final image and I didn’t really care about saving details since those will come from the stack I’ll describe later.
The base layer for the sky is shown below (one image with extra noise reduction):
I then added the ground layer to the stack above it. You can see that the sky is blurry due to the long exposure I used for the ground. That longer exposure at a lower ISO has much less noise than the images I captured for the sky.
For those that aren’t familiar with stacking layers in Photoshop, the layer pallet is shown below. The ground layer is shown on top of the sky layer which means that only pixels in the Ground layer will be visible.
The next step is to create a layer mask. A layer mask is applied to a layer and selects which portions of that layer visible in the final image. Anything shown in white in a layer mask is visible and anything black is hidden allowing the lower layers to show through. I created the layer mask for this image using a selection. I started by using the Quick Select tool (as shown below) and dragged it just above the horizon to select the sky.
The Quick Select tool does a good initial job but can usually be improved with a little effort. After selecting the sky above, I clicked the “Select and Mask” button near the top of the screen. This provides some options for refining the selection. I used the Refine Edge Brush (identified below) and dragged it along the edges to refine the selection. I start with the options shown in the right panel for my blending selections and they usually work well: 1px radius, select “smart radius”, 1px smooth and 1px feather. Click OK when done.
Once I defined the selection, I inverted it (shift+control+i or shift+command+i) to select the ground instead of the sky and then used it to create my layer mask. With the ground selection active and the ground layer selected (click on it in the layer pallet), click the mask icon in the layer pallet (looks like a white rectangle with a hole in it) to apply the selection as a mask. The result is shown below. You can see the black and white mask applied to the Ground layer. The black on the top portion allows the pixels from the sky layer to show through.
As you can see above, the sky didn’t blend well with the water. To fix this I used a soft brush to paint black into the mask over the water (revealing more of the sky layer) to blend them more carefully. I used the quick selection tool on the rocks to protect them from being affected. I did this by actually selecting the rocks (similar to the method described above) and then inverting it so that any painting I do won’t affect them. I then clicked on the mask itself (see below) to make sure I painted on the mask rather than the image and used a black brush with a low opacity to paint along the horizon until I liked the blend. The resulting mask and layers are shown below.
Here is a close-up of the final mask.
And here is how the image looked after this blending. It has a careful blend of the sky and ground, using a low-noise ground exposure and a single sky image (that is lacking detail because of the extra noise reduction).
Now if you were just going to blend two exposures (or a single exposure processed separately for the sky and ground) you would be done blending and could simply work on finishing the image. In this case, though, I will average three separate sky exposures to reduce the overall noise due to the high ISO used and combine the result with the image above.
The next step is to create a better sky image with more detail and less noise. The image above used a single image of the sky and added extra noise reduction. This is relatively easy to blend with the ground but loses detail due to the noise reduction. You can use multiple images of the sky in a smart object to average out the noise while retaining the detail. That is the process I used here. First I opened all three sky images as layers in a separate Photoshop file. I masked out the ground in each layer (so the alignment would just be applied to the stars) as shown below. This mask doesn’t need to be particularly accurate – you just want to hide the ground because it has moved relative to the stars in each successive exposure and it is the stars that you want to be sharp.
With all three layers selected, select Edit∙Auto Align Layers from the menu to line them up.
Here are the options I use for aligning, essentially just selecting “auto”.
Next I deleted the layer masks and here is the result. You can see a white line along the right edge, showing how the images had to be rotated to line up.
With the three layers selected I right-clicked on the layer pallet and selected “Convert to Smart Object”. This provides some extra capabilities within Photoshop to blend layers.
High-ISO noise in an image has a random noise pattern. By average the data in several images you can remove much of that random noise and leave the actual details intact. This is achieved by changing the stack mode of the 3-layer smart object. From the menu, select Layer∙Smart Objects∙Stack Mode∙Median. This will average the information in the layers.
I then flattened the layers in my image and copied the result into my base image from Figure 9 above. The new layer is labelled “Stacked Sky” below. Note that because the sky images were aligned to the stars, the ground in this layer is blurry. We’ll have to mask that out.
Finally I did a manual blend of the smart object (sky stack) with the base, staying away from the ground, and did a slight crop to remove some of the edge issues created from alignment. The blend wasn’t particularly challenging because the image has some clouds along the horizon. Also, I had used one of the sky images for the base so the colors match well. For a more complex blend, I would have selected the sky to create the mask and then painted black with a small brush along the horizon to remove the land. The resulting layer stack is shown below. This image now includes the long exposure, low ISO ground layer, a bit of noise reduced sky along the horizon, and then 3 sky images aligned and averaged to reduce noise in the sky.
And this produces the final result shown at the top of this post. In my next post, I’ll look at a variation of this technique using a star tracker to get a detailed, low noise image of the sky.