@eteq - I see your point, the dictionary is perhaps not the best way after all. Just to be sure I understand where this is going to be used: would this be mostly in the `SkyCoordinate` class, so that it knows what other data are required? If so, would it be an idea to make this a private attribute, `_frame_attr_names`? Or, in other words, does this need to be part of the API, or is it really more a good way to implement it? (Please feel free to ignore this and go with what you have; you have thought about this much more than I have!)