code: def wpeak(self, wavelengths=None):
          
          x = self._validate_wavelengths(wavelengths)
          return x[self(x) == self.tpeak(wavelengths=wavelengths)][0]
docs: Calculate
          :ref:`wavelength at peak throughput <synphot-formula-tpeak>`.
  
          If there are multiple data points with peak throughput
          value, only the first match is returned.
  
          Parameters
          ----------
          wavelengths : array-like, `~astropy.units.quantity.Quantity`, or `None`
              Wavelength values for sampling.
              If not a Quantity, assumed to be in Angstrom.
              If `None`, ``self.waveset`` is used.
  
          Returns
          -------
          wpeak : `~astropy.units.quantity.Quantity`
              Wavelength at peak throughput.