Introduction

Polarization observations are one of the important science drivers for the ALMA telescope. One of the most important areas where polarization observations will play a significant role is star formation. Probably mosaic mapping of extended polarized emission will dominate polarization work. Therefore, optimization for such observations is crucial.

The March 2000 ASAC report proposed the requirement for 0.1% polarization mapping fidelity after calibration. Although synchrotron polarization is often >10%, polarization of dust emission seems typically to be <5%. Hence, requirements for dust polarization determinations are more stringent than in the perhaps more familiar synchrotron polarization case. Also, simply detecting polarized dust emission is insufficient; position angles must be measured to a high degree of significance. For example, interstellar turbulence can lead to tangling of magnetic field lines and therefore to a dispersion $\Delta\theta$ in the position angles of polarized dust emission. Measurement of $\Delta\theta$ can lead to an estimate of magnetic field strengths, since the degree of field tangling depends on the ratio of magnetic to turbulent energies, and turbulent energy can be estimated from spectral line widths. For 1% polarization of dust emission, an uncertainty at the 0.1% level implies a $1\sigma$ uncertainty in position angle of about 6$^\circ$, about the largest uncertainty for the results to be useful.

This requirement of 0.1% polarization fidelity after calibration does not lead directly to specifications on receivers, since the combination of instrumental polarization induced by receivers and antennas both play a role, along with the procedures for calibration. We discuss these separately.