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SPEX: the Spectropolarimeter for Planetary Exploration

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Author: Rieetjens, J.H.H. · Snik, F. · Stam, D.M. · Smit, J.M. · Harten, G. van · Keller, C.U. · Verlaan, A.L. · Laan, E.C. · Horst, R. ter · Navarro, R. · Wielinga, K. · Moon, S.G. · Voors, R.
Type:article
Date:2010
Publisher: SPIE
Institution: TNO Industrie en Techniek
Source:Cugny, B.Armandillo, E.Karafolas, N., International Conference on Space Optics, ICSO 2010, 4-8 October 2010, Rhodes, Greece, 10565
series:
Proceedings of SPIE - The International Society for Optical Engineering
Identifier: 784913
doi: doi:10.1117/12.2309231
ISBN: 9781510616196
Keywords: Interplanetary spacecraft · Atmospheric particles · Degree of linear polarization · Microphysical property · Multiple-order retarders · Planetary exploration · Polarization observations · Polarizing beam splitters · Vertical distributions · Space & Scientific Instrumentation · Industrial Innovation

Abstract

We present SPEX, the Spectropolarimeter for Planetary Exploration, which is a compact, robust and low-mass spectropolarimeter designed to operate from an orbiting or in situ platform. Its purpose is to simultaneously measure the radiance and the state (degree and angle) of linear polarization of sunlight that has been scattered in a planetary atmosphere and/or reflected by a planetary surface with high accuracy. The degree of linear polarization is extremely sensitive to the microphysical properties of atmospheric or surface particles (such as size, shape, and composition), and to the vertical distribution of atmospheric particles, such as cloud top altitudes. Measurements as those performed by SPEX are therefore crucial and often the only tool for disentangling the many parameters that describe planetary atmospheres and surfaces. SPEX uses a novel, passive method for its radiance and polarization observations that is based on a carefully selected combination of polarization optics. This method, called spectral modulation, is the modulation of the radiance spectrum in both amplitude and phase by the degree and angle of linear polarization, respectively. The polarization optics consists of an achromatic quarter-wave retarder, an athermal multiple-order retarder, and a polarizing beam splitter. We will show first results obtained with the recently developed prototype of the SPEX instrument, and present a performance analysis based on a dedicated vector radiative transport model together with a recently developed SPEX instrument simulator.