Characterizing the polarization response of the ALMA primary beam

Mon, November 09, 2015

Most observations of linear polarization with the SMA and CARMA include only on-axis polarization calibration. This has not posed much of a problem in the past, considering the sensitivity limits and the relatively large fields of view of the two arrays. However, the era of ALMA is upon us, and with it come both extremely sensitive observations and smaller fields of view. Consequently, we set out to characterize the wide-field polarization performance of the 12m dishes at ALMA at Bands 3, 6, and 7 (3 mm, 1 mm, and 850 um, respectively). Here I will present the results obtained by observing a bright, highly polarized quasar (3C279) in an 11×11 grid of off-axis pointings, covering a region spanning the full-width at half-maximum (FWHM) of the primary beam in each band. Our main findings were that after on-axis calibration of all off-axis pointings in all bands, and within the FWHM of the primary beam, the error in polarization fraction Pfrac,err = Pfrac,on − Pfrac,off < 0.01; the error in position angle χerr = χon − χoff < 4º; and the band-averaged off-axis leakage amplitudes are |D| < 0.1. I will conclude by briefly mentioning other polarization commissioning activities, both current and future.

Dr. Hull got his PhD from Berkeley, where he worked with the ATA and CARMA, including working with Dick Plambeck to install the CARMA dual-polarization receiver system. He is now an NRAO Jansky Fellow at Harvard CfA, where he works on star formation, dust polarization, and polarization calibration using ALMA. He has also eaten at every Indian restaurant in Berkeley.