Characterizing the accuracy of ALMA linear-polarization mosaics
  We characterize the accuracy of linear-polarization mosaics made using the
Atacama Large Millimeter/submillimeter Array (ALMA). First, we observed the
bright, highly linearly polarized blazar 3C 279 at Bands 3, 5, 6, and 7 (3 mm,
1.6 mm, 1.3 mm, and 0.87 mm, respectively). At each band, we measured the
blazar's polarization on an 11$\times$11 grid of evenly-spaced offset pointings
covering the full-width at half-maximum (FWHM) area of the primary beam. After
applying calibration solutions derived from the on-axis pointing of 3C 279 to
all of the on- and off-axis data, we find that the residual polarization errors
across the primary beam are similar at all frequencies: the residual errors in
linear polarization fraction $P_\textrm{frac}$ and polarization position angle
$\chi$ are $\lesssim$0.001 ($\lesssim$0.1% of Stokes $I$) and
$\lesssim$1$^\circ$ near the center of the primary beam; the errors increase to
$\sim$0.003-0.005 ($\sim$0.3-0.5% of Stokes $I$) and $\sim$1-5$^\circ$ near the
FWHM as a result of the asymmetric beam patterns in the (linearly polarized)
$Q$ and $U$ maps. We see the expected double-lobed "beam squint" pattern in the
circular polarization (Stokes $V$) maps. Second, to test the polarization
accuracy in a typical ALMA project, we performed observations of continuum
linear polarization toward the Kleinmann-Low nebula in Orion (Orion-KL) using
several mosaic patterns at Bands 3 and 6. We show that after mosaicking, the
residual off-axis errors decrease as a result of overlapping multiple
pointings. Finally, we compare the ALMA mosaics with an archival 1.3 mm CARMA
polarization mosaic of Orion-KL and find good consistency in the polarization
patterns.
