In the previous entry, I just stacked 58 photos without any additional thoughts to get a nice simple photo. It is time now to go one step further. What are the steps required to stack astrophotos together?

When I was in South Africa, I got a set of photos from the Milky Way at night, without any landscape, just the sky, and I was wondering about how to get the most of this set. It’s not perfect, I could have gone for a higher focal length with the aperture I had, for more definition and closer sky, but 28mm at f/2, 20s, each image is clear enough and even if the focus could also have been better, it’s a good set to practice what is the best workflow for sky stacking.

The modern workflow

I don’t think there is one and unique workflow, and most free tools like Deep Sky Stacker can do all of them, but I’ve had some issues with them. Basically DSS is not managing to align some photos properly, and I think it’s using some algorithms that are to much for what it should do. Also I like learning new things, so…

With modern cameras, there is no need for the old preprocessing workflow. No need for black images, as the camera does noise reduction, and the dematricers will apply geometry and luminosity corrections. No need for these anymore.

What is needed first is light pollution reduction. I think this is the best place to remove sky pollution because later in the process, the image histogram can be stretched, and light pollution reduction on an aligned stack and a stretched histogram just doesn’t make sense to me due to the non linearities that get introduced.

Once this is done, we can align the images together, and to be fair, this is a complex question. What should align? How to select the best features that need to be aligned? How to do that robustly and with a non linear transformation? How do we keep the ground, the moon and planets fixed?

Then once this is done, there is the question of actually stacking everything. There are different ways of doing it, but it’s basically an average process for the stars and for the fixed elements, we need to use the reference image.

Before assembling the two images, there still need to be a step to stretch the stars histogram and fix any final lighting issue. Then after the two images, stars and fixed elements, can be stitched together.

Conclusion

At first I wanted to present in one article the process to stack the images when there are no fixed elements, but it makes more sense to to smaller articles.

I don’t know how to make a full workflow with fixed elements, but it’s going to be an interesting journey in discovering one. I may end up writing more than Python scripts to stack photos, as I have too many issues with Deep Sky Stacker (one being that my main computer is a Mac), but it will depend on how people feel about my workflow.