SIMBAD references

Atmospheric propagation errors are a main constraint on the accuracy of very-long-baseline-interferometry astrometry. For relative astrometry, differential techniques can mitigate these errors, but their effectiveness diminishes with decreasing elevation and increasing angular separations between target and calibrator, among others. The MultiView technique addresses atmospheric spatial-structure errors by observing multiple calibrators around the target and interpolating at the target position, thereby reducing atmospheric errors more effectively than phase referencing with only one calibrator. The first MultiView realisation at 1.6 GHz involved cyclically observing all calibrators and the target, fitting a phase plane from calibrator solutions in each cycle, and is a well-established technique. This implementation reduces on-target time and is constricted by the short atmospheric coherence time at high frequencies. We propose a new realisation, serial MultiView, which rotates the phase plane iteratively based on the time series of calibrator residual phases. This new strategy obviates the necessity of observing all calibrators within each cycle, thereby shortening the observing cycle and offering considerable potential at higher frequencies where the temporal structure is the dominant source of errors. Additionally, by incorporating time-domain information in the iterations, phase ambiguities can be accurately and automatically identified. We verify the astrometric accuracy of serial MultiView at 5 GHz by comparing it to conventional MultiView, achieving <10 μas error in R.A. direction, and show the calibration overhead can be reduced in both approaches. This approach enables efficient, high-accuracy differential astrometry and artifact-reduced imaging for astrophysical studies, and we provide a user-friendly tool for it.