Design of an optical beam combiner for dual band observation with ALMA

The aim of this work is to improve high-frequency calibration data on long baseline observations for the ALMA antennas. A dual-frequency atmospheric phase error calibration method is proposed and will be implemented by the simultaneous observation in two ALMA Bands, specifically 6 and 9, coupled by means external optics in a few baselines. This method is envisioned to demonstrate the advantage of receiving signals simultaneously at different frequencies from the same point of the sky. It will permit an increase of accuracy in determining the phase correction needed to reduce the effects of the atmosphere. While maintaining the existing receiver optics, an optical layout that couples Bands 6 and 9 is proposed. Here we demonstrate that very limited impact on the existing ALMA system is needed. Furthermore, we present in detail the optical layout, made within the formalism of ray optics, and a detailed tolerance analysis. The initial results demonstrate the feasibility of the proposed system.