FunctionTransformWithFiniteDifference¶

class
astropy.coordinates.
FunctionTransformWithFiniteDifference
(func, fromsys, tosys, priority=1, register_graph=None, finite_difference_frameattr_name='obstime', finite_difference_dt=<Quantity 1. s>, symmetric_finite_difference=True)[source]¶ Bases:
astropy.coordinates.FunctionTransform
A coordinate transformation that works like a
FunctionTransform
, but computes velocity shifts based on the finitedifference relative to one of the frame attributes. Note that the transform function should not change the differential at all in this case, as any differentials will be overridden.When a differential is in the from coordinate, the finite difference calculation has two components. The first part is simple the existing differential, but reorientation (using finitedifference techniques) to point in the direction the velocity vector has in the new frame. The second component is the “induced” velocity. That is, the velocity intrinsic to the frame itself, estimated by shifting the frame using the
finite_difference_frameattr_name
frame attribute a small amount (finite_difference_dt
) in time and recalculating the position.Parameters:  finite_difference_frameattr_name : str or None
The name of the frame attribute on the frames to use for the finite difference. Both the to and the from frame will be checked for this attribute, but only one needs to have it. If None, no velocity component induced from the frame itself will be included  only the reorientation of any existing differential.
 finite_difference_dt :
Quantity
or callable If a quantity, this is the size of the differential used to do the finite difference. If a callable, should accept
(fromcoord, toframe)
and return thedt
value. symmetric_finite_difference : bool
If True, the finite difference is computed as \(\frac{x(t + \Delta t / 2)  x(t + \Delta t / 2)}{\Delta t}\), or if False, \(\frac{x(t + \Delta t)  x(t)}{\Delta t}\). The latter case has slightly better performance (and more stable finite difference behavior).
 All other parameters are identical to the initializer for
 `FunctionTransform`.
Attributes Summary
finite_difference_frameattr_name
Methods Summary
__call__
(self, fromcoord, toframe)Does the actual coordinate transformation from the fromsys
class to thetosys
class.Attributes Documentation

finite_difference_frameattr_name
¶
Methods Documentation

__call__
(self, fromcoord, toframe)[source]¶ Does the actual coordinate transformation from the
fromsys
class to thetosys
class.Parameters:  fromcoord : fromsys object
An object of class matching
fromsys
that is to be transformed. toframe : object
An object that has the attributes necessary to fully specify the frame. That is, it must have attributes with names that match the keys of the dictionary that
tosys.get_frame_attr_names()
returns. Typically this is of classtosys
, but it might be some other class as long as it has the appropriate attributes.
Returns:  tocoord : tosys object
The new coordinate after the transform has been applied.