Source code for astropy.coordinates.builtin_frames.fk5
# Licensed under a 3-clause BSD style license - see LICENSE.rst
import erfa
from astropy.coordinates.attributes import TimeAttribute
from astropy.coordinates.baseframe import base_doc, frame_transform_graph
from astropy.coordinates.transformations import DynamicMatrixTransform
from astropy.utils.decorators import format_doc
from .baseradec import BaseRADecFrame, doc_components
from .utils import EQUINOX_J2000, get_jd12
__all__ = ["FK5"]
doc_footer = """
Other parameters
----------------
equinox : `~astropy.time.Time`
The equinox of this frame.
"""
[docs]
@format_doc(base_doc, components=doc_components, footer=doc_footer)
class FK5(BaseRADecFrame):
"""
A coordinate or frame in the FK5 system.
Note that this is a barycentric version of FK5 - that is, the origin for
this frame is the Solar System Barycenter, *not* the Earth geocenter.
The frame attributes are listed under **Other Parameters**.
"""
equinox = TimeAttribute(default=EQUINOX_J2000, doc="The equinox time")
@staticmethod
def _precession_matrix(oldequinox, newequinox):
"""
Compute and return the precession matrix for FK5 based on Capitaine et
al. 2003/IAU2006. Used inside some of the transformation functions.
Parameters
----------
oldequinox : `~astropy.time.Time`
The equinox to precess from.
newequinox : `~astropy.time.Time`
The equinox to precess to.
Returns
-------
newcoord : array
The precession matrix to transform to the new equinox
References
----------
Hilton, J. et al., 2006, Celest.Mech.Dyn.Astron. 94, 351
"""
# Multiply the two precession matrices (without frame bias) through J2000.0
fromepoch_to_J2000 = erfa.bp06(*get_jd12(oldequinox, "tt"))[1].swapaxes(-2, -1)
J2000_to_toepoch = erfa.bp06(*get_jd12(newequinox, "tt"))[1]
return J2000_to_toepoch @ fromepoch_to_J2000
# This is the "self-transform". Defined at module level because the decorator
# needs a reference to the FK5 class
@frame_transform_graph.transform(DynamicMatrixTransform, FK5, FK5)
def fk5_to_fk5(fk5coord1, fk5frame2):
return fk5coord1._precession_matrix(fk5coord1.equinox, fk5frame2.equinox)
