docs: def getZernike(self, index):
          """getZernike
  
          Retrieve a map representing the index-th Zernike polynomial
  
          Args:
              index (int): The index of Zernike map to be generated,
                  following Noll 1976 ordering.
  
          Returns:
              np.array: A map representing the index-th Zernike polynomial
  
          """
          if index not in list(self._dictCache.keys()):
              self._dictCache[index]= self._polar(index, self._rhoMap,
                                                  self._thetaMap)
          return self._dictCache[index]