SkyCoord

class astropy.coordinates.SkyCoord(*args, **kwargs)

Bases: object

High-level object providing a flexible interface for celestial coordinate representation, manipulation, and transformation between systems.

The SkyCoord class accepts a wide variety of inputs for initialization. At a minimum these must provide one or more celestial coordinate values with unambiguous units. Typically one also specifies the coordinate frame, though this is not required. The general pattern is for spherical representations is:

SkyCoord(COORD, [FRAME], keyword_args ...)
SkyCoord(LON, LAT, [FRAME], keyword_args ...)
SkyCoord(LON, LAT, [DISTANCE], frame=FRAME, unit=UNIT, keyword_args ...)
SkyCoord([FRAME], <lon_attr>=LON, <lat_attr>=LAT, keyword_args ...)

It is also possible to input coordinate values in other representations such as cartesian or cylindrical. In this case one includes the keyword argument representation='cartesian' (for example) along with data in x, y, and z.

Parameters:

frame : BaseCoordinateFrame class or string, optional

Type of coordinate frame this SkyCoord should represent. Defaults to to ICRS if not given or given as None.

unit : Unit, string, or tuple of Unit or str, optional

Units for supplied LON and LAT values, respectively. If only one unit is supplied then it applies to both LON and LAT.

obstime : valid Time initializer, optional

Time of observation

equinox : valid Time initializer, optional

Coordinate frame equinox

representation : str or Representation class

Specifies the representation, e.g. ‘spherical’, ‘cartesian’, or ‘cylindrical’. This affects the positional args and other keyword args which must correspond to the given representation.

**keyword_args

Other keyword arguments as applicable for user-defined coordinate frames. Common options include:

ra, dec : valid Angle initializer, optional

RA and Dec for frames where ra and dec are keys in the frame’s representation_component_names, including ICRS, FK5, FK4, and FK4NoETerms.

l, b : valid Angle initializer, optional

Galactic l and b for for frames where l and b are keys in the frame’s representation_component_names, including the Galactic frame.

x, y, z : float or Quantity, optional

Cartesian coordinates values

w, u, v : float or Quantity, optional

Cartesian coordinates values for the Galactic frame.

Examples

The examples below illustrate common ways of initializing a SkyCoord object. For a complete description of the allowed syntax see the full coordinates documentation. First some imports:

>>> from astropy.coordinates import SkyCoord  # High-level coordinates
>>> from astropy.coordinates import ICRS, Galactic, FK4, FK5  # Low-level frames
>>> from astropy.coordinates import Angle, Latitude, Longitude  # Angles
>>> import astropy.units as u

The coordinate values and frame specification can now be provided using positional and keyword arguments:

>>> c = SkyCoord(10, 20, unit="deg")  # defaults to ICRS frame
>>> c = SkyCoord([1, 2, 3], [-30, 45, 8], "icrs", unit="deg")  # 3 coords

>>> coords = ["1:12:43.2 +1:12:43", "1 12 43.2 +1 12 43"]
>>> c = SkyCoord(coords, FK4, unit=(u.deg, u.hourangle), obstime="J1992.21")

>>> c = SkyCoord("1h12m43.2s +1d12m43s", Galactic)  # Units from string
>>> c = SkyCoord("galactic", l="1h12m43.2s", b="+1d12m43s")

>>> ra = Longitude([1, 2, 3], unit=u.deg)  # Could also use Angle
>>> dec = np.array([4.5, 5.2, 6.3]) * u.deg  # Astropy Quantity
>>> c = SkyCoord(ra, dec, frame='icrs')
>>> c = SkyCoord(ICRS, ra=ra, dec=dec, obstime='2001-01-02T12:34:56')

>>> c = FK4(1 * u.deg, 2 * u.deg)  # Uses defaults for obstime, equinox
>>> c = SkyCoord(c, obstime='J2010.11', equinox='B1965')  # Override defaults

>>> c = SkyCoord(w=0, u=1, v=2, unit='kpc', frame='galactic', representation='cartesian')

As shown, the frame can be a BaseCoordinateFrame class or the corresponding string alias. The frame classes that are built in to astropy are ICRS, FK5, FK4, FK4NoETerms, and Galactic. The string aliases are simply lower-case versions of the class name, and allow for creating a SkyCoord object and transforming frames without explicitly importing the frame classes.

Attributes Summary

frame
representation

Methods Summary

from_name(name[, frame])	Given a name, query the CDS name resolver to attempt to retrieve coordinate information for that object.
match_to_catalog_3d(catalogcoord[, nthneighbor])	Finds the nearest 3-dimensional matches of this coordinate to a set of catalog coordinates.
match_to_catalog_sky(catalogcoord[, nthneighbor])	Finds the nearest on-sky matches of this coordinate in a set of catalog coordinates.
position_angle(other)	Computes the on-sky position angle (East of North) between this SkyCoord and another.
separation(other)	Computes on-sky separation between this coordinate and another.
separation_3d(other)	Computes three dimensional separation between this coordinate and another.
to_string([style])	A string representation of the coordinates.
transform_to(frame)	Transform this coordinate to a new frame.

Attributes Documentation

frame

representation

Methods Documentation

classmethod from_name(name, frame=u'icrs')

Given a name, query the CDS name resolver to attempt to retrieve coordinate information for that object. The search database, sesame url, and query timeout can be set through configuration items in astropy.coordinates.name_resolve – see docstring for get_icrs_coordinates for more information.

Parameters:

name : str

The name of the object to get coordinates for, e.g. 'M42'.

frame : str or BaseCoordinateFrame class or instance

The frame to transform the object to.

Returns:

coord : SkyCoord

Instance of the SkyCoord class.

match_to_catalog_3d(catalogcoord, nthneighbor=1)

Finds the nearest 3-dimensional matches of this coordinate to a set of catalog coordinates.

This finds the 3-dimensional closest neighbor, which is only different from the on-sky distance if distance is set in this object or the catalogcoord object.

Parameters:

catalogcoord : SkyCoord or BaseCoordinateFrame

The base catalog in which to search for matches. Typically this will be a coordinate object that is an array (i.e., catalogcoord.isscalar == False)

nthneighbor : int, optional

Which closest neighbor to search for. Typically 1 is desired here, as that is correct for matching one set of coordinates to another. The next likely use case is 2, for matching a coordinate catalog against itself (1 is inappropriate because each point will find itself as the closest match).

Returns:

idx : integer array

Indices into catalogcoord to get the matched points for each of this object’s coordinates. Shape matches this object.

sep2d : Angle

The on-sky separation between the closest match for each element in this object in catalogcoord. Shape matches this object.

dist3d : Quantity

The 3D distance between the closest match for each element in this object in catalogcoord. Shape matches this object.

See also

astropy.coordinates.match_coordinates_3d

Notes

This method requires SciPy to be installed or it will fail.

match_to_catalog_sky(catalogcoord, nthneighbor=1)

Finds the nearest on-sky matches of this coordinate in a set of catalog coordinates.

Parameters:

catalogcoord : SkyCoord or BaseCoordinateFrame

The base catalog in which to search for matches. Typically this will be a coordinate object that is an array (i.e., catalogcoord.isscalar == False)

nthneighbor : int, optional

Which closest neighbor to search for. Typically 1 is desired here, as that is correct for matching one set of coordinates to another. The next likely use case is 2, for matching a coordinate catalog against itself (1 is inappropriate because each point will find itself as the closest match).

Returns:

idx : integer array

Indices into catalogcoord to get the matched points for each of this object’s coordinates. Shape matches this object.

sep2d : Angle

The on-sky separation between the closest match for each element in this object in catalogcoord. Shape matches this object.

dist3d : Quantity

The 3D distance between the closest match for each element in this object in catalogcoord. Shape matches this object.

See also

astropy.coordinates.match_coordinates_sky

Notes

This method requires SciPy to be installed or it will fail.

position_angle(other)

Computes the on-sky position angle (East of North) between this SkyCoord and another.

Parameters:

other : SkyCoord

The other coordinate to compute the position angle to. It is treated as the “head” of the vector of the position angle.

Returns:

pa : Angle

The (positive) position angle of the vector pointing from self to other. If either self or other contain arrays, this will be an array following the appropriate numpy broadcasting rules.

Examples

>>> c1 = SkyCoord(0*u.deg, 0*u.deg)
>>> c2 = SkyCoord(1*u.deg, 0*u.deg)
>>> c1.position_angle(c2).degree
90.0
>>> c3 = SkyCoord(1*u.deg, 1*u.deg)
>>> c1.position_angle(c3).degree  
44.995636455344844

separation(other)

Computes on-sky separation between this coordinate and another.

Parameters:

other : SkyCoord or BaseCoordinateFrame

The coordinate to get the separation to.

Returns:

sep : Angle

The on-sky separation between this and the other coordinate.

Notes

The separation is calculated using the Vincenty formula, which is stable at all locations, including poles and antipodes [R4].

[R4]	http://en.wikipedia.org/wiki/Great-circle_distance

separation_3d(other)

Computes three dimensional separation between this coordinate and another.

Parameters:

other : SkyCoord or BaseCoordinateFrame

The coordinate to get the separation to.

Returns:

sep : Distance

The real-space distance between these two coordinates.

Raises:

ValueError

If this or the other coordinate do not have distances.

to_string(style=u'decimal', **kwargs)

A string representation of the coordinates.

The default styles definitions are:

'decimal': 'lat': {'decimal': True, 'unit': "deg"}
           'lon': {'decimal': True, 'unit': "deg"}
'dms': 'lat': {'unit': "deg"}
       'lon': {'unit': "deg"}
'hmsdms': 'lat': {'alwayssign': True, 'pad': True, 'unit': "deg"}
          'lon': {'pad': True, 'unit': "hour"}

See to_string() for details and keyword arguments (the two angles forming the coordinates are are both Angle instances). Keyword arguments have precedence over the style defaults and are passed to to_string().

Parameters:

style : {‘hmsdms’, ‘dms’, ‘decimal’}

The formatting specification to use. These encode the three most common ways to represent coordinates. The default is decimal.

kwargs

Keyword args passed to to_string().

transform_to(frame)

Transform this coordinate to a new frame.

Parameters:

frame : str or BaseCoordinateFrame class / instance or SkyCoord instance

The frame to transform this coordinate into.

Returns:

coord : SkyCoord

A new object with this coordinate represented in the frame frame.

Raises:

ValueError

If there is no possible transformation route.