ƒISPEx, the Integral-Field Imager and Spectrometer for Planetary Exploration

With the introduction of visible and infrared Imaging Spectrometers about 20 years ago, we have assisted to a dramatic enhancement of the scientific return achieved by space missions launched for the exploration of the Solar System. This innovative class of instruments are in fact able to conjugate together the imaging capabilities of the cameras with the spectral resolution accomplished by spectrographs. By exploiting such capabilities, planetary scientists can retrieve and map the physical and chemical properties of a target body and correlate them with surface morphological features or with dynamical structures of the atmospheres. It is not by chance that such instruments have become an essential payload on many planetary exploration missions since they have demonstrated their capabilities to allow a better understanding of the composition, processes and evolution of Solar System bodies. In the meanwhile, ongoing photonic research has made available new devices whose technology is mature enough to allow to investigate new optical concepts for color cameras and imaging spectrometers architectures to be implemented in a single integrated instrument (⨏ISPEx), which we present here. The ⨏ISPEx camera design uses a Liquid Crystal Tunable Filter (LCTF) to ensure high flexibility in the selection of the bandpass (between 0.42-0.73 µm) and bandwidth (35 nm at 0.55 µm) of the color filters, which can then be adapted to very different scientific targets. The camera has a broad 3.3°x1.6° FOV and an IFOV of 14 µm allowing to reach a spatial resolution of 0.14 m/px from 10 km distance. Conversely, the ⨏ISPEx 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 use Coded Mask Optical Reformatters (CMOR) based on optical fibers bundles to collect the full hyperspectral cube through a single acquisition resulting in a great reduction of the acquisition time with respect to traditional whiskbroom and pushbroom instruments. The FOV/IFOV are 0.4° (circular)/100 µrad and 0.28°x0.28° (square)/225 µrad for the VIS and IR channel spectrometers, respectively. These values correspond to a 70 m-wide circular hyperspectral image made of 4500 pixels at 1 m/px resolution for the VIS channel and to a 50x50 m image made of 484 pixels at 2.25 m/px for the IR channel for acquisitions made from a distance of 10 Km. These requirements are optimized for remote sensing of asteroids and comets from close distances. Camera’s and spectrometers’ FOV are co-located on the same boresight thanks to the use of a common Three Mirror Afocal Telescope operating with a 2.4X beam reduction factor (entrance pupil diameter 84 mm). By means of a beamsplitter and a dichroic filter the collimated beam generated by the telescope is split to the camera and spectrometers’ pupils. Thanks to the solutions we are developing, the camera and imaging spectrometers will operate at the same time on the same boresight, resulting in a great simplification of their operations. In this respect, the ⨏ISPEx will offer the advantage to allow a 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 possible to apply sharpening algorithms to spectrometer’s ones. Apart this, an integral-field spectrometer will keep the capabilities of more traditional whiskbroom and pushbroom 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 temporal scale of the investigated event. This is the case of observations enquiring into the dynamical evolution of planetary atmospheres, lightning events, outbursts, hypervelocity 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 flyby phases, where the distance from the target and illumination geometry are rapidly changing, resulting in limited time 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 flexibility similar to the one achieved by a camera. Within this study we are defining the configuration of the ⨏ISPEx 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 limited to the 0.4-1.0 µm spectral range to conduct performance tests at system level on LCTF and CMOR devices. We gratefully acknowledge financial contribution from the Agreement ASI-INAF n.2018-16-HH.0.