Milky Way Panorama Processing

September 20, 2020  •  1 Comment

Milky Way Over the Spanish Peaks, March 2018

I recently reworked my first Milky Way image from the original raw files (separate post in the works), and in the process I learned a completely new approach to processing night sky photos. I decided to apply this process to my first Milky Way Arch panorama photo to see if I could improve the final image quality. (Note: this post is mostly a way for me to keep track of my edits.)

This image was captured in March 2018 using the Sony a6000 and Samyang 12mm f/2 lens mounted on a Panosaurus panoramic head. The first image was captured at 4:13 am, and the final image was captured at 5:33 am. But the actual photos used for the final image were all captured within a ten minute window from 5:10 to 5:20 am. Most of the time before that was spent waiting for some clouds to clear out of the southern part of the sky. I also spent quite a while picking cactus thorns out of my leg after I sat down on a cactus. The perils of night photography!

Panorama Base Images

The photo is a merge of two separate six-frame panoramas, one for the sky and the other for the landscape. The sky images were captured with an exposure time of 25 seconds and ISO 5000 while the foreground was captured with six frames with an exposure time of about 60 seconds (manually controlled in Bulb mode, so the exposure times varied a little between frames) also at ISO 5000. A panorama head (like the Panosaurus) is a big help at night. You can decide in advance how much to rotate the head between frames based on the focal length of the lens. Then in the field, the pano head can be set to mark the start position and you only have to pay attention to the marks for rotating the head. I used my phone screen to provide just enough light to see the marks between exposures. Some pano heads have click detents so it's not even necessary to be able to see the marks on the head to rotate the camera.

Base images for the sky (top) and foreground (bottom)

If I had it to do over again, I would have tried capturing multiple exposures of each frame for stacking, but on this particular night I'm not sure that would have been an option. Given the sky conditions and the equipment I was using, I'm happy with the quality of the raw exposures and just have to make the best of what I have to work with.

Major Processing Steps

1. Merge base images for the foreground and sky panoramas.  

2. Align and mask foreground and sky panoramas.

3. Apply adjustments to blend and match foreground and sky exposures.

4. Enhance the Milky Way.

5. Apply Noise Reduction and Star Boost.

6. Final adjustments in Lightroom.

Merge Base Images

The foreground and sky panoramas were separately merged in Lightroom, then these were composited in Photoshop. For this round of processing, I decided to use the spherical projection for merging the panoramas because it allows me to use a 2:1 aspect ratio for the final crop for printing later on. My original edit used the cylindrical projection which resulted in a 16:9 aspect ratio for the final image. The screenshot below shows the merged panoramas of the foreground exposures; deceivingly, both are about 12,000 pixels wide although the cylindrical projection pano has a higher megapixel count (79 MP vs. 59 MP).

Cylindrical (top) and Spherical (bottom) Projections

Unfortunately, my a6000 produced some strange color shifts between the different panorama frames of the sky image that became very visible in the merged panorama. I did not take the time to correct this problem in my original edit and ended up spending a lot of time trying to fix it afterwards. This time around, I individually color corrected each raw file using the White Balance dropper on an area of dark sky in ACR, then merged the exported (rather than raw) images in Photoshop. Another problem was cloud cover that obscured a large portion the Milky Way core. Fortunately, I was able to use an earlier exposure of the same sky panorama frame to blend in and reduce the amount of cloud cover in the first frame. Even this single frame blending was affected by the color shift issue, so I first had to carefully match the colors of the two exposures before blending them. After merging the panorama frames using the spherical projection in Photoshop, I used the Adaptive Wide Angle filter to straighten the horizon.

Color Shift Apparent in the Merged Panorama

Align and mask foreground and sky

Once the base panoramas have been created, the first step in processing them is to select both of the merged panorama DNG files in Lightroom, then open as Smart Objects in Photoshop (in this case, the sky pano was merged in Photoshop so only the foreground was imported as a Smart Object). This allows for making adjustments to the raw files from within Photoshop when needed. I stacked the images in a single file in Photoshop, then preliminarily aligned the layers so that the foreground features of the landscape image hide the same features in the sky image. A layer mask will be used later to reveal the sky, but first the raw adjustments need to be made in Adobe Camera Raw.

I use the approach recommended by Wayne Pinkston for raw edits: set the White Balance and apply Sharpening and Noise Reduction in ACR, then perform the rest of the editing in Photoshop. He recommends setting White Balance by making the darkest part of the sky as neutral as possible. This is easy in ACR because you can select an area of the photo using the White Balance tool rather than just a spot which is all Lightroom allows.

The foreground exposure was imported as a Smart Object, and I followed Wayne's advice to keep raw edits to a minimum. I set a custom raw Profile to one that looked the clearest, and applied Sharpening and Noise Reduction only in ACR. Unfortunately, at this point I had to rasterize the foreground layer so that I could align the foreground and sky. Auto-Align did not work, so the layers were aligned manually using the Scale and Rotate Transform tools. I placed the foreground layer on top with the horizon positioned slightly above the corresponding features in the Sky layer so that masking the sky from the foreground layer would reveal the sky layer beneath.

Preliminary Alignment of Foreground and Sky Panoramas

The most tedious part of the edit was masking along the horizon. This was accomplished with the help of some luminosity and color masks along with a good bit of detail work by hand. Once the foreground was masked, I made corrections separately to the foreground and sky layers to clean up distractions. For the foreground layer, these were primarily stray lights that were scattered across the landscape. Since it was such a dark night, the lights of vehicles or houses were very bright and distracting. In the sky, I tried to remove some of the remaining clouds in the right side of the image and also removed the light trail of an airplane. For these edits, I mostly used selections of nearby areas to cover up the distractions, although for many of the small lights in the foreground I just used the Clone Stamp tool.

Blend and Match Exposures

Once the layers were cleaned up, the next task was to adjust the Sky layer to get the right color balance and enhance the Milky Way. Once again, I followed Wayne's recommendations here and kept it simple. I used a Vibrance layer set to +20 to enhance the colors, Levels to darken the sky, and a Color Balance layer to adjust the colors by adding blue to the shadows and midtones and a slight amount of green to the midtones.

The harder task was adjusting the foreground to match the sky. For this adjustment, I followed a tip from Stanley Harper and used a Color Lookup layer to apply the "NightfromDay" LUT included with Photoshop. I only used an opacity of 12% on this layer, but it was just enough to darken the landscape a bit. This was followed by a Color Balance layer to pull some blue out of the deep shadows, especially the mountains, and also add in just a hint of magenta and cyan. Finally, I used a Levels adjustment to darken the landscape just a little more so that it blends in with the sky. I would say the Color Balance layer is the key to making the foreground blend in with the sky. Because of the long exposure time for the foreground images, I had quite a few hot pixels, so I used Blake Rudis' Broken Pixel Correction to remove those. I also used his Contrast+ adjustment to enhance the details in the landscape.

Enhance the Milky Way

At this point, the foreground and sky exposures are blended and matched, so it is time to enhance the Milky Way. I started with a Curves adjustment to add contrast and darken the sky. This was applied with a gradient mask to blend the adjustment in to the foreground. Next, I added a Black and White adjustment layer using the Blue Filter preset and Luminance blend mode to enhance the contrast in the Milky Way. After this, a Hue/Saturation layer was used to add some saturation (+33) to the Milky Way, and another Curves layer added some brightness. I then used Blake Rudis' Milky Way Pop with an opacity of 33 which really deepens the colors and contrast. All of these adjustments were applied to the Milky Way only using a feathered mask. I added Blake Rudis' Nightshade 1 with an opacity of 8% to apply a slight color grade to the entire image. Then finally, I finished up the Milky Way adjustments with a Clarity adjustment (+50) applied to a stamp layer with 66% opacity and applied an Orton Effect Bright using Raya Pro to the entire image with an opacity of 12%.

Noise Reduction and Star Boost

Noise reduction was applied as the almost-final step. I used Nik Dfine on a stamp layer which did an excellent job of reducing noise, but also softened the foreground a bit too much. To limit the noise reduction, I used an opacity of 55% and also used BlendIf to exclude noise reduction from the highlights (stars).

The final adjustment in Photoshop was a Star Boost to enhance the brightness of the stars and Milky Way. I'm not sure where I learned this technique, and I had to improvise it for this image because I am not sure how I created it the first time. Star Boost uses the Overlay blend mode on a solid white layer with a Brights luminosity mask along with BlendIf to exclude the effect from darker areas. The overall effect is controlled with opacity, and the layer can be duplicated to subtly build up the effect.

Final adjustments in Lightroom

In Lightroom, I made some final global adjustments (exposure and contract bump, negative clarity, add vibrance, and add some red and blue saturation in Calibration) and added a slight vignette.

And then after coming back to this image after a couple of months, I realized that the color shifts so apparent in the sky exposures also occurred in the foreground images. For some reason, I never saw it before, but now I can't not see it. Since correcting the colors in the base images would require reworking the entire image from scratch, I used the gradient filter tools in Lightroom to address the worst areas. Since the foreground is so dark anyway, the color problems are difficult to see unless you are specifically looking for them.

When compared to my original edit of this image, this revised edit is far superior. Partly this is because I did not take the time to correct the color shifts in the raw exposures in the original edit, but following Wayne Pinkston's recommendations to go easy on the raw adjustments really improves the final image quality. Also, I think this image works better as a 2:1 panorama, and the arch looks more natural to me in the wider format.

Comparison of Final Images: Original (top) and Revised (bottom)

 


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