Using and Designing Coordinate Frames

In astropy.coordinates, as outlined in the Overview of astropy.coordinates Concepts, subclasses of BaseCoordinateFrame (“frame classes”) define particular coordinate frames. They can (but do not have to) contain representation objects storing the actual coordinate data. The actual coordinate transformations are defined as functions that transform representations between frame classes. This approach serves to separate high-level user functionality (see Using the SkyCoord High-Level Class) and details of how the coordinates are actually stored (see Using and Designing Coordinate Representations) from the definition of frames and how they are transformed.

Using Frame Objects

Frames without Data

Frame objects have two distinct (but related) uses. The first is storing the information needed to uniquely define a frame (e.g., equinox, observation time). This information is stored on the frame objects as (read-only) Python attributes, which are set when the object is first created:

>>> from astropy.coordinates import ICRS, FK5
>>> FK5(equinox='J1975')
<FK5 Frame (equinox=J1975.000)>
>>> ICRS()  # has no attributes
<ICRS Frame>
>>> FK5()  # uses default equinox
<FK5 Frame (equinox=J2000.000)>

The specific names of attributes available for a particular frame (and their default values) are available as the class method get_frame_attr_names:

>>> FK5.get_frame_attr_names()
OrderedDict([('equinox', <Time object: scale='tt' format='jyear_str' value=J2000.000>)])

You can access any of the attributes on a frame by using standard Python attribute access. Note that for cases like equinox, which are time inputs, if you pass in any unambiguous time string, it will be converted into an Time object (see Inferring input format):

>>> f = FK5(equinox='J1975')
>>> f.equinox
<Time object: scale='tt' format='jyear_str' value=J1975.000>
>>> f = FK5(equinox='2011-05-15T12:13:14')
>>> f.equinox
<Time object: scale='utc' format='isot' value=2011-05-15T12:13:14.000>

Frames with Data

The second use for frame objects is to store actual realized coordinate data for frames like those described above. In this use, it is similar to the SkyCoord class, and in fact, the SkyCoord class internally uses the frame classes as its implementation. However, the frame classes have fewer “convenience” features, thereby streamlining the implementation of frame classes. As such, they are created similarly to SkyCoord objects. One suggested way is to use with keywords appropriate for the frame (e.g., ra and dec for equatorial systems):

>>> from astropy import units as u
>>> ICRS(ra=1.1*u.deg, dec=2.2*u.deg)  
<ICRS Coordinate: (ra, dec) in deg
    (1.1, 2.2)>
>>> FK5(ra=1.1*u.deg, dec=2.2*u.deg, equinox='J1975')  
<FK5 Coordinate (equinox=J1975.000): (ra, dec) in deg
    (1.1, 2.2)>

These same attributes can be used to access the data in the frames as Angle objects (or Angle subclasses):

>>> coo = ICRS(ra=1.1*u.deg, dec=2.2*u.deg)
>>> coo.ra  
<Longitude 1.1 deg>
>>> coo.ra.value  
<Longitude 0.07333333 hourangle>

You can use the representation_type attribute in conjunction with the representation_component_names attribute to figure out what keywords are accepted by a particular class object. The former will be the representation class in which the system is expressed (e.g., spherical for equatorial frames), and the latter will be a dictionary mapping names for that frame to the attribute name on the representation class:

>>> import astropy.units as u
>>> icrs = ICRS(1*u.deg, 2*u.deg)
>>> icrs.representation_type
<class 'astropy.coordinates.representation.SphericalRepresentation'>
>>> icrs.representation_component_names
OrderedDict([('ra', 'lon'), ('dec', 'lat'), ('distance', 'distance')])

You can get the data in a different representation if needed:

>>> icrs.represent_as('cartesian')  
<CartesianRepresentation (x, y, z) [dimensionless]
     (0.99923861, 0.01744177, 0.0348995)>


In previous versions of Astropy, both the frame attribute and the argument to frame classes that are now named representation_type used to be simply representation. The name of this attribute/argument is confusing as it points to the representation class, not the object containing the underlying frame data (which is accessed via the frame attribute .data). To clarify, we have renamed representation to representation_type. In this version 3.0, we have only changed the references to this attribute in the documentation. In the next major version, we will issue a deprecation warning. In two major versions, we will remove the .representation attribute and representation= argument.

The representation of the coordinate object can also be changed directly, as shown below. This does nothing to the object internal data which stores the coordinate values, but it changes the external view of that data in two ways: (1) the object prints itself in accord with the new representation, and (2) the available attributes change to match those of the new representation (e.g., from ra, dec, distance to x, y, z). Setting the representation_type thus changes a property of the object (how it appears) without changing the intrinsic object itself which represents a point in 3D space.:

>>> from astropy.coordinates import CartesianRepresentation
>>> icrs.representation_type = CartesianRepresentation
>>> icrs  
<ICRS Coordinate: (x, y, z) [dimensionless]
    (0.99923861, 0.01744177, 0.0348995)>
>>> icrs.x  
<Quantity 0.99923861>

The representation can also be set at the time of creating a coordinate and affects the set of keywords used to supply the coordinate data. For example, to create a coordinate with Cartesian data do:

>>> ICRS(x=1*u.kpc, y=2*u.kpc, z=3*u.kpc, representation_type='cartesian')  
<ICRS Coordinate: (x, y, z) in kpc
    (1., 2., 3.)>

For more information about the use of representations in coordinates see the Representations section, and for details about the representations themselves see Using and Designing Coordinate Representations.

There are two other ways to create frame classes with coordinates. A representation class can be passed in directly at creation, along with any frame attributes required:

>>> from astropy.coordinates import SphericalRepresentation
>>> rep = SphericalRepresentation(lon=1.1*u.deg, lat=2.2*u.deg, distance=3.3*u.kpc)
>>> FK5(rep, equinox='J1975')  
<FK5 Coordinate (equinox=J1975.000): (ra, dec, distance) in (deg, deg, kpc)
    (1.1, 2.2, 3.3)>

A final way is to create a frame object from an already existing frame (either one with or without data), using the realize_frame method. This will yield a frame with the same attributes, but new data:

>>> f1 = FK5(equinox='J1975')
>>> f1
<FK5 Frame (equinox=J1975.000)>
>>> rep = SphericalRepresentation(lon=1.1*u.deg, lat=2.2*u.deg, distance=3.3*u.kpc)
>>> f1.realize_frame(rep)  
<FK5 Coordinate (equinox=J1975.000): (ra, dec, distance) in (deg, deg, kpc)
    (1.1, 2.2, 3.3)>

You can check if a frame object has data using the has_data attribute, and if it is present, it can be accessed from the data attribute:

>>> ICRS().has_data
>>> cooi = ICRS(ra=1.1*u.deg, dec=2.2*u.deg)
>>> cooi.has_data
<UnitSphericalRepresentation (lon, lat) in deg
    (1.1, 2.2)>

All of the above methods can also accept array data (in the form of class:Quantity, or other Python sequences) to create arrays of coordinates:

>>> ICRS(ra=[1.5, 2.5]*u.deg, dec=[3.5, 4.5]*u.deg)  
<ICRS Coordinate: (ra, dec) in deg
    [(1.5, 3.5), (2.5, 4.5)]>

If you pass in mixed arrays and scalars, the arrays will be broadcast over the scalars appropriately:

>>> ICRS(ra=[1.5, 2.5]*u.deg, dec=[3.5, 4.5]*u.deg, distance=5*u.kpc)  
<ICRS Coordinate: (ra, dec, distance) in (deg, deg, kpc)
    [(1.5, 3.5, 5.), (2.5, 4.5, 5.)]>

Similar broadcasting happens if you transform to another frame. For example:

>>> import numpy as np
>>> from astropy.coordinates import EarthLocation, AltAz
>>> coo = ICRS(ra=180.*u.deg, dec=51.477811*u.deg)
>>> lf = AltAz(location=EarthLocation.of_site('greenwich'),
...            obstime=['2012-03-21T00:00:00', '2012-06-21T00:00:00'])
>>> lcoo = coo.transform_to(lf)  # this can load finals2000A.all 
>>> lcoo  
<AltAz Coordinate (obstime=['2012-03-21T00:00:00.000' '2012-06-21T00:00:00.000'], location=(3980608.9024681724, -102.47522910648239, 4966861.273100675) m, pressure=0.0 hPa, temperature=0.0 deg_C, relative_humidity=0.0, obswl=1.0 micron): (az, alt) in deg
    [( 94.71264944, 89.21424252), (307.69488825, 37.98077771)]>

Above, the shapes — () for coo and (2,) for lf — were broadcast against each other. If you wish to determine the positions for a set of coordinates, you will need to make sure that the shapes allow this:

>>> coo2 = ICRS(ra=[180., 225., 270.]*u.deg, dec=[51.5, 0., 51.5]*u.deg)
>>> coo2.transform_to(lf)
Traceback (most recent call last):
ValueError: operands could not be broadcast together...
>>> coo2.shape
>>> lf.shape
>>> lf2 = lf[:, np.newaxis]
>>> lf2.shape
(2, 1)
>>> coo2.transform_to(lf2)  
<AltAz Coordinate (obstime=[['2012-03-21T00:00:00.000' '2012-03-21T00:00:00.000'
 ['2012-06-21T00:00:00.000' '2012-06-21T00:00:00.000'
  '2012-06-21T00:00:00.000']], location=(3980608.9024681724, -102.47522910648239, 4966861.273100675) m, pressure=0.0 hPa, temperature=0.0 deg_C, relative_humidity=0.0, obswl=1.0 micron): (az, alt) in deg
    [[( 93.09845202, 89.21613119), (126.85789652, 25.46600543),
      ( 51.37993229, 37.18532521)],
     [(307.71713699, 37.99437658), (231.37407871, 26.36768329),
      ( 85.42187335, 89.69297997)]]>


Frames without data have a shape that is determined by their frame attributes. For frames with data, the shape always is that of the data; any non-scalar attributes are broadcast to have matching shapes (as can be seen for obstime in the last line above).

Transforming between Frames

To transform a frame object with data into another frame, use the transform_to method of an object, and provide it the frame you wish to transform to. This frame can either be a frame class, in which case the default attributes will be used, or a frame object (with or without data):

>>> cooi = ICRS(1.5*u.deg, 2.5*u.deg)
>>> cooi.transform_to(FK5)  
<FK5 Coordinate (equinox=J2000.000): (ra, dec) in deg
    (1.50000661, 2.50000238)>
>>> cooi.transform_to(FK5(equinox='J1975'))  
<FK5 Coordinate (equinox=J1975.000): (ra, dec) in deg
    (1.17960348, 2.36085321)>

The Reference/API includes a list of all of the frames built into astropy.coordinates, as well as the defined transformations between them. Any transformation that has a valid path, even if it passes through other frames, can be transformed too. To programmatically check for or manipulate transformations, see the TransformGraph documentation.

Defining a New Frame

Users can add new coordinate frames by creating new classes that are subclasses of BaseCoordinateFrame. Detailed instructions for subclassing are in the docstrings for that class. The key aspects are to define the class attributes default_representation and frame_specific_representation_info along with frame attributes as Attribute class instances (or subclasses like TimeAttribute). If these are defined, there is often no need to define an __init__ function, as the initializer in BaseCoordinateFrame will probably behave in the way you want. As an example:

>>> from astropy.coordinates import BaseCoordinateFrame, Attribute, TimeAttribute, RepresentationMapping
>>> import astropy.coordinates.representation as r
>>> class MyFrame(BaseCoordinateFrame):
...     # Specify how coordinate values are represented when outputted
...      default_representation = r.SphericalRepresentation
...      # Specify overrides to the default names and units for all available
...      # representations (subclasses of BaseRepresentation).
...      frame_specific_representation_info = {
...          r.SphericalRepresentation: [RepresentationMapping(reprname='lon', framename='R', defaultunit=u.rad),
...                                      RepresentationMapping(reprname='lat', framename='D', defaultunit=u.rad),
...                                      RepresentationMapping(reprname='distance', framename='DIST', defaultunit=None)],
...          r.UnitSphericalRepresentation: [RepresentationMapping(reprname='lon', framename='R', defaultunit=u.rad),
...                                          RepresentationMapping(reprname='lat', framename='D', defaultunit=u.rad)],
...          r.CartesianRepresentation: [RepresentationMapping(reprname='x', framename='X'),
...                                      RepresentationMapping(reprname='y', framename='Y'),
...                                      RepresentationMapping(reprname='z', framename='Z')]
...      }
...      # Specify frame attributes required to fully specify the frame
...      location = Attribute(default=None)
...      equinox = TimeAttribute(default='B1950')
...      obstime = TimeAttribute(default=None, secondary_attribute='equinox')

>>> c = MyFrame(R=10*u.deg, D=20*u.deg)
>>> c  
<MyFrame Coordinate (location=None, equinox=B1950.000, obstime=B1950.000): (R, D) in rad
    (0.17453293, 0.34906585)>
>>> c.equinox
<Time object: scale='tt' format='byear_str' value=B1950.000>

If you also want to support velocity data in your coordinate frame, see the velocities documentation at Creating Frame Objects with Velocity Data.

You can also define arbitrary methods for any added functionality you want your frame to have that is unique to that frame. These methods will be available in any SkyCoord that is created using your user-defined frame.

For examples of defining frame classes, the first place to look is at the source code for the frames that are included in astropy (available at astropy.coordinates.builtin_frames). These are not “magic” in any way, and use all of the same API and features available to user-created frames.


See also Create a new coordinate class (for the Sagittarius stream) for a more annotated example of defining a new coordinate frame.

Customizing Display of Attributes

While the default repr for coordinate frames is suitable for most cases, you may want to customize how frame attributes are displayed in certain cases. To do this you can define a method named _astropy_repr_in_frame. This method should be defined on the object that is set to the frame attribute itself, not the Attribute descriptor.

For example, you could have an object Spam which you have as an attribute of your frame:

>>> class Spam:
...     def _astropy_repr_in_frame(self):
...         return "<A can of Spam>"

If your frame has this class as an attribute:

>>> class Egg(BaseCoordinateFrame):
...     can = Attribute(default=Spam())

When it is displayed by the frame it will use the result of _astropy_repr_in_frame:

>>> Egg()
<Egg Frame (can=<A can of Spam>)>

Defining Transformations

A frame may not be too useful without a way to transform coordinates defined within it to or from other frames. Fortunately, astropy.coordinates provides a framework to do just that. The key concept for these transformations is the frame transform graph, available as astropy.coordinates.frame_transform_graph, an instance of the TransformGraph class. This graph (in the “graph theory” sense, not “plot”), stores all the transformations between all of the built-in frames, as well as tools for finding shortest paths through this graph to transform from any frame to any other. All of the power of this graph is available to user-created frames as well, meaning that once you define even one transform from your frame to some frame in the graph, coordinates defined in your frame can be transformed to any other frame in the graph.

The transforms themselves are represented as CoordinateTransform objects or their subclasses. The useful subclasses/types of transformations are:

  • FunctionTransform

    A transform that is defined as a function that takes a frame object of one frame class and returns an object of another class.

  • AffineTransform

    A transformation that includes a linear matrix operation and a translation (vector offset). These transformations are defined by a 3x3 matrix and a 3-vector for the offset (supplied as a Cartesian representation). The transformation is applied to the Cartesian representation of one frame and transforms into the Cartesian representation of the target frame.

  • StaticMatrixTransform

  • DynamicMatrixTransform

    The matrix transforms are AffineTransform transformations without a translation (i.e., a rotation). The static version is for the case where the matrix is independent of the frame attributes (e.g., the ICRS->FK5 transformation, because ICRS has no frame attributes). The dynamic case is for transformations where the transformation matrix depends on the frame attributes of either the to or from frame.

Generally, it is not necessary to use these classes directly. Instead, use methods on frame_transform_graph that can be used as function decorators. Define functions that either do the actual transformation (for FunctionTransform), or that compute the necessary transformation matrices to transform. Then decorate the functions to register these transformations with the frame transform graph:

from astropy.coordinates import frame_transform_graph

@frame_transform_graph.transform(DynamicMatrixTransform, ICRS, FK5)
def icrs_to_fk5(icrscoord, fk5frame):

@frame_transform_graph.transform(DynamicMatrixTransform, FK5, ICRS)
def fk5_to_icrs(fk5coord, icrsframe):

If the transformation to your coordinate frame of interest is not representable by a matrix operation, you can also specify a function to do the actual transformation, and pass the FunctionTransform class to the transform graph decorator instead:

@frame_transform_graph.transform(FunctionTransform, FK4NoETerms, FK4)
def fk4_no_e_to_fk4(fk4noecoord, fk4frame):

Furthermore, the frame_transform_graph does some caching and optimization to speed up transformations after the first attempt to go from one frame to another, and shortcuts steps where relevant (for example, combining multiple static matrix transforms into a single matrix). Hence, in general, it is better to define whatever are the most natural transformations for a user-defined frame, rather than worrying about optimizing or caching a transformation to speed up the process.

For a demonstration of how to define transformation functions that also work for transforming velocity components, see Transforming Frames with Velocities.