Prototyping for the Spectropolarimeter for Planetary EXploration (SPEX): calibration and sky measurements Gerard van Harten a , Frans Snik a , Jeroen H. H. Rietjens b , J. Martijn Smit b , Jozua de Boer a , Renia Diamantopoulou a , Otto P. Hasekamp b , Daphne M. Stam b , Christoph U. Keller a , Erik C. Laan c , Ad L. Verlaan c , Willem A. Vliegenthart c , Rik ter Horst d , Ram´on Navarro d , Klaas Wielinga e , Sandro Hannemann f , Scott G. Moon f , Robert Voors g a Sterrekundig Instituut Utrecht, Princetonplein 5, 3584 CC Utrecht, the Netherlands; b SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands; c TNO Science & Industry, Stieltjesweg 1, 2628 CK Delft, the Netherlands; d NOVA-ASTRON, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, the Netherlands; e Mecon Engineering BV, Koopmanslaan 25, 7005 BK Doetinchem, the Netherlands; f cosine, Niels Bohrweg 11, 2333 CA Leiden, the Netherlands; g Dutch Space, Newtonweg 1, 2333 CP Leiden, the Netherlands ABSTRACT We present the Spectropolarimeter for Planetary EXploration (SPEX), a high-accuracy linear spectropolarime- ter measuring from 400 to 800 nm (with 2 nm intensity resolution), that is compact (∼ 1 liter), robust and lightweight. This is achieved by employing the unconventional spectral polarization modulation technique, op- timized for linear polarimetry. The polarization modulator consists of an achromatic quarter-wave retarder and a multiple-order retarder, followed by a polarizing beamsplitter, such that the incoming polarization state is encoded as a sinusoidal modulation in the intensity spectrum, where the amplitude scales with the degree of linear polarization, and the phase is determined by the angle of linear polarization. An optimized combination of birefringent crystals creates an athermal multiple-order retarder, with a uniform retardance across the field of view. Based on these specifications, SPEX is an ideal, passive remote sensing instrument for characterizing planetary atmospheres from an orbiting, air-borne or ground-based platform. By measuring the intensity and polarization spectra of sunlight that is scattered in the planetary atmosphere as a function of the single scattering angle, aerosol microphysical properties (size, shape, composition), vertical distribution and optical thickness can be derived. Such information is essential to fully understand the climate of a planet. A functional SPEX proto- type has been developed and calibrated, showing excellent agreement with end-to-end performance simulations. Calibration tests show that the precision of the polarization measurements is at least 2 · 10 -4 . We performed multi-angle spectropolarimetric measurements of the Earth’s atmosphere from the ground in conjunction with one of AERONET’s sun photometers. Several applications exist for SPEX throughout the solar system, a.o. in orbit around Mars, Jupiter and the Earth, and SPEX can also be part of a ground-based aerosol monitoring network. Keywords: planet, atmosphere, aerosol, dust, Earth, Mars, remote sensing, spectropolarimetry, spectral mod- ulation 1. INTRODUCTION Atmospheric aerosol research is gaining more and more attention, mainly because of climate models showing their large impact on the Earth’s climate, whereas the actual aerosol input for the models is poorly known [1]. Aerosols, 0.2–200 μm sized particles suspended in the atmosphere, such as tiny water droplets, sand and sea salt, have a direct effect on the climate by absorbing and reflecting incoming sunlight, thereby cooling our planet. Moreover, some types of aerosols act as cloud condensation nuclei, with an increase of such aerosol leading Further author information: (Send correspondence to G.v.H or F.S.) G.v.H.: E-mail: g.vanharten@astro-uu.nl F.S.: E-mail: f.snik@astro-uu.nl Polarization Science and Remote Sensing V, edited by Joseph A. Shaw, J. Scott Tyo, Proc. of SPIE Vol. 8160, 81600Z · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.893741 Proc. of SPIE Vol. 8160 81600Z-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/21/2015 Terms of Use: http://spiedl.org/terms