doppler_optical

astropy.units.doppler_optical(rest)[source] [edit on github]

Return the equivalency pairs for the optical convention for velocity.

The optical convention for the relation between velocity and frequency is:

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

Parameters:
rest : Quantity

Any quantity supported by the standard spectral equivalencies (wavelength, energy, frequency, wave number).

References

NRAO site defining the conventions

Examples

>>> import astropy.units as u
>>> CO_restfreq = 115.27120*u.GHz  # rest frequency of 12 CO 1-0 in GHz
>>> optical_CO_equiv = u.doppler_optical(CO_restfreq)
>>> measured_freq = 115.2832*u.GHz
>>> optical_velocity = measured_freq.to(u.km/u.s, equivalencies=optical_CO_equiv)
>>> optical_velocity  
<Quantity -31.20584348799674 km / s>