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Prototyping for the Spectropolarimeter for Planetary EXploration (SPEX): calibration and sky measurements

Author: Harten, G. van · Snik, F. · Rietjens, J.H.H. · Smit, J.M. · Boer, J. de · Diamantopoulou, R. · Hasekamp, O.P. · Stam, D.M. · Keller, C.U. · Laan, E.C. · Verlaan, A.L. · Vliegenthart, W.A. · Horst, R. ter · Navarro, R. · Wielinga, K. · Hannemann, S. · Moon, S.G. · Voors, R.
Type:article
Date:2011
Source:Polarization Science and Remote Sensing V, 21-22 August 2011, San Diego, CA, USA
series:
Proceedings of SPIE - The International Society for Optical Engineering
Identifier: 443002
ISBN: 9780819487704
Article number: 81600Z
Keywords: Aviation · Remote sensing · Spectral modulation · Spectropolarimetry · A-thermal · AERONET · Aerosol monitoring networks · Birefringent crystals · Calibration tests · Degree of linear polarization · End-to-end performance · Field of views · Ground based · High-accuracy · In-orbit · Intensity spectrum · Planetary exploration · Polarization spectra · Polarizing beam splitters · Quarter waves · Retardance · Single scattering · Sun photometers · Vertical distributions · Atmospheric aerosols · Earth (planet) · Modulation · Optimization · High Tech Systems & Materials · Industrial Innovation · Physics & Electronics · SSE - Space Systems Engineering · TS - Technical Sciences

Abstract

We present the Spectropolarimeter for Planetary EXploration (SPEX), a high-accuracy linear spectropolarimeter 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, optimized 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 prototype 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.