DeepSkyStacker
This is the github repository for all the DeepSkyStacker source code.
DeepSkyStacker is a freeware created in 2006 by Luc Coiffier
It was open sourced in 2018 and is maintained by a small and dedicated team: David Partidge, Tony Cook and Mat Draper.
Copyright: the source is open, though it doesn't mean that you can create a commercial software from it. That's about the only limitation