THERMAP: a mid-infrared spectro-imager based on an uncooled micro-bolometer for space missions to small bodies of the solar system

Abstract

We report on the feasibility study of a mid-infrared (8-18 µm) spectro-imager called THERMAP, based on an uncooled micro-bolometer detector array. Due to the recent technological development of these detectors, which have undergone significant improvements in the last decade, we wanted to test their performances for the Marco Polo R ESA Cosmic Vision mission. In this study, we demonstrate that the new generation of uncooled micro-bolometer detectors has all the imaging and spectroscopic capabilities to fulfill the scientific objectives of this mission. In order to test the imaging capabilities of the detector, we set up an experiment based on a 640x480 ULIS micro-bolometer array, a germanium objective and a black body. Using the results of this experiment, we show that calibrated radiometric images can be obtained down to at least 255 K (lower limit of our experiment), and that two calibration points are sufficient to determine the absolute scene temperature with an accuracy better than 1.5 K. Adding flux attenuating neutral density mid-infrared filters (transmittance: 50%, 10%, 1%) to our experiment, we were able to evaluate the spectroscopic performances of the detector. Our results show that we can perform spectroscopic measurements in the wavelength range 8-16 µm with a spectral resolution of R~40-80 for a scene temperature <300 K, the typical surface temperature of a Near Earth Asteroid at 1 AU from the Sun. The mid-infrared spectro-imager THERMAP, based on the above detector, is therefore well suited for the Marco Polo R mission.