'Moffat full width at half maximum.
Derivation of the formula is available in
`this notebook by Yoonsoo Bach <http://nbviewer.jupyter.org/github/ysbach/AO_2017/blob/master/04_Ground_Based_Concept.ipynb#1.2.-Moffat>`_.'

@property
def fwhm(self):

    return ((2.0 * self.gamma) * np.sqrt(((2.0 ** (1.0 / self.alpha)) - 1.0)))
