I recently tried out a new software program for star stacking called Kandao Raw+. Up to this point I had been using Starry Landscape Stacker which has worked very well so I was a bit skeptical as to whether Kandao Raw+ could actually bring anything new to the table. But I was pleasantly surprised and the short summary is that Kandao Raw+ works extremely well and even handles problematic cases which cause difficulty for programs like Starry Landscape Stacker and Sequator.
How does Kandao Raw+ work?
Kandao Raw+ (KR) uses computational photography algorithms similar to what Apple and Google developed for smartphones to increase image quality. This approach combines multiple images taken in rapid succession to reduce noise and increase detail. Kandao Raw+ wasn’t specifcally for designed for star stacking but it is a natural use case.
I don’t know the exact details of the KR algorithm but if it works like other computational algorithms it will do sub image alignment even with irregular motion. There are probably a bunch of other image processing tricks such as handheld super resolution and night sight but you may need to be a scientist in computational imaging to understand what is happening at a detailed level (this is beyond my paygrade).
Blending trees with motion
The hardest example for star stacking is photographing the night sky through trees, vegetation, or other foreground objects that move. This has caused problems for me in Starry Landscape Stacker (SLS) where I have to do a lot of clone stamping to fix the resulting artifacts. However, Kandao Raw+ handles this case like a champ and really impressed me.
For example, here is a test scene taken with a 14-24mm Sigma dg dn lens at 14mm, f/2.8, 13s, ISO 3200. This is a single image although I took a stack of 15 images. There was some wind which caused the branches to move between shots.
Lets look at a few crops:
Crop 1: gaps and movement in tree canopy
Below I’ve taken a crop from the scene where the edge of the tree canopy is visible as well as gaps between the branches. Shown below are the single exposure, Starry Landscape Stacker and Kandao Raw+ images.
Because SLS can’t get the mask perfect to isolate the tree branches, we see a number of anomalies in the stacked image:
- There is streaking in the sky near the edge of the tree canopy where the leaves and branches move.
- Some branches exhibit a bit of ghosting or duplication in the image.
- Stars that would appear between the branches in small openings are removed.
On the other hand, Kandao Raw+ does a great job and looks just like the single exposure except that there’s lower noise and a bit more motion blur (to be expected as stacking is the equivalent of doing a long exposure). The artifacts made by SLS are simply not present.
Crop 2: thin branches
Small thin branches are often problematic for stacking software because they are hard to select with a foreground mask and if they move from shot to shot, the software often doesn’t detect and handle the movement properly.
Shown below are the single exposure, Starry Landscape Stacker and Kandao Raw+ images.
Here SLS removed some branches completely from the scene and sometimes left black smudges where the branches should be. SLS also removed the middle portions of branches leaving detached “orphans” in the air.
Kandao Raw+ shines in this example. Because it can align small segments of the image, the trees branches do not need to be perfectly still nor do they need regular/predictable motion like the stars. As with the previous example, KR also correctly handles the case where stars appear in a small gap between branches.
Crop 3: large stars
Around large stars, I noticed that Kandao Raw can generate some artifacts and it appears to emphasize the noise (instead of reducing it) resulting in a salt & pepper ring around the star. This does not happen with Starry Landscape Stacker.
What are the differences between Kandao Raw+, Starry Landscape Stacker, and Sequator?
From a workflow perspective, here’s how I see the differences:
- The simplest program to use and only requires loading the images.
- Completely automatic and does not require the user to generate a mask for the sky vs ground regions.
- Works on raw files.
- May generate some small artifacts around large stars.
- Available for both mac & windows and is free (download it here).
Starry Landscape Stacker
- One of the first stacking programs available that was relatively easy to use (lots of tutorials available).
- Requires selecting the sky (although it provides some automatic selection algorithms and tools for manual revision).
- Supports dark and flat frames.
- Costs $40 on mac app store (no Windows version).
- The main advantage is that Sequator is available for Windows and is free (no Mac version).
- You must manually select the sky region. Unlike SLS there are no automated tools to help in this process.
I think Kandao Raw+ is very promising for star stacking. It eliminated one of my pain points, stacking stars when there are foreground objects that both move and occlude the sky. This is particularly important to me because I love photographing objects like trees which may sway in the wind.
Because of the simplicity of KR, availablity for mac/windows, and price (free) I suspect that it may become much more popular and perhaps even a standard in landscape astrophotography.
Finally, I’ve just started to use Kandao Raw+ and I probably haven’t fully discovered all of the pros and cons of the software. But as shown in example 3, Kandao Raw+ can generate artifacts around the larger stars. These probably wouldn’t be difficult to correct but you would still need to manually find them and retouch them. I suspect there may be other cases where the software fails – I’ll update this article as I come across any issues.