1 mm Dual-polarization Science with CARMA Charles L. H. Hull 1 , Richard Plambeck 2 , and Gregory Engargiola 3 1 Radio Astronomy Laboratory, University of California, Berkeley. 601 Campbell Hall, Berkeley, CA 94720, USA. TEL: +1-510-642-5275. FAX: +1-510-642-3411. EMAIL: chat@astro.berkeley.edu. 2 plambeck@astro.berkeley.edu 3 greg@astro.berkeley.edu Abstract We have constructed dual polarization 1 mm receivers for the CARMA array. A key science goal for these receivers is to map the linearly polarized thermal emission from dust grains in interstellar clouds, which may be used to infer the magnetic-field morphologies in these regions. Polarization fractions of a few percent are typical of this radia- tion. For an aperture synthesis array like CARMA it is advantageous to measure this weak linear polarization by cross-correlating the signals from circularly polarized feeds. To observe circular polarization, we install a waveguide polarizer between the feed horn and the OMT (orthomode transducer), at a temperature of 4 K, to convert incoming circularly polarized signals into linearly polarized signals. The receivers use waveguide OMTs to split the incoming radiation into orthogonal linear polarizations. The two OMT outputs are coupled to separate SIS (superconductor- insulator-superconductor) mixers and WBA-13 LNAs (low-noise amplifiers). 1 Introduction CARMA (the Combined Array for Research in Millimeter-wave Astronomy) is an aperture-synthesis telescope in Cedar Flat, CA that is operated by Caltech, UC Berkeley, the University of Illinois at Urbana-Champaign, the University of Maryland, and the University of Chicago (1). It is a 23-element heterogeneous array, comprising six 10-meter, nine 6-meter, and eight 3.5-meter antennas. The 6- and 10-meter dishes currently observe in the 1 and 3 mm bands; soon all 23 antennas will operate at both 3 mm and 1 cm. With the installation of the 1 mm dual-polarization system described herein, CARMA will be able to access the wealth of polarization information contained in observed 1 mm radiation. Of particular interest is the linearly polarized emission from dust grains in interstellar clouds, which are bright at 1 mm. Mapping polarization will allow us to probe the morphology of magnetic fields in those regions—in particular, in the collapsing envelopes and extended outflows around deeply embedded Class 0 protostellar cores—and to identify scientifically interesting targets that can be followed up with the unprecedented resolution and sensitivity of ALMA. Figure 1 shows a photo of one of the 1 mm dual-polarization receivers that is mounted in the dewars on the 10- and 6-m telescopes at CARMA. The receiver consists of a feed horn; a waveguide circular-to-linear polarizer; a waveguide orthomode transducer (OMT); two mixers with superconductor-insulator-superconductor (SIS) devices designed by the NRAO for ALMA’s band 6 (2); and two wide-band, low-noise WBA13 intermediate frequency (IF) amplifiers. The entire module is installed in a dewar, the innermost stage of which is cooled to 4 K. In the following sections, we give brief descriptions of the various components of the 1 mm dual-polarization receivers. 2 The polarizer The waveguide polarizer we use converts circularly polarized light to linearly polarized light. It has a 2-section design, using half-wave and quarter-wave retarder sections rotated axially by 59.5 ◦ with respect to one other to achieve broadband performance (3). The design was inspired by the broadband waveguide polarizer described by Kovac and Carlstrom (4) and by Lilie (5), and is discussed in detail by Plambeck (6). Figure 2 shows the mandrel that was used 978-1-4244-6051-9/11/$26.00 ©2011 IEEE