Design and Initial Performance of SHARP, a Polarimeter for the SHARCII Camera at the Calte.pdf

Design and Initial Performance of SHARP, a Polarimeter for the SHARC-II Camera at the Caltech Submillimeter Observatory H. Li a*, C. D. Dowell b,c, L. Kirby d, G. Novak a, and J. E. Vaillancourt c,e a Northwestern University, Dept. of Physics and Astronomy, Evanston, IL, 60208, USA b Jet Propulsion Laboratory, MS 169-506, 4800 Oak Grove Dr., Pasadena, CA,91109, USA c California Institute of Technology, Mail Code 320-47, 1200 E. California Blvd., Pasadena, CA, 91125, USA d University of Chicago, Enrico Fermi Institute and Dept. of Astronomy and Astrophysics, Chicago, IL, 60637, USA e University of Chicago, Enrico Fermi Institute, Chicago, IL, 60637 ABSTRACT We have developed a fore-optics module that converts the SHARC-II camera at the Caltech Submillimeter Observatory into a sensitive imaging polarimeter at wavelengths of 350 and 450 m. We refer to this module as “SHARP”. SHARP splits the incident radiation into two orthogonally polarized beams that are then re-imaged onto opposite ends of the 32  12 pixel detector array in SHARC-II. A rotating half-wave plate is used j ust upstream from the crossed grid. The effect of SHARP is to convert SHARC-II into a dual-beam 12  12 pixel polarimeter. A novel feature of SHARPs design is the use of a crossed grid in a submillimeter polarimeter. Here we describe the detailed optical design of SHARP and present results of tests carried out during our first few observing runs. At 350 m, the beam size (9 arcseconds), throughput (75%), and instrumental polarization ( 1%) are all very close to our design goals. Keywords: Submillimeter, Polarization. 1. INTRODUCTION The earliest detections of far-infrared