A Cryogenic Half-Wave Plate Module to Measure Polarization at Multiple FIR Passbands

A Cryogenic Half-Wave Plate Module to Measure Polarization at Multiple FIR PassbandsTimothy S. Rennick*, John E. Vaillancourt**, Roger H. Hildebrand +, and Stephen J. Heimsath*AbstractOne of the key components in a far-infrared polarimeter that is being designed at The University ofChicago is a locally-powered half-wave plate module. This compact, lightweight, and reliable module willoperate at cryogenic temperatures, rotating a half-wave plate about its axis within the optical path. Bydoing so, polarization measurements can be made. Further, by utilizing multiple half-wave plate moduleswithin the polarimeter, multiple wavelengths or passbands can be studied. In this paper, we describe thedesign and performance of a relatively inexpensive prototype module that was assembled and testedsuccessfully, outline the difficulties that had to be overcome, and recommend improvements to futuremodules. This effort now lays some of the groundwork for a next-generation polarimeter for far-infraredastronomy.IntroductionInstruments designed for far-infrared (FIR) and submillimeter astronomy generally operate with thedetectors and most of the optical elements at cryogenic temperature. When the design calls for insertingany of several lenses or filters into the optical path, there must be suitable actuators, bearings, andsensors for the moving parts. The actuators have usually consisted of external, room temperature, motorsconnected to the moving parts by insulating shafts passing through vacuum seals and working their waythrough tight spaces at times via right angle gearing to the cold parts. This approach has been workablefor relatively simple instruments, but has become increasingly difficult as the instruments have becomemore complex.