Calculate the relative and absolute airmass.

        Automatically chooses zenith or apparant zenith
        depending on the selected model.

        Parameters
        ----------
        times : None or DatetimeIndex, default None
            Only used if solar_position is not provided.
        solar_position : None or DataFrame, default None
            DataFrame with with columns 'apparent_zenith', 'zenith'.
        model : str, default 'kastenyoung1989'
            Relative airmass model

        Returns
        -------
        airmass : DataFrame
            Columns are 'airmass_relative', 'airmass_absolute'

def get_airmass(self, times=None, solar_position=None,
                    model='kastenyoung1989'):
        """
        Calculate the relative and absolute airmass.

        Automatically chooses zenith or apparant zenith
        depending on the selected model.

        Parameters
        ----------
        times : None or DatetimeIndex, default None
            Only used if solar_position is not provided.
        solar_position : None or DataFrame, default None
            DataFrame with with columns 'apparent_zenith', 'zenith'.
        model : str, default 'kastenyoung1989'
            Relative airmass model

        Returns
        -------
        airmass : DataFrame
            Columns are 'airmass_relative', 'airmass_absolute'
        """

        if solar_position is None:
            solar_position = self.get_solarposition(times)

        if model in atmosphere.APPARENT_ZENITH_MODELS:
            zenith = solar_position['apparent_zenith']
        elif model in atmosphere.TRUE_ZENITH_MODELS:
            zenith = solar_position['zenith']
        else:
            raise ValueError('{} is not a valid airmass model'.format(model))

        airmass_relative = atmosphere.get_relative_airmass(zenith, model)

        pressure = atmosphere.alt2pres(self.altitude)
        airmass_absolute = atmosphere.get_absolute_airmass(airmass_relative,
                                                           pressure)

        airmass = pd.DataFrame(index=solar_position.index)
        airmass['airmass_relative'] = airmass_relative
        airmass['airmass_absolute'] = airmass_absolute

        return airmass