Shortly, all the photos are opened as layers, a rotation point is identified and all the relevant layers are then one by one rotated around that point until the stars in the pictures match.
I am able to open all the photos in layers in GIMP, identify where the rotation point is, but I cannot figure out what tool to use to rotate the layers to the right position. The tool in Photoshop is making the rotating layer transparent so that you can see what is underneath and be able to see when the stars position match. I could not find how to do the same with the rotation tool in GIMP.
Instead of aligning in GIMP, you can align using Hugin, then export the aligned images.
To rotate in GIMP, use the Rotate tool (Shift+r) or the Unified Transform Tool (Shift+t), move the center of rotation onto a star, and then rotate.
Make sure you’re using GIMP-2.10.6 if possible to avoid bugs present in earlier releases.
Here’s a screencast to show how to rotate around a pivot point to align two images using the Rotate tool and the Unified Transform tool. Note: When using the latter, click outside of the canvas to rotate.
You can actually set the % transparency of the layer in the rotation tool, so you can see the layer underneath while rotating.
Another tool for stacking stars is Siril, which specializes in astrophotography: FreeAstro
It is FOSS.
Great, I did not noticed there is a Transparency also in the tool options. I was only trying the layer transparency.
Thanks for the point!
In the meanwhile I tried the suggestion from Morgan to use another software to align, and it also works fine.
So, there are two answers to my question now:
use the transparency option of the rotation tool
use a different software for the alignment.
Both work, there is probably less work with the alignment (if you have a lot of photos) when you use a different tool to align the photos.
Thank you all for the hints. I can now do what I wanted.
