Optical design of a freeform hyperspectral camera for CubeSats

This article explores the development of a compact freeform hyperspectral imaging system designed to fit within a 2U CubeSat form factor, aimed at terrestrial observation. The proposed instrument operates over a spectral bandwidth ranging from 400nm to 1100nm, with a field of view (FoV) of 14°. The primary design objective is to achieve both high spectral and spatial resolution, while strictly adhering to the volume constraints inherent to CubeSat platforms. Additionally, the system must deliver high optical performance in terms of Modulation Transfer Function (MTF) and minimize Keystone and Smile distortions. The final optical layout consists of a reflective telescope based on a Three-Mirror Anastigmat (TMA) configuration, coupled with a spectrometer designed in an Offner-Chrisp configuration. To enhance imaging performance and correct for higher-order aberrations within the limited space envelope, freeform optics were employed, specifically, Zernike Fringe polynomials were applied to all mirror surfaces. This approach allows for better control of wavefront errors and facilitates compact folding of the optical path, which is essential for integration into the CubeSat's constrained volume. A final MTF greater than 0.56 was achieved at 1100nm; Smile and Keystone spectral distortion were kept below 2nm and 0.39nm, respectively. The combination of advanced optical design and hyperspectral capabilities in a miniaturized package presents a promising solution for Earth observation applications.