This is just a tip that might make @keflavich's life easier in the future, and I'm fine with merging without this change.  You might consider using `pytest.mark.parameterize` here (see http://pytest.org/latest/example/parametrize.html).  In a nearly identical test  in `coordinates/tests/test_arrays.py`, I use that in `test_array_coordinates_transformations` to make separate tests for each coordinate, but without having to re-write the tests or iterate over anything.  It just makes the tests slightly easier to write and easier to interpret if they do fail.