Space-based spectrometers are of high importance for Earth observation and greenhouse gas sensing. We present a novel freeform pushbroom imaging spectrometer, covering the visible (400 – 1100 nm), near-infrared (1100 – 1700 nm) and thermal (8 – 14 μm) wavelength range, showing a full field-of-view of 120°, while fitting within a single CubeSat Unit. The design is composed of a freeform 2-mirror telescope, followed by a freeform spectrometer unit. The freeform telescope guides the light to the spectrometer entrance slit, after which a collimating mirror is present, and the light is split to the 3 spectrometer channels. Each spectrometer channel focuses the light onto a 2D detector providing both spatial and spectral information. The visible spectrometer channel comprises a transmission diffraction grating followed by 2 freeform mirrors, enabling a nadir spatial resolution of 9.87 km and a spectral resolution of 5.18 nm. The near-infrared and thermal spectrometer each comprise a reflective diffraction grating and 2 freeform mirrors, featuring a nadir spatial resolution of 4.13 km and 9.31 km, and a spectral resolution of 2.74 nm and 48.27 nm, respectively. All mirrors are described using XY polynomials, up to the 4th order, enabling a close to diffraction-limited performance. Consequently, this design might benefit future space missions enabling an improved Earth observation and climate monitoring.
Earth observation and greenhouse gas sensing from space provides vital information for climate and climate change monitoring, indicating the importance of novel spaceborne telescopes and spectrometers. We present a novel freeform pushbroom imaging spectrometer enabling the sensing of water vapor, carbon dioxide and methane in the atmosphere, while fitting within 2 CubeSats Units. The design comprises a 2-mirror freeform telescope, combined with a near-infrared (1100 – 1700 nm) spectrometer featuring 3 freeform mirrors and a reflective grating, providing both spatial and spectral information using a 2D detector. All mirrors are described and optimized using XY polynomials, enabling a nearly diffraction-limited performance. The novel design is exceeding the state-of-the-art, by showing a full FOV of 120°, a spatial resolution of 2.6 km, and a spectral resolution of 13 nm. According to our knowledge, our novel design shows the widest field-of-view that has ever been realized for space-based telescopes, nearly reaching Earth observation from limb to limb from an altitude of about 700 km. The freeform telescope mirrors were manufactured in-house using high-precision 5-axis milling and 5-axis ultraprecision diamond tooling. Finally, a laboratory proof-of-concept demonstrator was realized validating the field-of-view and focusing spot sizes, paving the way for future space missions that target wide field-ofview imaging and/or an enhanced climate monitoring.
Wide field-of-view imaging optics offers a huge potential for space-based Earth observation and climate change monitoring by capturing global data. We present the design and proof-of-concept demonstration of a freeform 2-mirror space-based telescope featuring a full field-of-view of 120°, nearly reaching Earth observation from limb to limb from a nominal altitude of 700 km, while showing a spatial resolution of 2.6 km, and fitting within 1 CubeSat unit. Our design benefits from freeform optics to maximize the field-of-view, while maintaining a diffraction-limited image quality and minimizing the system dimensions. Particularly, both mirror surfaces were accurately modelled and optimized using an XY polynomial description. Subsequently, the mirrors are manufactured using high-precision 5-axis milling and ultraprecision diamond tooling, after which a laboratory demonstrator setup of the telescope was realized. We believe this design paves the way towards future space missions enabling an improved Earth observation, leading to an enhanced monitoring of climate and climate change.
Climate change monitoring is still a major challenge, which is currently typically addressed using radiometers monitoring the radiative fluxes at the top of the atmosphere. To improve the current state-of-the-art monitoring instruments, we pursue the development of novel space instrumentation, combining a radiometer with two additional imagers, improving the spatial resolution to a few kilometers allowing scene identification, while enabling a spectral distinction between the reflected solar radiation (RSR) using a visible to near-infrared (400 – 1100 nm) camera, and the Earth’s emitted thermal radiation using a thermal infrared (8 – 14 μm) camera. In this paper, we present a novel camera design optimized towards RSR monitoring, while targeting a compact design and minimizing the number of aspheric components. More specifically, our optimized imaging design shows a wide field of view (138°) enabling to observe the Earth from limb to limb, a compact volume fitting within 1 CubeSat Unit (1U), a wide spectral range (400 – 900 nm) to retrieve the RSR with a certainty of more than 95%, a spatial resolution better than 5 km at nadir, and a close to diffraction-limited performance. After optimization towards the nominal design, possible design alternatives are considered and discussed, enabling a cost-efficient design choice. Following, the mechanical and optical design tolerances are evaluated using a statistical Monte Carlo analysis, indicating a robust and tolerant design that can be manufactured using ultra-precision diamond tooling. Finally, stray-light analysis was performed enabling evaluation of ghost reflection and evaluating the necessity of an anti-reflection coating. Consequently, we can conclude our proposed imaging designs show a promising performance optimized towards Earth observation, paving the way to an improved climate change monitoring.
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