Suppressing Beam Systematics in Antenna-Coupled TES Bolometers for CMB Polarimetry

We have deployed arrays of antenna-coupled TES bolometers for cosmic microwave background (CMB) polarimetry in the BICEP2 and Keck array experiments and will deploy similar detectors in SPIDER and Polar-1. Each pixel receives optical power centered at 146 GHz with a 20% bandwidth (?10 dB edges) through an integrated dual-polarized phased-array antenna. In past deployments, these detectors have shown offsets in the two polarizations’ beam centroids (differential ‘steering’), but we have redesigned the feed networks by strategically spacing lines and adding intentional phase delays to suppress this unwanted effect. We expect that the focal planes in this season’s deployment of Keck and in SPIDER will have detectors with less than 5′ steering, which is 0.5% of the 14.2° nominal antenna’s FWHM (i.e. without lenses). We have also redesigned the antenna-array’s illumination pattern to suppress side-lobe response in anticipation of the Polar-1 experiment, reducing spillover by a factor of 2.5.