After performing the Gaussian fits on short baselines , we derived amplitude and phase self - calibration solutions and applied them to all baselines . For the amplitude self - calibration , we used a total flux density of 1 Jy , which is comparable to the average of historical values at 1.3 cm ( e.g. , Bower et al . 2015b ) . This choice of normalization does not affect our remaining analysis , which only relies on the fractional visibility amplitudes relative to the total flux density . In addition , we averaged the data across all IFs , to maximize our sensitivity . Figure 2 shows the resulting visibility measurements , including upper limits on baselines to Spektr - R ( computed by PIMA ) and highlighting the amplitudes for our most sensitive ground baselines . These baselines are expected to have signals that are dominated by refractive noise ; they have / up to 8.4 after the final incoherent averaging in time , indicating a reliable detection of image substructure . For each figure , we show the expected envelope of the ensembleaverage image and the predicted " renormalized refractive noise"ˆr ef ( u ) = Δˆ(u ) 2 1/2 , whereˆis the complex visibility function of the source after centering the image and normalizing the total flux density ( for details , see Appendix A of J18 ) .