Proceedings Article | 29 August 2022
G. Filacchione, M. Tarabini, E. Mazzotta Epifani, M. Ciarniello, G. Piccioni, A. Raponi, B. Saggin, Z. Kanuchova, P. Palumbo, I. Guerri, A. Taiti, I. Ficai-Veltroni, M. Barilli, S. Pelli, F. Cosi, A. Galiano, M. Zambelli, D. Biondi, A. Boccaccini, F. Nuccilli, M. Giusti
KEYWORDS: Spectroscopy, Sensors, Objectives, Infrared spectroscopy, Imaging systems, Cameras, Asteroids, Telescopes, Infrared imaging, Infrared telescopes
The ⨏ISPEx instrument is an innovative optical payload for planetary exploration which integrates a Color Camera and an Integral Field Spectrometer. The instrument uses a Three Mirror Afocal Telescope operating with a 2.4X beam reduction factor (input pupil diameter 84 mm, output pupil diameter 35 mm) to feed a collimated light beam to the camera's and spectrometers' entrance pupils. The FOVs of the two channels are aligned on a common boresight utilizing a beamsplitter and a dichroic filter. This solution, apart from saving mass by adopting a single telescope, allows operating the camera and imaging spectrometer in unison resulting in a great simplication of their operations. By selecting the polarization voltage applied to a Liquid Crystal Tunable Filter (LCTF) placed on the entrance pupil of the camera objective, it is possible to select the optical bandpass (between 0.42-0.73 μm) and the corresponding bandwidth (from 16 nm at 0.42 μm to 48 nm at 0.73 μm). This solution allows avoiding mechanical parts like in filters wheels while guaranteeing high spectral sampling. The camera optical design ensures a 3.3° x 1.6° FOV and an IFOV of 14 μrad corresponding to a spatial resolution of 0.14 m/px from a 10 km distance. The spectral channel is further divided in the VIS (0.4-1.05 μm spectral range, 3.2 nm/band sampling) and IR (0.95-5.0 μm, 8 nm/band) integral field imaging spectrometers. These channels use custom-made Coded Mask Optical Reformatters (CMOR) built from optical fibers bundles to collect the hyperspectral cube through a single acquisition. This innovative solution affords a great saving of the acquisition time with respect to instruments operating in whiskbroom or pushbroom mode. The FOV/IFOV are 0.4° (circular)/100 μrad and 0.28° x 0.28° (square)/225 μrad for the VIS and IR spectrometers, respectively. From a 10 km distance, the VIS spectrometer builds a 70 m-wide circular hyperspectral image made of 4000 pixels at 1 m/px resolution from 10 km distance; the IR channel acquires a 50x50 m image made of 484 pixels at 2.25 m/px. These requirements have been optimized for remote sensing of asteroids and comets from close distances but the instrument can be adapted also for other observation scenarios. In this respect, the ISPEx will offer the advantage to allow better exploitation of the data collected by the three channels resulting in a tremendous advantage for many scientific investigations. With this synergic approach, it will be possible to analyze high-resolution images to constrain morphology interpretation of a target, while hyperspectral data collected at the same time allow the retrieval of composition and physical properties. The availability of camera images makes it possible to apply sharpening algorithms to spectrometer ones. Apart from this, an integral-field spectrometer will keep the capabilities of more traditional whiskbroom and push broom spectrometers but it will overcome them when the target scene is rapidly evolving and changing during the acquisition: traditional instruments are limited by the fact that the cube acquisition process may take a time longer than the time scale of the investigated event. This is the case of observations enquiring into the dynamical evolution of planetary atmospheres, lightning events, outbursts, hyper-velocity impact, or fast-moving targets. By operating with fast readout detectors, an Integral Field spectrometer can adequately resolve the four dimensions of data (2D spatial, spectral and temporal) opening the possibility to perform time-resolved hyperspectral movies. Another substantial advantage of Integral Field spectrometers is their better operability during fast yby phases, where the distance from the target and illumination geometry are rapidly changing, resulting in limited periods suitable to observe the target with optimal conditions. By collecting the entire hyperspectral cube in a fraction of the time necessary to complete the scan for a traditional scanning spectrometer, an Integral Field spectrometer can reach a level of imaging exibility similar to the one achieved by a camera. Within this study, we are defining the configuration of the ISPEx5:0μm 0:4 space model operating in the 0.4-5.0 μm spectral range including optical performance analyses, and thermomechanical and electronic architecture. Moreover, we are realizing a development breadboard ISPEx1:0μm 0:4 limited to the 0.4-1.0 μm spectral range to conduct performance tests at the system level on LCTF and CMOR devices.
