TMAP: designing a thermal infrared multi-spectral imager for an Io mission

Abstract

TMAP is a high spatial resolution thermal imager, optimized for observing Io, derived from BepiColombo’s Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS). TMAP has three detectors with a beam split at image plane level: a multispectral imager using an uncooled microbolometer and two radiometer line detectors provide greater precision than the imager for background temperatures (80–150 K) as well as a precise temperatures measurement of active lava flows at 2μm. The TMAP microbolometer and radiometer operate primarily in a pushbroom imaging configuration. The optical design of TMAP is simplified from MERTIS, replacing the spectrometer with simple stripe filters on the microbolometer. Minor modifications of the three-mirror anastigmat (TMA) telescope optics and updating the microbolometer improve the spatial resolution by over 5x. TMAP has a fully reflective, gold-coated F/2.5 TMA optical path with 135-mm focal length. A flip-panel in the optical path between the detectors and mirror 3 serves as a calibration target and follows the standard design used in a wide range of flight instruments. TMAP will measure thermal infrared emission of SO2-free areas of silicate to characterize spectral shape and locate the Christiansen feature emissivity maximum with 8 stripe filters spanning 4.5–16 μm. TMAP will classify eruptions over a significant portion of Io using a 2μm filter on the radiometer and two 5μm bands of the bolometer. TMAP will measure the surface radiance both with the microbolometer between 5 and 16 μm and with the radiometer at wavelength ≥18μm.