SPEXone is a compact multi-angle spectropolarimeter that measures both spectral intensity and the state of linear polarization of light scattered by aerosols in the Earth’s atmosphere at five different viewing angles simultaneously. This enables a very accurate quantification and characterization of atmospheric aerosols, helping us to better understand their effects on global climate and air quality. Building upon the success of its predecessor SPEXone, which has been launched in 2024 as part of the NASA PACE observatory, a second and improved instrument, SPEXone Second Generation, has been built within the ESA PRODEX program. Most recently, the integrated instrument underwent full on-ground characterization and calibration in ambient conditions at SRON. This contribution gives an overview of the measurements and presents preliminary results from the characterization and calibration campaign, focusing on the instrument performance. A few key performance aspects such as straylight, spatial and spectral resolution are discussed, with data from SPEXone for PACE serving as a comparison. The result of the analysis shows excellent image quality and indicates an improvement in the amount of diffuse straylight.
In this contribution, the detector-characterization results and some of the on-ground calibration plans are presented for an adjusted and improved SPEXone satellite instrument. SPEXone is a highly compact multi-angle space spectro-polarimeter developed by a Dutch consortium for the NASA PACE observatory scheduled for launch early 2024. This instrument will enable detailed characterization of the microphysical properties of fine particulate matter or aerosols in the atmosphere from low Earth orbit, which is essential for climate, ecosystem, and human-health science. A successor to the SPEXone instrument is currently being developed, with a wider swath as the main change (250 km instead of 100 km), and with several design improvements to reduce straylight. The detector firmware was adjusted to enable the required higher frame rate, and to make the readout more robust. The detector was characterized in a similar way as for PACE, though even more extensively based on lessons learned. In particular, full illumination measurements were complemented with partial illumination measurements, where parts of the detector are covered using dedicated detector masks, to investigate peculiar signal-induced offset effects that were observed only late for PACE. Additionally, direct memory measurements were performed using time-dependent illumination generated using a fast electronic shutter. Following the detector characterization, instrument-calibration preparations have started. The instrument will be fully calibrated in ambient, complemented with a highly selective set of measurements in vacuum. The approach followed will be similar to PACE, but modifications will be made to deal with the increased swath. Important improvements will be implemented to improve the data quality, such as increased number of wavelengths for straylight measurements.
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.
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