class_docstr: Class implementing the MTG FCI L1c Filehandler.
  
  This class implements the Meteosat Third Generation (MTG) Flexible
  Combined Imager (FCI) Level-1c NetCDF reader.
  It is designed to be used through the :class:`~satpy.Scene`
  class using the :mod:`~satpy.Scene.load` method with the reader
  ``"fci_l1c_nc"``.
docstr: Get the auxiliary data arrays using the index map.
docstr_tok: ["Get","the","auxiliary","data","arrays","using","the","index","map","."]
code: def _get_dataset_aux_data(self, dsname):
      """Get the auxiliary data arrays using the index map."""
      # get index map
      index_map = self._get_dataset_index_map(
          _get_channel_name_from_dsname(dsname)
      )
      # subtract minimum of index variable (index_offset)
      index_map -= np.min(self.get_and_cache_npxr("index"))
  
      # get lut values from 1-d vector variable
      lut = self._get_aux_data_lut_vector(
          _get_aux_data_name_from_dsname(dsname)
      )
  
      # assign lut values based on index map indices
      aux = index_map.data.map_blocks(
          self._getitem, lut.data, dtype=lut.data.dtype
      )
      aux = xr.DataArray(
          aux,
          dims=index_map.dims,
          attrs=index_map.attrs,
          coords=index_map.coords,
      )
  
      # filter out out-of-disk values
      aux = aux.where(index_map >= 0)
  
      return aux
  
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  import logging
  from datetime import timedelta
  from functools import cached_property
  import dask.array
  import numpy
  import xarray
  from netCDF4 import default_fillvals
  from pyresample import geometry
  from satpy.readers._geos_area import get_geos_area_naming
  from satpy.readers.eum_base import get_service_mode
  from .netcdf_utils import NetCDF4FsspecFileHandler
label_desc: Use image node_id 18 for calling the FCIL1cNCFileHandler obj's underlying member method code with example usage: obj._get_dataset_aux_data(dsname) and returns: aux