docs: def pivot_wavelength(self):
          """Get the bandpass' pivot wavelength.
  
          Unlike calc_pivot_wavelength(), this function will use a cached
          value if available.
  
          """
          wl = self.registry._pivot_wavelengths.get((self.telescope, self.band))
          if wl is not None:
              return wl
  
          wl = self.calc_pivot_wavelength()
          self.registry.register_pivot_wavelength(self.telescope, self.band, wl)
          return wl