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.
Recently a spectrometer concept has been invented which uses compressive sensing in combination with photonic crystal filters. Here we present an adaption of this concept in push-broom configuration for earth observation. This implementation allows for a compact design, while maintaining a high spatial resolution and high signal-to-noise ratio compared to other traditional implementations. The photonic crystals have a unique transmission profile and act as a spectral filter, which allows for the computational reconstruction of the input spectrum with a limited number of filters. We show, using simulations, that our approach is able to reconstruct input radiance spectra with high accuracy and assess the performance for different number of filter sets. We furthermore show proof-of-principle measurements of the transmission profile of a manufactured photonic crystal. Future research will focus on the effect of noise on the reconstruction algorithm as well as further filter set optimization by combining the filter selection process with trace gas concentration retrieval.
Aerosol quantification is of paramount importance for climate research, health and many other fields. The best method for measuring and characterizing aerosol from space is the application of a multi-angle polarimeter. A Dutch consortium has developed and delivered the so called SPEXone instrument for the NASA PACE (Plankton, Aerosol, Cloud, ocean Ecosystem) mission, to be launched early 2024. SPEXone is based on the polarization modulation of the spectrum, allowing full characterization of the state of linear polarization of the incoming light. Earth is viewed under five angles, producing ten modulated spectra, projected on a single detector. A polarimetric accuracy of 0.3% is achieved, with the instrument of about 10 dm3 volume and 10 kg mass. Based on the SPEXone design and experience, an upgraded instrument is being developed. Main change is the wider swath applied, from the 100 km swath for PACE to the present 250 km. This impacts the five telescopes, being integrated in one telescope unit. Other changes in the design are based on lessons learned, in particular the reduction and avoidance of stray light. The detector readout is adjusted for higher frame rate and more robust readout. These changes do not impact the instrument's budgets for mass, volume and power. In this paper, we will explain the principle of the SPEXone multi-angle spectropolarimeter instrument, the improvements with respect to the PACE version and its development status. The instrument can be flown as a stand-alone instrument for aerosol detection, as well as a support instrument where aerosol corrections are relevant, e.g., for high accuracy detection and quantification of methane and CO2.
In this paper, we discuss the setup of a confocal nanoscope in reflection using a super-oscillatory lens (SOL)which offers a sub-diffracted focal spot with an ultra-short depth of focus (≈100 nm). A tolerance of 20 nm defocussing in a 10 μm travel range is thus necessary, translating to an alignment requirement of the stage, with respect to the optical axis, below 2 mrad. We discuss an iterative procedure to fine tune the stage movement to achieve this requirement. We also demonstrate the necessity of the alignment by imaging a 5μm long 1D array of rectangular shaped Au nanostructures with a periodicity of 500 nm
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