High-resolution VLBI astrometry of pulsar scintillation screens with the ?-? transform

The recent development of ?-? techniques in pulsar scintillometry has opened the door for new high-resolution imaging techniques of the scattering medium. By solving the phase retrie v al problem and reco v ering the wavefield from a pulsar dynamic spectrum, the Doppler shift, time delay, and phase offset of individual images can be determined. Ho we ver, the results of phase retrie v al from a single dish are only known up to a constant phase rotation, which introduces extra parameters when doing astrometry using very long baseline interferometry. We present an extension to previous 0 - 0 methods using the interferometric visibilities between multiple stations to calibrate the wavefields. When applied to existing data for PSR B0834 + 06, we measure the ef fecti v e screen distance and lens orientation with fiv e times greater precision than w as possible in previous w ork.