SkyCoord, Frame and Angles as Dataset¶

SkyCoord and Frame fully support conversion to datasets preserving:

Representation
Differential
Frame
Angle (Longitude and Latitude)

Conversion to dataset supports optionally providing axis (xarray coordinate) labels.

SkyCoordinate representation and differential types for both display and data are detected. In other words, radial_velocity when implicit is not recorded as a data_var.

display differential_type is 'sphericalcoslat', but
data differential_type is 'unitsphericalcoslat'

Convert Dataset to SkyCoord¶

SkyCoord datasets use a frame attribute for converting to a SkyCoordinate.

from astropy_xarray.coordinates import dataset_to_skycoord, load_frame

ds = skycoord_to_dataset(
    sky_position, coords={"field_label": ("field", ["a", "b", "c", "d"])}
)

# frame specific info stored as dataset attribute
assert load_frame(ds.attrs["frame"]) == sky_position.replicate_without_data()

# dataset
result = dataset_to_skycoord(ds)
display(result)
np.testing.assert_array_equal(result.ra, sky_position.ra)
np.testing.assert_array_equal(result.dec, sky_position.dec)
np.testing.assert_array_equal(result.distance, sky_position.distance)

<SkyCoord (ICRS): (ra, dec, distance) in (deg, deg, pc)
    [(2., 4., 4.), (6., 7., 3.), (7., 4., 6.), (4., 3., 4.)]
 (pm_ra_cosdec, pm_dec) in mas / yr
    [(1., 1.), (1., 1.), (1., 1.), (1., 1.)]>

# accessor available
ds.astropy.to_skycoord()

<SkyCoord (ICRS): (ra, dec, distance) in (deg, deg, pc)
    [(2., 4., 4.), (6., 7., 3.), (7., 4., 6.), (4., 3., 4.)]
 (pm_ra_cosdec, pm_dec) in mas / yr
    [(1., 1.), (1., 1.), (1., 1.), (1., 1.)]>

Serialization and Deserialization¶

Using dequantifing, all type metadata can be converted entirely to json-like attributes.

This allows for compatibility with simple serialization implementations (no custom type handling required).

SkyCoordinate Quirks¶

# SkyCoord frame doesn't strongly infer directional reprentation and differential types
import astropy.units as u
from astropy.coordinates import SkyCoord

sky_direction = SkyCoord(
    ra=[2, 6, 7, 4] * u.deg,
    dec=[4, 7, 4, 3] * u.deg,
    pm_ra_cosdec=[1, 1, 1, 1] * u.mas / u.yr,
    pm_dec=[1, 1, 1, 1] * u.mas / u.yr,
    frame="icrs",
)
assert (
    sky_direction.representation_type.name == "spherical"
)  #  may have expected "unitspherical"
assert (
    sky_direction.differential_type.name == "sphericalcoslat"
)  # may have expected "unitsphericalcoslat"

display(sky_direction)

# additional members are therefore implicitly available
display(sky_direction.distance)
display(sky_direction.radial_velocity)

display(sky_direction)

<SkyCoord (ICRS): (ra, dec) in deg
    [(2., 4.), (6., 7.), (7., 4.), (4., 3.)]
 (pm_ra_cosdec, pm_dec) in mas / yr
    [(1., 1.), (1., 1.), (1., 1.), (1., 1.)]>

\[[1,~1,~1,~1] \; \mathrm{}\]

\[[-1.3877788 \times 10^{-17},~2.7755576 \times 10^{-17},~-1.3877788 \times 10^{-17},~-1.3877788 \times 10^{-17}] \; \mathrm{\frac{mas}{rad\,yr}}\]

<SkyCoord (ICRS): (ra, dec) in deg
    [(2., 4.), (6., 7.), (7., 4.), (4., 3.)]
 (pm_ra_cosdec, pm_dec) in mas / yr
    [(1., 1.), (1., 1.), (1., 1.), (1., 1.)]>

sky_position = SkyCoord(
    ra=[2, 6, 7, 4] * u.deg,
    dec=[4, 7, 4, 3] * u.deg,
    distance=[4, 3, 6, 4] * u.parsec,
    pm_ra_cosdec=[1, 1, 1, 1] * u.mas / u.yr,
    pm_dec=[1, 1, 1, 1] * u.mas / u.yr,
    frame="icrs",
)
display(sky_position)
# using positional representation with direction differentials causes accessor of
# `radial_velocity` to mutate data of the skycoordinate
display(sky_position.radial_velocity)
# skycoord_to_dataset will then subsequently include these radial velocity values when this occurs
display(sky_position)

<SkyCoord (ICRS): (ra, dec, distance) in (deg, deg, pc)
    [(2., 4., 4.), (6., 7., 3.), (7., 4., 6.), (4., 3., 4.)]
 (pm_ra_cosdec, pm_dec) in mas / yr
    [(1., 1.), (1., 1.), (1., 1.), (1., 1.)]>

\[[-2.6314897 \times 10^{-19},~0,~-1.3157449 \times 10^{-18},~-2.6314897 \times 10^{-19}] \; \mathrm{\frac{km}{s}}\]

<SkyCoord (ICRS): (ra, dec, distance) in (deg, deg, pc)
    [(2., 4., 4.), (6., 7., 3.), (7., 4., 6.), (4., 3., 4.)]
 (pm_ra_cosdec, pm_dec, radial_velocity) in (mas / yr, mas / yr, km / s)
    [(1., 1., -2.63148973e-19), (1., 1.,  0.00000000e+00),
     (1., 1., -1.31574486e-18), (1., 1., -2.63148973e-19)]>

# `represent_as` can also update cached data
sky_direction = SkyCoord(
    ra=[2, 6, 7, 4] * u.deg,
    dec=[4, 7, 4, 3] * u.deg,
    distance=[1, 1, 1, 1] * u.dimensionless_unscaled,
    pm_ra_cosdec=[1, 1, 1, 1] * u.mas / u.yr,
    pm_dec=[1, 1, 1, 1] * u.mas / u.yr,
    frame="icrs",
)
assert sky_direction.representation_type.name == "spherical"
assert sky_direction.differential_type.name == "sphericalcoslat"
sky_direction_old_str = str(sky_direction)

sky_direction.represent_as("spherical", "sphericalcoslat")  # cache updated
assert sky_direction.representation_type.name == "spherical"
assert sky_direction.differential_type.name == "sphericalcoslat"
assert str(sky_direction) != sky_direction_old_str  # string method changed

# To avoid mutable cache quirks, use explicit repr and diff types for unit sphericals
sky_position = SkyCoord(
    ra=[2, 6, 7, 4] * u.deg,
    dec=[4, 7, 4, 3] * u.deg,
    distance=[1, 1, 1, 1] * u.dimensionless_unscaled,
    pm_ra_cosdec=[1, 1, 1, 1] * u.mas / u.yr,
    pm_dec=[1, 1, 1, 1] * u.mas / u.yr,
    frame=ICRS(
        representation_type="spherical",
        differential_type="unitsphericalcoslat",
    ),
)
sky_position_old_str = str(sky_position)
sky_position.represent_as("spherical", "sphericalcoslat")
assert str(sky_position) == sky_position_old_str

# astropy skycoord frame type casting between directional and positional
# representation may alias data
sky_direction = SkyCoord(
    ra=[2, 6, 7, 4] * u.deg,
    dec=[4, 7, 4, 3] * u.deg,
    pm_ra_cosdec=[1, 1, 1, 1] * u.mas / u.yr,
    pm_dec=[1, 1, 1, 1] * u.mas / u.yr,
    frame="icrs",
)
print("SkyCoord:", sky_direction.ra.data[1])

# astropy-xarray dataset prevents unit conversion loss
ds = skycoord_to_dataset(sky_direction, coords={"field": ["a", "b", "c", "d"]})
print("Dataset:", ds.ra.data[1].value)

SkyCoord: 6.000000000000001
Dataset: 6.0