SkyCoord/BaseFrame.represent_as does not treat differentials very logically #11477
Description
Right now, things get transformed a bit too eagerly to having all velocity components present, while really we should not make up numbers:
from astropy.coordinates import SkyCoord
from astropy import units as u
sc = SkyCoord(10*u.deg, 10*u.deg, radial_velocity=-10*u.km/u.s)
sc
<SkyCoord (ICRS): (ra, dec) in deg
(10., 10.)
(radial_velocity) in km / s
(-10.,)>
sc.data.differentials
{'s': <RadialDifferential (d_distance) in km / s
(-10.,)>}
sc.represent_as('spherical')
<SphericalRepresentation (lon, lat, distance) in (rad, rad, )
(0.17453293, 0.17453293, 1.)
(has differentials w.r.t.: 's')>
sc.represent_as('spherical').differentials
{'s': <SphericalDifferential (d_lon, d_lat, d_distance) in (km rad / s, km rad / s, km / s)
(2.22044605e-16, -2.22044605e-16, -10.)>}
Here, there is no reason to move to a SphericalDifferential and set the proper motions to zero - rather, just stick with the RadialDifferential since that is compatible with SphericalRepresentation. Needs some adjustment in BaseFrame.represent_as (and perhaps in _get_repr_classes).
p.s. The current process is blocking #11467.
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