The Polychromatic Polarization Modutor Alfred G. de Wijn, Steven Tomczyk, Roberto Casini, Peter G. Nelson High Altitude Observatory, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307, USA ABSTRACT An increasing number of astronomical applications depend on the measurement of polarized light. For example, our knowledge of solar magnetism relies heavily on our ability to measure and interpret polarization signatures introduced by magnetic field. Many new instruments have consequently focused considerable attention on polarimetry. For solar applications, spectro-polarimeters in particular are often designed to observe the solar atmosphere in multiple spectral lines simultaneously, thus requiring that the polarization modulator employed is efficient at all wavelengths of interest. We present designs of polarization modulators that exhibit near-optimal modulation characteristics over broad spectral ranges. Our design process employs a computer code to optimize the efficiency of the modulator at specified wavelengths. We will present several examples of modulator designs based on rotating stacks of Quartz waveplates and Ferroelectric Liquid Crystals (FLCs). An FLC-based modulator of this design was recently deployed for the ProMag instrument at the Evans Solar Facility of NSO/SP. We show that this modulator behaves according to its design. Keywords: Polarimetry 1. INTRODUCTION Our knowledge of solar magnetism relies heavily on our ability to detect and interpret the polarization signatures of mag- netic fields in solar spectral lines. The Zeeman effect was first used to identify sunspots as regions of strong magnetism. 1 Since then, the number of physical mechanisms applied to the interpretation of solar spectral line profiles has increased significantly. The Hanle effect observed in linear polarization has been exploited to diagnose weak turbulent magnetic fields. 2–4 A rich spectrum of linear polarization observed near the solar limb named the “second solar spectrum” 5 has been used to constrain magnetic fields and scattering physics. Observations of lines like Mn which display hyperfine splitting have been used to break the degeneracy between magnetic flux and field for weak magnetic fields. 6–8 Information about plasma kinetics during solar flares is encoded in the linear polarization of H lines through impact polarization. 9 Polarimeters are finding their way into other fields of astrophysics as well. Zeeman-Doppler imaging allows for dis- entangling magnetic field distribution on the surface a rotating star. 10 A polarimeter was recently installed on the HARPS instrument for planet finding. 11 There is now a strong drive toward instruments that observe over broad spectral ranges. In the context of understanding magnetism in the solar atmosphere, observations of spectral lines formed at different heights are needed to constrain the field geometry in three dimensions, and simultaneous observation of multiple spectral lines greatly enhances the diagnostic potential of Zeeman-effect observations through application of a line-ratio technique. 12 Improving technologies (e.g., IR detector arrays) have made it possible to take advantage of the increased sensitivity of the Zeeman effect with wavelength through the observation of infrared spectral lines. 13, 14 It follows that the next generation of polarimeters must have the capability to observe in a variety of spectral lines over a wide wavelength range, coupled with the ability to observe several lines simultaneously. This is reflected in the design of the recently completed Spectro-Polarimeter for InfraRed and Optical Regions (SPINOR, 15 ), installed at the Dunn Solar Telescope (DST) of the National Solar Observatoryon Sacramento Peak (NSO/SP, Sunspot, NM), that can observe between 430 and 1600nm. The instruments planned for the Advanced Technology Solar Telescope and the European Solar Telescope will span from the UV to the near-IR. 16, 17 One immediate instrument requirement stemming from this need for wavelength diversity is that the polarization modu- lation scheme must be efficient at all wavelengths of interest. Typically, one attempts to achieve this goal by achromatizing the polarimetric response of a modulator. Here, instead, we present a new paradigm for the design of efficient polarization dwijn@ucar.edu; phone 1 303 497 2171 Ground-based and Airborne Instrumentation for Astronomy III, edited by Ian S. McLean, Suzanne K. Ramsay, Hideki Takami, Proc. of SPIE Vol. 7735, 77354A · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.857745 Proc. of SPIE Vol. 7735 77354A-1 Downloaded from SPIE Digital Library on 28 Dec 2011 to 128.117.65.55. Terms of Use: http://spiedl.org/terms