def _xy_idx(self):
        """
        1D arrays of x and y indices of unmasked pixels in the cutout
        reference frame.
        """

        yidx, xidx = np.indices(self._data.shape)
        return xidx[~self.mask].ravel(), yidx[~self.mask].ravel()
1D arrays of x and y indices of unmasked pixels in the cutout
        reference frame.