code: def boundaries_skycoord(self, healpix_index, step):
          
          if self.frame is None:
              raise NoFrameError("boundaries_skycoord")
          lon, lat = self.boundaries_lonlat(healpix_index, step)
          representation = UnitSphericalRepresentation(lon, lat, copy=False)
          return SkyCoord(self.frame.realize_frame(representation))
docs: Return the celestial coordinates of the edges of HEALPix pixels
  
          This returns the celestial coordinates of points along the edge of each
          HEALPIX pixel. The number of points returned for each pixel is ``4 * step``,
          so setting ``step`` to 1 returns just the corners.
  
          This method requires that a celestial frame was specified when
          initializing HEALPix.  If you don't know or need the celestial frame,
          you can instead use :meth:`~astropy_healpix.HEALPix.boundaries_lonlat`.
  
          Parameters
          ----------
          healpix_index : `~numpy.ndarray`
              1-D array of HEALPix pixels
          step : int
              The number of steps to take along each edge.
  
          Returns
          -------
          skycoord : :class:`~astropy.coordinates.SkyCoord`
              The celestial coordinates of the HEALPix pixel boundaries