Add option to project separation onto an ellipse

[eteq]

Right now separation only gives the angle between two points. Another common "separation" in astronomy is the distance projected along a particular axis (e.g. the major axis of a galaxy). This would be particularly useful because of the rather sordid history of various people confusing which way a PA goes ("degrees east of north" I think is the most common convention).

Also useful (at least in galaxy contexts) might be an "elliptical" distance (where the separation along the major and minor axes is weighted by the ellipticity).

This should probably await until some of the significant coordinate internal changes are done, so I'm not targeting a particular milestone.

[balbinot]

Well I need exactly this for my research. I have a working code to do this (all conventions taken to account). I can try to contribute.

Do you think the coordinates API is mature enough to receive this addition?

[eteq]

@balbinot - the general API for defining coordinates will be changing in the upcoming version, but this part of it (separations) probably will not be. So contributing your code is definitely worthwhile - we will then be able to adapt it to the new scheme as we make the changes to the existing separation stuff.

[eteq]

Note that this was partially implemented in #2487, which did the circular/PA version. However, it would still be useful to add the "ellipse" part. So I've renamed this issue appropriately.

I've also tagged this effort-low/package-novice, as it's fairly straightforward either as an extra keyword to position_angle or as a new method to be called something like elliptical_pa.

[smeyerre]

Hi @eteq, I was looking for something I could start on for a "first issue" and I thought I'd try this if it's still open. I just had a clarification question: I assume by "separation along the major and minor axes" you mean the distance projected along those axes. My question is: how should that be 'weighted' by the ellipticity? What's the weight function?

[Himanshu-Garg]

is this still open?
interested in giving it a try