docs: def halfmax_points(self):
          """Get the bandpass' half-maximum wavelengths. These can be used to
          compute a representative bandwidth, or for display purposes.
  
          Unlike calc_halfmax_points(), this function will use a cached value if
          available.
  
          """
          t = self.registry._halfmaxes.get((self.telescope, self.band))
          if t is not None:
              return t
  
          t = self.calc_halfmax_points()
          self.registry.register_halfmaxes(self.telescope, self.band, t[0], t[1])
          return t