code: def lonlat_to_healpix(self, lon, lat, return_offsets=False):
          
          return lonlat_to_healpix(lon, lat, self.nside,
                                   return_offsets=return_offsets, order=self.order)
docs: Convert longitudes/latitudes to HEALPix indices (optionally with offsets)
  
          Parameters
          ----------
          lon, lat : :class:`~astropy.units.Quantity`
              The longitude and latitude values as :class:`~astropy.units.Quantity` instances
              with angle units.
          return_offsets : bool
              If `True`, the returned values are the HEALPix pixel as well as
              ``dx`` and ``dy``, the fractional positions inside the pixel. If
              `False` (the default), only the HEALPix pixel is returned.
  
          Returns
          -------
          healpix_index : `~numpy.ndarray`
              1-D array of HEALPix indices
          dx, dy : `~numpy.ndarray`
              1-D arrays of offsets inside the HEALPix pixel in the range [0:1] (0.5
              is the center of the HEALPix pixels). This is returned if
              ``return_offsets`` is `True`.