Optical Design and Analysis of Theia: A Scientific-Grade Multispectral Imager for Nanosatellites

This article describes the optical design and analysis of Theia: a miniature, scientific-grade, multispectral Earth observation imager for nanosatellites that can be used for quantitative remote sensing. The instrument is designed to take images with a 5% radiometric accuracy throughout its three-year lifetime. The sustained accuracy is achieved with a post-launch calibration module. The instrument can capture frames at a 33-m ground sampling distance at an orbit height of 650 km. The design can take diffraction-limited images with a modulation transfer function of at least 0.13 at the sensor’s Nyquist frequency, which is comparable with Sentinel-2’s MultiSpectral Instrument. The instrument works in two user-defined bands, fits inside one CubeSat unit, and weighs 600 g. The successful analysis of the system is presented, including optical performance, radiometric, stray light, and tolerance analyses.