This is coming out of the discussion in #4254 . Basically, that got merged without a thorough review of the actual transformation methodology (there was code review, but not necessarily a methodology/mathematical correctness review). So, when possible, someone should review that code for the details of how the transformations are implemented. (In particular for #4254, there's a specific question at #4254 (diff) that probably requires someone who's willing to go in and wrap their head around the geometry of the situation.)

More generally, this issue encapsulates my concern that quite a bit of the transformation code in coordinates has not been "conceptually" reviewed. That is, all of the code has been reviewed from a coding style/interface perspective, but not necessarily a review of the mathematical/scientific content.

I realize this is a pretty big ask because the transform code is quite complex and involves a lot of overlapping frames and concepts... But specifically because of that, I think it's quite important that we make use of the hive mind on these topics.

Hopefully someone is willing to volunteer for this - I shouldn't do so because I am the original author of the majority of this stuff. I'm milestoning this for v1.2 mainly just as a way to give whoever might volunteer a deadline: as we all know, deadlines are the best way to get things done!

cc @adrn @mhvk @astrofrog @taldcroft @bmorris3