code: def characterize_psf(self):
          
          l,u = max(self.zrange[0], self.param_dict['psf-zslab']), self.zrange[1]
  
          size_l, drift_l = self.measure_size_drift(l, size=self.support)
          size_u, drift_u = self.measure_size_drift(u, size=self.support)
  
          self.drift_poly = np.polyfit([l, u], [drift_l, drift_u], 1)
docs: Get support size and drift polynomial for current set of params