The unit module has machinery for supporting equivalences between different units in certain contexts, namely when equations can uniquely relate a value in one unit to a different unit. A good example is the equivalence between wavelength, frequency, and energy for specifying a wavelength of radiation. Normally these units are not convertible, but when understood as representing light, they are convertible in certain contexts. Here we describe how to use the equivalencies included in astropy.units and how to define new equivalencies.

Equivalencies are used by passing a list of equivalency pairs to the equivalencies keyword argument of Quantity.to() Unit.to() or Unit.get_converter() methods. The list can be supplied directly, but astropy contains several functions that return appropriate lists so constructing them is often not necessary. Alternatively, if a larger piece of code needs the same equivalencies, you can set them for a given context.

Built-In Equivalencies#

How to Convert Parallax to Distance#

The length unit parsec is defined such that a star one parsec away will exhibit a 1-arcsecond parallax. (Think of the name as a contraction between parallax and arcsecond.)

The parallax() function handles conversions between parallax angles and length.

In general, you should not be able to change units of length into angles or vice versa, so to() raises an exception:

>>> from astropy import units as u
>>> (0.8 * u.arcsec).to(u.parsec)  
Traceback (most recent call last):
UnitConversionError: 'arcsec' (angle) and 'pc' (length) are not convertible

To trigger the conversion between parallax angle and distance, provide parallax() as the optional keyword argument (equivalencies=) to the to() method.

>>> (0.8 * u.arcsec).to(u.parsec, equivalencies=u.parallax())
<Quantity 1.25 pc>

Angles as Dimensionless Units#

Angles are treated as a physically distinct type, which usually helps to avoid mistakes. However, this is not very handy when working with units related to rotational energy or the small angle approximation. (Indeed, this double-sidedness underlies why radians went from a supplementary to derived unit.) The function dimensionless_angles() provides the required equivalency list that helps convert between angles and dimensionless units. It is somewhat different from all others in that it allows an arbitrary change in the number of powers to which radians is raised (i.e., including zero and thus dimensionless).


Normally the following would raise exceptions:

>>> u.degree.to('')  
Traceback (most recent call last):
UnitConversionError: 'deg' (angle) and '' (dimensionless) are not convertible
>>> (u.kg * u.m**2 * (u.cycle / u.s)**2).to(u.J)  
Traceback (most recent call last):
UnitConversionError: 'cycle2 kg m2 / s2' and 'J' (energy) are not convertible

But when passing the proper conversion function, dimensionless_angles(), it works.

>>> u.deg.to('', equivalencies=u.dimensionless_angles())  
>>> (0.5e38 * u.kg * u.m**2 * (u.cycle / u.s)**2).to(u.J,
...                            equivalencies=u.dimensionless_angles())  
<Quantity 1.9739208802178715e+39 J>
>>> import numpy as np
>>> np.exp((1j*0.125*u.cycle).to('', equivalencies=u.dimensionless_angles())) 
<Quantity  0.70710678+0.70710678j>

In an example with complex numbers you may well be doing a fair number of similar calculations. For such situations, there is the option to set default equivalencies.

In some situations, this equivalency may behave differently than anticipated. For instance, it might at first seem reasonable to use it for converting from an angular velocity \(\omega\) in radians per second to the corresponding frequency \(f\) in hertz (i.e., to implement \(f=\omega/2\pi\)). However, attempting this yields:

>>> (1*u.rad/u.s).to(u.Hz, equivalencies=u.dimensionless_angles())  
<Quantity 1. Hz>
>>> (1*u.cycle/u.s).to(u.Hz, equivalencies=u.dimensionless_angles())  
<Quantity 6.283185307179586 Hz>

Here, we might have expected ~0.159 Hz in the first example and 1 Hz in the second. However, dimensionless_angles() converts to radians per second and then drops radians as a unit. The implicit mistake made in these examples is that the unit Hz is taken to be equivalent to cycles per second, which it is not (it is just “per second”). This realization also leads to the solution: to use an explicit equivalency between cycles per second and hertz:

>>> (1*u.rad/u.s).to(u.Hz, equivalencies=[(u.cy/u.s, u.Hz)])  
<Quantity 0.15915494309189535 Hz>
>>> (1*u.cy/u.s).to(u.Hz, equivalencies=[(u.cy/u.s, u.Hz)])  
<Quantity 1. Hz>

Spectral Units#

spectral() is a function that returns an equivalency list to handle conversions between wavelength, frequency, energy, and wave number.

As mentioned with parallax units, we pass a list of equivalencies (in this case, the result of spectral()) as the second argument to the to() method and wavelength, and then frequency and energy can be converted.

>>> ([1000, 2000] * u.nm).to(u.Hz, equivalencies=u.spectral())  
<Quantity [2.99792458e+14, 1.49896229e+14] Hz>
>>> ([1000, 2000] * u.nm).to(u.eV, equivalencies=u.spectral())  
<Quantity [1.23984193, 0.61992096] eV>

These equivalencies even work with non-base units:

>>> # Inches to calories
>>> from astropy.units import imperial
>>> imperial.inch.to(imperial.Cal, equivalencies=u.spectral())  

Spectral (Doppler) Equivalencies#

Spectral equivalencies allow you to convert between wavelength, frequency, energy, and wave number, but not to velocity, which is frequently the quantity of interest.

It is fairly convenient to define the equivalency, but note that there are different conventions. In these conventions \(f_0\) is the rest frequency, \(f\) is the observed frequency, \(V\) is the velocity, and \(c\) is the speed of light:

  • Radio \(V = c \frac{f_0 - f}{f_0} ; f(V) = f_0 ( 1 - V/c )\)

  • Optical \(V = c \frac{f_0 - f}{f } ; f(V) = f_0 ( 1 + V/c )^{-1}\)

  • Relativistic \(V = c \frac{f_0^2 - f^2}{f_0^2 + f^2} ; f(V) = f_0 \frac{\left(1 - (V/c)^2\right)^{1/2}}{(1+V/c)}\)

These three conventions are implemented as doppler_optical(), doppler_radio(), and doppler_relativistic().


To define an equivalency:

>>> restfreq = 115.27120 * u.GHz  # rest frequency of 12 CO 1-0 in GHz
>>> freq_to_vel = u.doppler_radio(restfreq)
>>> (116e9 * u.Hz).to(u.km / u.s, equivalencies=freq_to_vel)  
<Quantity -1895.4321928669085 km / s>

Spectral Flux and Luminosity Density Units#

There is also support for spectral flux and luminosity density units, their equivalent surface brightness units, and integrated flux units. Their use is more complex, since it is necessary to also supply the location in the spectrum for which the conversions will be done, and the units of those spectral locations. The function that handles these unit conversions is spectral_density(). This function takes as its arguments the Quantity for the spectral location.


To perform unit conversions with spectral_density():

>>> (1.5 * u.Jy).to(u.photon / u.cm**2 / u.s / u.Hz,
...                 equivalencies=u.spectral_density(3500 * u.AA)) 
<Quantity 2.6429112e-12 ph / (Hz s cm2)>
>>> (1.5 * u.Jy).to(u.photon / u.cm**2 / u.s / u.micron,
...                 equivalencies=u.spectral_density(3500 * u.AA))  
<Quantity 6467.95791275 ph / (micron s cm2)>
>>> a = 1. * (u.photon / u.s / u.angstrom)
>>> a.to(u.erg / u.s / u.Hz,
...      equivalencies=u.spectral_density(5500 * u.AA))  
<Quantity 3.6443382634999996e-23 erg / (Hz s)>
>>> w = 5000 * u.AA
>>> a = 1. * (u.erg / u.cm**2 / u.s)
>>> b = a.to(u.photon / u.cm**2 / u.s, u.spectral_density(w))
>>> b  
<Quantity 2.51705828e+11 ph / (s cm2)>
>>> b.to(a.unit, u.spectral_density(w))  
<Quantity 1. erg / (s cm2)>

Brightness Temperature and Surface Brightness Equivalency#

There is an equivalency between surface brightness (flux density per area) and brightness temperature. This equivalency is often referred to as “Antenna Gain” since, at a given frequency, telescope brightness sensitivity is unrelated to aperture size, but flux density sensitivity is, so this equivalency is only dependent on the aperture size. See Tools of Radio Astronomy for details.


The brightness temperature mentioned here is the Rayleigh-Jeans equivalent temperature, which results in a linear relation between flux and temperature. This is the convention that is most often used in relation to observations, but if you are interested in computing the exact temperature of a blackbody function that would produce a given flux, you should not use this equivalency.


The brightness_temperature() equivalency requires the beam area and frequency as arguments. Recalling that the area of a 2D Gaussian is \(2 \pi \sigma^2\) (see wikipedia), here is an example:

>>> beam_sigma = 50*u.arcsec
>>> omega_B = 2 * np.pi * beam_sigma**2
>>> freq = 5 * u.GHz
>>> (1*u.Jy/omega_B).to(u.K, equivalencies=u.brightness_temperature(freq))  
<Quantity 3.526295144567176 K>

If you have beam full-width half-maxima (FWHM), which are often quoted and are the values stored in the FITS header keywords BMAJ and BMIN, a more appropriate example converts the FWHM to sigma:

>>> beam_fwhm = 50*u.arcsec
>>> fwhm_to_sigma = 1. / (8 * np.log(2))**0.5
>>> beam_sigma = beam_fwhm * fwhm_to_sigma
>>> omega_B = 2 * np.pi * beam_sigma**2
>>> (1*u.Jy/omega_B).to(u.K, equivalencies=u.brightness_temperature(freq))  
<Quantity 19.553932298231704 K>

You can also convert between Jy/beam and K by specifying the beam area:

>>> (1*u.Jy/u.beam).to(u.K, u.brightness_temperature(freq, beam_area=omega_B))  
<Quantity 19.553932298231704 K>

Beam Equivalency#

Radio data, especially from interferometers, is often produced in units of Jy/beam. Converting this number to a beam-independent value (e.g., Jy/sr), can be done with the beam_angular_area() equivalency.


To convert units of Jy/beam to Jy/sr:

>>> beam_fwhm = 50*u.arcsec
>>> fwhm_to_sigma = 1. / (8 * np.log(2))**0.5
>>> beam_sigma = beam_fwhm * fwhm_to_sigma
>>> omega_B = 2 * np.pi * beam_sigma**2
>>> (1*u.Jy/u.beam).to(u.MJy/u.sr, equivalencies=u.beam_angular_area(omega_B))  
<Quantity 15.019166691021288 MJy / sr>

Note that the radio_beam package deals with beam input/output and various operations more directly.

Temperature Energy Equivalency#

The temperature_energy() equivalency allows conversion between temperature and its equivalent in energy (i.e., the temperature multiplied by the Boltzmann constant), usually expressed in electronvolts. This is used frequently for observations at high-energy, be it for solar or X-ray astronomy.


To convert between temperature and its equivalent in energy:

>>> t_k = 1e6 * u.K
>>> t_k.to(u.eV, equivalencies=u.temperature_energy())  
<Quantity 86.17332384960955 eV>

Thermodynamic Temperature Equivalency#

This thermodynamic_temperature() equivalency allows conversion between Jy/beam and “thermodynamic temperature”, \(T_{CMB}\), in Kelvins.


To convert between Jy/beam and thermodynamic temperature:

>>> nu = 143 * u.GHz
>>> t_k = 0.002632051878 * u.K
>>> t_k.to(u.MJy / u.sr, equivalencies=u.thermodynamic_temperature(nu))  
<Quantity 1. MJy / sr>

By default, this will use the \(T_{CMB}\) value for the default cosmology in astropy, but it is possible to specify a custom \(T_{CMB}\) value for a specific cosmology as the second argument to the equivalency:

>>> from astropy.cosmology import WMAP9
>>> t_k.to(u.MJy / u.sr, equivalencies=u.thermodynamic_temperature(nu, T_cmb=WMAP9.Tcmb0))  
<Quantity 0.99982392 MJy / sr>

Molar Mass AMU Equivalency#

The molar_mass_amu() equivalency allows conversion between the atomic mass unit and the equivalent g/mol. For context, refer to the NIST definition of SI Base Units.


To convert between atomic mass unit and the equivalent g/mol:

>>> x = 1 * (u.g / u.mol)
>>> y = 1 * u.u
>>> x.to(u.u, equivalencies=u.molar_mass_amu()) 
<Quantity 1.0 u>
>>> y.to(u.g/u.mol, equivalencies=u.molar_mass_amu()) 
<Quantity 1.0 g / mol>

Pixel and Plate Scale Equivalencies#

These equivalencies are for converting between angular scales and either linear scales in the focal plane or distances in units of the number of pixels.


Suppose you are working with cutouts from the Sloan Digital Sky Survey, which defaults to a pixel scale of 0.4 arcseconds per pixel, and want to know the true size of something that you measure to be 240 pixels across in the cutout image:

>>> sdss_pixelscale = u.pixel_scale(0.4*u.arcsec/u.pixel)
>>> (240*u.pixel).to(u.arcmin, sdss_pixelscale)  
<Quantity 1.6 arcmin>

Or maybe you are designing an instrument for a telescope that someone told you has an inverse plate scale of 7.8 meters per radian (for your desired focus), and you want to know how big your pixels need to be to cover half an arcsecond. Using plate_scale():

>>> tel_platescale = u.plate_scale(7.8*u.m/u.radian)
>>> (0.5*u.arcsec).to(u.micron, tel_platescale)  
<Quantity 18.9077335632719 micron>

The pixel_scale() equivalency can also work in more general context, where the scale is specified as any quantity that is reducible to <composite unit>/u.pix or u.pix/<composite unit> (that is, the dimensionality of u.pix is 1 or -1). For instance, you may define the dots per inch (DPI) for a digital image to calculate its physical size:

>>> dpi = u.pixel_scale(100 * u.pix / u.imperial.inch)
>>> (1024 * u.pix).to(u.cm, dpi)  
<Quantity 26.0096 cm>

Photometric Zero Point Equivalency#

The zero_point_flux() equivalency provides a way to move between photometric systems (i.e., those defined relative to a particular zero-point flux) and absolute fluxes. This is most useful in conjunction with support for Magnitudes and Other Logarithmic Units.


Suppose you are observing a target with a filter with a reported standard zero point of 3631.1 Jy:

>>> target_flux = 1.2 * u.nanomaggy
>>> zero_point_star_equiv = u.zero_point_flux(3631.1 * u.Jy)
>>> u.Magnitude(target_flux.to(u.AB, zero_point_star_equiv))  
<Magnitude 22.30195136 mag(AB)>

Temperature Equivalency#

The temperature() equivalency allows conversion between the Celsius, Fahrenheit, Rankine and Kelvin.


To convert between temperature scales:

>>> temp_C = 0 * u.Celsius
>>> temp_Kelvin = temp_C.to(u.K, equivalencies=u.temperature())
>>> temp_Kelvin  
<Quantity 273.15 K>
>>> temp_F = temp_C.to(u.imperial.deg_F, equivalencies=u.temperature())
>>> temp_F  
<Quantity 32. deg_F>
>>> temp_R = temp_C.to(u.imperial.deg_R, equivalencies=u.temperature())
>>> temp_R  
<Quantity 491.67 deg_R>


You can also use u.deg_C instead of u.Celsius.

Mass-Energy Equivalency#

In a special relativity context it can be convenient to use the mass_energy() equivalency. For instance:

>>> (1 * u.g).to(u.eV, u.mass_energy())  
<Quantity 5.60958865e+32 eV>

Doppler Redshift Equivalency#

Conversion between Doppler redshift and radial velocity can be done with the doppler_redshift() equivalency.


To convert Doppler redshift (unitless) to km/s:

>>> z = 0.1 * u.dimensionless_unscaled
>>> z.to(u.km / u.s, u.doppler_redshift())  
<Quantity 28487.0661448 km / s>

However, it cannot take the cosmological redshift unit from astropy.cosmology.units because the latter should not be interpreted the same since the recessional velocity from the expansion of space can exceed the speed of light; see Hubble’s law: Redshift velocity and recessional velocity for more information.

Magnetic Flux Density and Field Strength Equivalency#

The magnetic_flux_field() equivalency allows conversion between Magnetic Flux Density (B) and its equivalent Magnetic Field Strength (H), governed by the equation

\[\mathbf{B} = \mu_r \mu_0 \mathbf{H}.\]

Where \(\mu_0\) is the vacuum permeability and \(\mu_r\) is the relative permeability of the medium. For a vacuum, \(\mu_r=1\).


To convert between Magnetic Flux Density (B) and its equivalent Magnetic Field Strength (H) in a vacuum.

>>> H = 1 * u.Oe
>>> H.to(u.G, u.magnetic_flux_field())  
<Quantity 1. G>
>>> H.to(u.T, u.magnetic_flux_field())  
<Quantity 0.0001 T>
>>> B = 1 * u.T
>>> B.to(u.A / u.m, equivalencies=u.magnetic_flux_field())  
<Quantity 795774.71502628 A / m>

Conversion in a medium with \(\mu_r=0.9\):

>>> H.to(u.G, u.magnetic_flux_field(mu_r=0.9))  
<Quantity 0.9 G>
>>> B.to(u.A / u.m, equivalencies=u.magnetic_flux_field(mu_r=0.9))  
<Quantity 884194.12780697 A / m>

Writing New Equivalencies#

An equivalence list is a list of tuples, where each tuple has four elements:

(from_unit, to_unit, forward, backward)

from_unit and to_unit are the equivalent units. forward and backward are functions that convert values between those units. forward and backward are optional, and if omitted then the equivalency declares that the two units should be taken as equivalent. The functions must take and return non-Quantity objects to avoid infinite recursion; See A More Complex Example: Spectral Doppler Equivalencies for more details.


Until 1964, the metric liter was defined as the volume of 1kg of water at 4°C at 760mm mercury pressure. Volumes and masses are not normally directly convertible, but if we hold the constants in the 1964 definition of the liter as true, we could build an equivalency for them:

>>> liters_water = [
...    (u.l, u.g, lambda x: 1000.0 * x, lambda x: x / 1000.0)
... ]
>>> u.l.to(u.kg, 1, equivalencies=liters_water)

Note that the equivalency can be used with any other compatible unit:

>>> imperial.gallon.to(imperial.pound, 1, equivalencies=liters_water)  

And it also works in the other direction:

>>> imperial.lb.to(imperial.pint, 1, equivalencies=liters_water)  

A More Complex Example: Spectral Doppler Equivalencies#

We show how to define an equivalency using the radio convention for CO 1-0. This function is already defined in doppler_radio(), but this example is illustrative:

>>> from astropy.constants import si
>>> restfreq = 115.27120  # rest frequency of 12 CO 1-0 in GHz
>>> freq_to_vel = [(u.GHz, u.km/u.s,
... lambda x: (restfreq-x) / restfreq * si.c.to_value('km/s'),
... lambda x: (1-x/si.c.to_value('km/s')) * restfreq )]
>>> u.Hz.to(u.km / u.s, 116e9, equivalencies=freq_to_vel)  
>>> (116e9 * u.Hz).to(u.km / u.s, equivalencies=freq_to_vel)  
<Quantity -1895.4321928669262 km / s>

Note that once this is defined for GHz and km/s, it will work for all other units of frequency and velocity. x is converted from the input frequency unit (e.g., Hz) to GHz before being passed to lambda x:. Similarly, the return value is assumed to be in units of km/s, which is why the value of c is used instead of the Constant.

Displaying Available Equivalencies#

The find_equivalent_units() method also understands equivalencies.


Without passing equivalencies, there are three compatible units for Hz in the standard set:

>>> u.Hz.find_equivalent_units()
  Primary name | Unit definition | Aliases
  Bq           | 1 / s           | becquerel    ,
  Ci           | 3.7e+10 / s    | curie        ,
  Hz           | 1 / s           | Hertz, hertz ,

However, when passing the spectral equivalency, you can see there are all kinds of things that Hz can be converted to:

>>> u.Hz.find_equivalent_units(equivalencies=u.spectral())
Primary name | Unit definition        | Aliases
  AU           | 1.49598e+11 m          | au, astronomical_unit            ,
  Angstrom     | 1e-10 m                | AA, angstrom                     ,
  Bq           | 1 / s                  | becquerel                        ,
  Ci           | 3.7e+10 / s            | curie                            ,
  Hz           | 1 / s                  | Hertz, hertz                     ,
  J            | m2 kg / s2             | Joule, joule                     ,
  Ry           | 2.17987e-18 m2 kg / s2 | rydberg                          ,
  cm           | 0.01 m                 | centimeter                       ,
  eV           | 1.60218e-19 m2 kg / s2 | electronvolt                     ,
  earthRad     | 6.3781e+06 m           | R_earth, Rearth                  ,
  erg          | 1e-07 m2 kg / s2       |                                  ,
  foe          | 1e+44 m2 kg / s2       | Bethe, bethe                     ,
  jupiterRad   | 7.1492e+07 m           | R_jup, Rjup, R_jupiter, Rjupiter ,
  k            | 100 / m                | Kayser, kayser                   ,
  lsec         | 2.99792e+08 m          | lightsecond                      ,
  lyr          | 9.46073e+15 m          | lightyear                        ,
  m            | irreducible            | meter                            ,
  micron       | 1e-06 m                |                                  ,
  pc           | 3.08568e+16 m          | parsec                           ,
  solRad       | 6.957e+08 m            | R_sun, Rsun                      ,

Using Equivalencies in Larger Pieces of Code#

Sometimes you may have an involved calculation where you are regularly switching back and forth between equivalent units. For these cases, you can set equivalencies that will by default be used, in a way similar to how you can enable other units.


To enable radians to be treated as a dimensionless unit use set_enabled_equivalencies() as a context manager:

>>> with u.set_enabled_equivalencies(u.dimensionless_angles()):
...    phase = 0.5 * u.cycle
...    c = np.exp(1j*phase)
>>> c  
<Quantity -1.+1.2246468e-16j>

To permanently and globally enable radians to be treated as a dimensionless unit use set_enabled_equivalencies() not as a context manager:

>>> u.set_enabled_equivalencies(u.dimensionless_angles())
<astropy.units.core._UnitContext object at ...>
>>> u.deg.to('')  

The disadvantage of the above approach is that you may forget to turn the default off (done by giving an empty argument).

set_enabled_equivalencies() accepts any list of equivalencies, so you could add, for example, spectral() and spectral_density() (since these return lists, they should indeed be combined by adding them together).