code: def measure_size_drift(self, z, size=31, zoffset=0.):
          
          drift = 0.0
          for i in range(self.measurement_iterations):
              psf, vec = self.psf_slice(z, size=size, zoffset=zoffset+drift)
              psf = psf / psf.sum()
  
              drift += moment(psf, vec[0], order=1)
              psize = [moment(psf, j, order=2) for j in vec]
          return np.array(psize), drift
docs: Returns the 'size' of the psf in each direction a particular z (px)