We present FYMOS, an all-aluminum, robust, light weight, freeform based, near infrared hYperspectral imager for MOisture Sensing. FYMOS was designed and built to remotely measure moisture content using spectral features from 0.7-1.7µm integrating an InGaAs sensor. The imaging system, operating at F/2.8, is based on the three-concentric-mirror (Offner) spectrograph configuration providing a spectral resolution of 8 nm optimized for broad spectral coverage with sufficient resolution to make assessments of water levels. To optimize the optical performance, whilst minimizing weight and size, the design incorporates a bespoke freeform blazed grating machined on a commercial 5 axis ultra precision diamond machine. We achieve a 30% improvement on the RMS wavefront error in the spatial and spectral fields compared to a conventional Offner-Chrisp design with similar aperture and the monolithic Primary/Tertiary mirror eases the manufacturing assembly whilst minimizing weight. We demonstrate the performance of FYMOS by measuring the evaporation rate of water on a soil sample and results are processed with a physical multilayer radiative transfer model (MARMIT) to estimate the mean water thickness.

中文翻译：

---

基于自由形式的高光谱成像仪，用于湿度感测（FYMOS）

我们提出了FYMOS，这是一种用于湿度感测的全铝，坚固，重量轻，基于自由形式的近红外高光谱成像仪。FYMOS的设计和制造是利用集成InGaAs传感器的0.7-1.7µm光谱特征来远程测量水分含量。该成像系统以F / 2.8运行，基于三同心镜（Offner）光谱仪配置，可提供8 nm的光谱分辨率，该光谱分辨率针对宽光谱范围进行了优化，并具有足够的分辨率来评估水位。为了优化光学性能，同时最小化重量和尺寸，该设计采用了在商用5轴超精密金刚石机床上加工的定制自由曲面闪耀光栅。与具有类似孔径的传统Offner-Chrisp设计相比，与传统的Offner-Chrisp设计相比，我们在空间和光谱领域的RMS波前误差提高了30％，并且单片主/第三镜简化了制造装配，同时使重量最小。我们通过测量土壤样品上水的蒸发速率来证明FYMOS的性能，并使用物理多层辐射传输模型（MARMIT）处理结果以估计平均水厚。