Solving the polarization problem in ALMA-VLBI observations

TL;DR

This paper introduces PolConvert, an algorithm for polarization conversion in ALMA-VLBI observations, enabling the integration of linear and circular basis data for improved calibration in high-resolution VLBI imaging.

Contribution

The paper presents a novel algorithm, PolConvert, that effectively converts mixed polarization basis visibilities, facilitating polarization calibration in ALMA-VLBI observations.

Findings

Successfully applied to realistic simulations

Validated with real VLBI data at 86 GHz

Improves polarization calibration accuracy

Abstract

The Atacama Large mm-submm Array (ALMA) is, by far, the most sensitive mm/submm telescope in the World. The ALMA Phasing Project (APP) will allow us to phase-up all the ALMA antennas and use them as one single VLBI station. This will be a key component of the Event Horizon Telescope (EHT), a Global VLBI array at millimeter wavelengths. A problem in the APP is the calibration and conversion of the polarization channels. Most VLBI stations record their signals in a circular basis, but the ALMA receivers record in a linear basis. The strategy that will be followed in the phased-ALMA VLBI observations will be to correlate in "mixed" basis (i.e., linear versus circular) and convert the visibilities to a pure circular basis after the correlation. We have developed an algorithm to perform such a polarization conversion of the VLBI visibilities. In these proceedings, we present the basics of the PolConvert algorithm and discuss on the polarization conversion in the general case were single dishes (besides phased arrays) record with linear receivers in VLBI observations. We show some results of PolConvert applied to realistic simulations, as well as a test with real VLBI observations at 86\,GHz between the Onsala radiotelescope (recording in linear basis) and the Effelsberg radiotelescope (recording in circular basis).