SpectralCoord¶

class
astropy.coordinates.
SpectralCoord
(value, unit=None, observer=None, target=None, radial_velocity=None, redshift=None, **kwargs)[source]¶ Bases:
astropy.coordinates.SpectralQuantity
A spectral coordinate with its corresponding unit.
Note
The
SpectralCoord
class is new in Astropy v4.1 and should be considered experimental at this time. Note that we do not fully support cases where the observer and target are moving relativistically relative to each other, so care should be taken in those cases. It is possible that there will be API changes in future versions of Astropy based on user feedback. If you have specific ideas for how it might be improved, please let us know on the astropydev mailing list or at http://feedback.astropy.org. Parameters
 value
ndarray
orQuantity
orSpectralCoord
Spectral values, which should be either wavelength, frequency, energy, wavenumber, or velocity values.
 unitastropy:unitlike
Unit for the given spectral values.
 observer
BaseCoordinateFrame
orSkyCoord
, optional The coordinate (position and velocity) of observer. If no velocities are present on this object, the observer is assumed to be stationary relative to the frame origin.
 target
BaseCoordinateFrame
orSkyCoord
, optional The coordinate (position and velocity) of target. If no velocities are present on this object, the target is assumed to be stationary relative to the frame origin.
 radial_velocity
Quantity
[:ref: ‘speed’], optional The radial velocity of the target with respect to the observer. This can only be specified if
redshift
is not specified. redshift
float
, optional The relativistic redshift of the target with respect to the observer. This can only be specified if
radial_velocity
cannot be specified. doppler_rest
Quantity
, optional The rest value to use when expressing the spectral value as a velocity.
 doppler_convention
str
, optional The Doppler convention to use when expressing the spectral value as a velocity.
 value
Attributes Summary
The coordinates of the observer.
Convert the
SpectralCoord
to aQuantity
.Radial velocity of target relative to the observer.
Redshift of target relative to observer.
The coordinates of the target being observed.
Methods Summary
replicate
([value, unit, observer, target, …])Return a replica of the
SpectralCoord
, optionally changing the values or attributes.to_rest
()Transforms the spectral axis to the rest frame.
A new
SpectralCoord
with the velocity of the observer altered, but not the position.with_radial_velocity_shift
([target_shift, …])Apply a velocity shift to this spectral coordinate.
Attributes Documentation

observer
¶ The coordinates of the observer.
If set, and a target is set as well, this will override any explicit radial velocity passed in.
 Returns
BaseCoordinateFrame
The astropy coordinate frame representing the observation.

radial_velocity
¶ Radial velocity of target relative to the observer.
 Returns
Quantity
[:ref: ‘speed’]Radial velocity of target.
Notes
This is different from the
.radial_velocity
property of a coordinate frame in that this calculates the radial velocity with respect to the observer, not the origin of the frame.

redshift
¶ Redshift of target relative to observer. Calculated from the radial velocity.
 Returns
astropy.units.Quantity
Redshift of target.

target
¶ The coordinates of the target being observed.
If set, and an observer is set as well, this will override any explicit radial velocity passed in.
 Returns
BaseCoordinateFrame
The astropy coordinate frame representing the target.
Methods Documentation

replicate
(value=None, unit=None, observer=None, target=None, radial_velocity=None, redshift=None, doppler_convention=None, doppler_rest=None, copy=False)[source]¶ Return a replica of the
SpectralCoord
, optionally changing the values or attributes.Note that no conversion is carried out by this method  this keeps all the values and attributes the same, except for the ones explicitly passed to this method which are changed.
If
copy
is set toTrue
then a full copy of the internal arrays will be made. By default the replica will use a reference to the original arrays when possible to save memory. Parameters
 value
ndarray
orQuantity
orSpectralCoord
, optional Spectral values, which should be either wavelength, frequency, energy, wavenumber, or velocity values.
 unitastropy:unitlike
Unit for the given spectral values.
 observer
BaseCoordinateFrame
orSkyCoord
, optional The coordinate (position and velocity) of observer.
 target
BaseCoordinateFrame
orSkyCoord
, optional The coordinate (position and velocity) of target.
 radial_velocity
Quantity
[:ref: ‘speed’], optional The radial velocity of the target with respect to the observer.
 redshift
float
, optional The relativistic redshift of the target with respect to the observer.
 doppler_rest
Quantity
, optional The rest value to use when expressing the spectral value as a velocity.
 doppler_convention
str
, optional The Doppler convention to use when expressing the spectral value as a velocity.
 copybool, optional
If
True
, andvalue
is not specified, the values are copied to the newSkyCoord
 otherwise a reference to the same values is used.
 value
 Returns
 sc
SpectralCoord
object
Replica of this object
 sc

with_observer_stationary_relative_to
(frame, velocity=None, preserve_observer_frame=False)[source]¶ A new
SpectralCoord
with the velocity of the observer altered, but not the position.If a coordinate frame is specified, the observer velocities will be modified to be stationary in the specified frame. If a coordinate instance is specified, optionally with nonzero velocities, the observer velocities will be updated so that the observer is comoving with the specified coordinates.
 Parameters
 frame
str
,BaseCoordinateFrame
orSkyCoord
The observation frame in which the observer will be stationary. This can be the name of a frame (e.g. ‘icrs’), a frame class, frame instance with no data, or instance with data. This can optionally include velocities.
 velocity
Quantity
orCartesianDifferential
, optional If
frame
does not contain velocities, these can be specified as a 3elementQuantity
. In the case where this is also not specified, the velocities default to zero. preserve_observer_framebool
If
True
, the final observer frame class will be the same as the original one, and ifFalse
it will be the frame of the velocity reference class.
 frame
 Returns
 new_coord
SpectralCoord
The new coordinate object representing the spectral data transformed based on the observer’s new velocity frame.
 new_coord

with_radial_velocity_shift
(target_shift=None, observer_shift=None)[source]¶ Apply a velocity shift to this spectral coordinate.
The shift can be provided as a redshift (float value) or radial velocity (
Quantity
with physical type of ‘speed’). Parameters
 target_shift
float
orQuantity
[:ref: ‘speed’] Shift value to apply to current target.
 observer_shift
float
orQuantity
[:ref: ‘speed’] Shift value to apply to current observer.
 target_shift
 Returns
SpectralCoord
New spectral coordinate with the target/observer velocity changed to incorporate the shift. This is always a new object even if
target_shift
andobserver_shift
are bothNone
.