Open source alternative for RegiStar (align, rotate, and combine images into single frame)
I've had success with Hugin. It's a panorama creator, but it can be used to rotate, resize, and relocate images into a coherent whole. It has a few different automated profiles, but sometimes they are a bit off with star stacks. You can manually select the guide stars though.

A really good program for after that is GIMP and the plugin G'MIC to stack and remove noise.

Unfortunately it's not a single package like RegiStar, but it's free and can do quite a lot!

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