SPEX: the spectropolarimeter for planetary exploration

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.

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

“…7a), with a maximum efficiency of about 90%. This is fully predicted by the SPEX instrument simulator [7], and can be attributed to the slit function of the spectrometer, in combination with spectral modulation, which is faster at the blue side of the spectrum, such that in the blue the measured intensity is an average over a larger part of the modulated input spectrum. In addition to this spectral dependence, the polarimetric efficiency is also a function of the AoLP of the incoming light, as shown in Fig.…”

“…In order to keep the instrument passive and energy efficient, temperature control is avoided, using the following solution: the retarder consists of two plates with opposite temperature effects. Measurements published in reference [7] show that a subtractive combination of MgF 2 and Al 2 O 3 (i.e. with their fast axes crossed), with a thickness ratio of 2.4 : 1, yields optimum performance over the wavelength range 350-800 nm.…”

“…For the SPEX prototype, only 3 fingers are filled with polarization optics. This section is largely based on the work in [7].…”

Spectropolarimetry for earth observations: a novel method for characterization of aerosols and clouds

“…For measuring the polarization state a new method is used, which encodes the degree and angle of linear polarization of the incoming light as a sinusoidal modulation of the intensity spectrum [1], [2], [3]. The principle is achieved using an optical configuration (see Fig.…”

Atmospheric aerosol characterization with the Dutch–Chinese FAST formation flying mission

“…5). SPEX has a truly innovative method for doing polarimetry: the degree and direction of linear polarization of the scattered and observed sunlight is encoded as a sinusoidal modulation into the flux spectrum 25,26 . Thus, from a single flux spectrum (measured in a specific viewing direction), the spectral dependence of the polarization and the flux itself can be retrieved.…”