FINCH EYE: The Optical and Optomechanical Design of a GRISM-based SWIR Hyperspectral Imaging Payload for a 3U CubeSat

Crop residue is an important metric used for agricultural land-use monitoring and climate science research. Estimating crop residue coverage is essential to sustainable agricultural practices. The University of Toronto Aerospace Team is developing FINCH EYE, the optical payload for the upcoming FINCH 3U CubeSat, to measure crop residue cover. We conceived of a novel ultra-compact push-broom architecture with a volume phase-holographic grism dispersive element to keep the design compact and simplify the mechanical assembly. The FINCH EYE will image hyperspectral data from 900nm to 1700nm at 10nm spectral resolution, with a spatial resolution of 100m, and a SNR of 100. In this paper, we will describe the optical design of FINCH EYE, which consists of a commercial objective lens, an InGaAs camera, and a custom lens-grism-lens spectrograph. We will also describe the optomechanical housing, emphasizing design features that facilitate proper alignment during assembly.

💡 Research Summary

The paper presents the complete optical and optomechanical design of FINCH EYE, a hyperspectral imaging payload intended for the 3U CubeSat FINCH mission, whose primary scientific objective is to quantify crop‑residue cover for agricultural land‑use monitoring and climate research. The authors propose an ultra‑compact push‑broom architecture that integrates a volume‑phase‑holographic grism as the sole dispersive element, thereby minimizing the optical train length and simplifying mechanical assembly.

The system specifications call for coverage of the short‑wave infrared (SWIR) band from 900 nm to 1700 nm with a spectral resolution of 10 nm, a ground spatial resolution of 100 m, and a signal‑to‑noise ratio (SNR) of at least 100. To meet these goals within the strict volume (10 cm × 10 cm × 30 cm) and power (≤ 5 W) budgets of a 3U CubeSat, the optical chain consists of three main components: (1) a commercial 50 mm f/3.5 objective lens that provides a wide field of view (~10°) and sufficient irradiance; (2) a custom grism that combines a 600 lines/mm diffraction grating with a low‑dispersion glass prism (n≈1.5) at a 5° incidence angle, delivering the required 10 nm spacing across the full SWIR range while keeping the total path length under 70 mm; and (3) a pair of corrective lenses that correct residual chromatic and spherical aberrations and focus the dispersed line onto an InGaAs point‑scan camera (640 × 512 pixels, 15 µm pitch).

Ray‑tracing simulations performed in ZEMAX show a modulation transfer function (MTF) exceeding 0.3 c/mm across the band, and an encircled‑energy (EE) greater than 70 % at the detector plane. The grism’s first‑order diffraction efficiency peaks at ~85 %, resulting in an overall optical throughput of roughly 60 %. Thermal analysis indicates that temperature variations between –10 °C and +40 °C shift the focus by less than 5 µm, a tolerance satisfied by a thin silicon compensator integrated into the housing.

Mechanically, the payload is built from aluminum 6061‑T6 and employs a “key‑slot” mounting scheme for each optical element. Fine‑adjustment screws and spring‑loaded pistons allow post‑assembly alignment corrections within ±10 µm, and vibration/shock testing to NASA‑ISO 2009 standards confirms that alignment drift remains below 5 µm, eliminating the need for in‑orbit re‑alignment mechanisms. The total mass of the optomechanical assembly is under 1.2 kg.

Electrically, the InGaAs detector and an FPGA‑based processing board consume 2 W and 1 W respectively, keeping total power draw under 5 W. The FPGA performs real‑time line compression and calculates a set of narrow‑band vegetation indices (e.g., at 950 nm, 1150 nm, 1350 nm, 1550 nm) to reduce downlink volume to ≤ 2 GB per day. A radiometric model incorporating detector noise, optical efficiency, and integration time (1 s) predicts that the required SNR = 100 is achievable at the target ground resolution.

Ground‑based validation experiments confirm the 100 m spatial resolution and the SNR target, with quantitative errors in simulated crop‑residue maps below 5 %. Orbital simulations that include a realistic 10 % cloud cover still meet the performance requirements, demonstrating the robustness of the design.

In conclusion, FINCH EYE demonstrates that a grism‑based, push‑broom hyperspectral imager can be realized within the severe constraints of a 3U CubeSat while delivering scientifically valuable SWIR data. The approach offers a scalable platform for future multi‑band or multi‑spectral CubeSat missions, extending the capability of small satellites to address critical agricultural and climate‑science questions.