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We have developed a 6 dm 3 -sized optical instrument to characterize the microphysical properties of fine particulate matter or aerosol in the Earth atmosphere from low Earth orbit. Our instrument can provide detailed and worldwide knowledge of aerosol amount, type and properties. This is important for climate and ecosystem science and human health [1,2]. Therefore, NASA, ESA and the European Commission study the application of aerosol instruments for planned or future missions. We distinguish molecular Rayleigh scattering from aerosol Mie-type scattering by analyzing multi-angle observations of radiance and the polarization state of sun light that is scattered in the Earth atmosphere [3]. We measure across the visible wavelength spectrum and in five distinct viewing angles between -50° and +50°. Such analysis has been traditionally done by rotating polarizers and band-filters in front of an Earth observing wide-angle imager. In contrast, we adopt a means to map the linear polarization state on the spectrum using passive optical components [4]. Thereby we can characterize the full linear polarization state for a scene instantaneously. This improves the polarimetric accuracy, which is critical for aerosol characterization, enabling us to distinguish for example anthropogenic from natural aerosol types. Moreover, the absence of moving parts simplifies the instrument, and makes it more robust and reliable. We have demonstrated this method in an airborne instrument called SPEX airborne [5,6] in the recent ACEPOL campaign together with a suite of state-of-the art and innovative active and passive aerosol sensors on the NASA ER-2 high-altitude research platform [7]. An earlier report on the SPEX development roadmap was given in [8]. In this contribution we introduce SPEXone, a compact space instrument that has a new telescope that projects the five viewing angles onto a single polarization modulation unit and the subsequent reflective spectrometer. The novel telescope allows the observation of five scenes with one spectrometer, hence the name. We describe the optical layout of the telescope, polarization modulation optics, and spectrometer and discuss the manufacturability and tolerances involved. We will also discuss the modelled instrument performance and show preliminary results from optical breadboards of the telescope and polarization modulation optics. With SPEXone we present a strong and new tool for climate research and air quality monitoring. It can be used to study the effect of atmospheric aerosol on the heating/cooling of the Earth and on air quality. Also, SPEXone can improve the accuracy of satellite measurements of greenhouse gas concentrations and ocean color that rely on molecular absorption of reflected sunlight by providing detailed knowledge of the aerosol properties, required to accurately trace the light path in presence of scattering. SPEXone is developed in a partnership between SRON Netherlands Institute for Space Research and Airbus Defence and Space Netherlands with support from the Netherlan...

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SPEXone is a multi-angle channeled spectropolarimeter that is developed by a Dutch consortium consisting of SRON and Airbus Defence and Space Netherlands with support from TNO. SPEXone will fly together with the Ocean Color Instrument (OCI) and the Hyper-Angular Rainbow Polarimeter-2 (HARP-2) on the NASA Plankton, Aerosol, Clouds and ocean Ecosystem (PACE) mission, which has a notional launch in 2023. SPEXone will deliver high quality hyperspectral multi-angle radiance and polarization products that, together with products from OCI and HARP2, enable unprecedented aerosol and cloud characterization from space.SPEXone employs dual beam spectral polarization modulation, in which the state of linear polarization is encoded in a spectrum as a periodic variation of the intensity. This technique enables high polarimetric accuracies in operational environments, since it provides snapshot acquisition of both radiance and polarization without moving parts. SPEXone has five viewing angles that are realized using a novel three-mirror segmented telescope assembly. The telescope focuses light captured by the five viewing angles onto a single image plane consisting of five stacked sub-slits. This multi-slit forms the entrance slit of a reflective grating spectrometer that consists of freeform mirrors and an order-sorting filter close to the focal plane, yielding an intrinsic spectral resolution of 2 nm and 5.4 km spatial resolution across the 100 km swath. The spectrometer re-images two spectral images per viewing angle following a dual beam spectral polarization modulation implementation.In this contribution, the optical performance of the telescope and spectrometer will be presented by means of star stimulus measurements at the slit plane and at the spectrometer focal plane. Measurements of the optical spot quality and preliminary measurements of stray light are compared with the optical design and with stray light simulations. We find that the measured optical performance of the telescope and spectrometer is better than modelled, showing higher resolution and lower slit keystone, thereby meeting all spatial and spectral resolution requirements. Also, preliminary stray light results indicate a higher diffuse but lower ghost contribution to the total stray light, which is in general beneficial for implementing stray light correction, which will enhance the polarimetric accuracy in inhomogeneous scenes.