In the framework of the ESA-JAXA BepiColombo mission to Mercury, the global mapping of the planet will be performed by the on-board Stereo Camera (STC), part of the SIMBIO-SYS suite . In this paper we propose a new technique for the validation of the 3D reconstruction of planetary surface from images acquired with a stereo camera.
STC will provide a three-dimensional reconstruction of Mercury surface. The generation of a DTM of the observed features is based on the processing of the acquired images and on the knowledge of the intrinsic and extrinsic parameters of the optical system.
The new stereo concept developed for STC needs a pre-flight verification of the actual capabilities to obtain elevation information from stereo couples: for this, a stereo validation setup to get an indoor reproduction of the flight observing condition of the instrument would give a much greater confidence to the developed instrument design.
STC is the first stereo satellite camera with two optical channels converging in a unique sensor. Its optical model is based on a brand new concept to minimize mass and volume and to allow push-frame imaging. This model imposed to define a new calibration pipeline to test the reconstruction method in a controlled ambient. An ad-hoc indoor set-up has been realized for validating the instrument designed to operate in deep space, i.e. in-flight STC will have to deal with source/target essentially placed at infinity.
This auxiliary indoor setup permits on one side to rescale the stereo reconstruction problem from the operative distance in-flight of 400 km to almost 1 meter in lab; on the other side it allows to replicate different viewing angles for the considered targets.
Neglecting for sake of simplicity the Mercury curvature, the STC observing geometry of the same portion of the planet surface at periherm corresponds to a rotation of the spacecraft (SC) around the observed target by twice the 20° separation of each channel with respect to nadir. The indoor simulation of the SC trajectory can therefore be provided by two rotation stages to generate a dual system of the real one with same stereo parameters but different scale.
The set of acquired images will be used to get a 3D reconstruction of the target: depth information retrieved from stereo reconstruction and the known features of the target will allow to get an evaluation of the stereo system performance both in terms of horizontal resolution and vertical accuracy.
To verify the 3D reconstruction capabilities of STC by means of this stereo validation set-up, the lab target surface should provide a reference, i.e. should be known with an accuracy better than that required on the 3D reconstruction itself. For this reason, the rock samples accurately selected to be used as lab targets have been measured with a suitable accurate 3D laser scanner.
The paper will show this method in detail analyzing all the choices adopted to lead back a so complex system to the indoor solution for calibration.
BepiColombo is one of the cornerstone missions of the European Space Agency dedicated to the exploration of the planet Mercury and it is expected to be launched in July 2016.
One of the BepiColombo instruments is the STereoscopic imaging Channel (STC), which is a channel of the Spectrometers and Imagers for MPO BepiColombo Integrated Observatory SYStem (SIMBIOSYS) suite: an integrated system for imaging and spectroscopic investigation of the Mercury surface. STC main aim is the 3D global mapping of the entire surface of the planet Mercury during the BepiColombo one year nominal mission.
The STC instrument consists in a novel concept of stereocamera: two identical cameras (sub-channels) looking at ±20° from nadir which share most of the optical components and the detector. Being the detector a 2D matrix, STC is able to adopt the push-frame acquisition technique instead of the much common push-broom one.
The camera has the capability of imaging in five different spectral bands: one panchromatic and four intermediate bands, in the range between 410 and 930 nm.
To avoid mechanisms, the technical solution chosen for the filters is the single substrate stripe-butted filter in which different glass pieces, with different transmission properties, are glued together and positioned just in front of the detector.
The useful field of view (FoV) of each sub-channel, though divided in 3 strips, is about 5.3° x 3.2°. The optical design, a modified Schmidt layout, is able to guarantee that over all the FoV the diffraction Ensquared Energy inside one pixel of the detector is of the order of 70-80%.
To effectively test and calibrate the overall STC channel, an ad hoc Optical Ground Support Equipment has been developed. Each of the sub-channels has to be separately calibrated, but also the data of one sub-channel have to be easily correlated with the other one.
In this paper, the experimental results obtained by the analysis of the data acquired during the preliminary onground optical calibration campaign on the STC Flight Model will be presented.
This analysis shows a good agreement between the theoretical expected performance and the experimental results.
The Stereo Camera (STC), mounted on-board the BepiColombo spacecraft, will acquire in push frame stereo mode the entire surface of Mercury. STC will provide the images for the global three-dimensional reconstruction of the surface of the innermost planet of the Solar System. The launch of BepiColombo is foreseen in 2018. STC has an innovative optical system configuration, which allows good optical performances with a mass and volume reduction of a factor two with respect to classical stereo camera approach. In such a telescope, two different optical paths inclined of ±20°, with respect to the nadir direction, are merged together in a unique off axis path and focused on a single detector. The focal plane is equipped with a 2k x 2k hybrid Si-PIN detector, based on CMOS technology, combining low read-out noise, high radiation hardness, compactness, lack of parasitic light, capability of snapshot image acquisition and short exposure times (less than 1 ms) and small pixel size (10 μm).
During the preflight calibration campaign of STC, some detector spurious effects have been noticed. Analyzing the images taken during the calibration phase, two different signals affecting the background level have been measured. These signals can reduce the detector dynamics down to a factor of 1/4th and they are not due to dark current, stray light or similar effects.
In this work we will describe all the features of these unwilled effects, and the calibration procedures we developed to analyze them.
The STereoscopic imaging Channel (STC) is one of the instruments on-board the BepiColombo mission, which is an ESA/JAXA Cornerstone mission dedicated to the investigation of the Mercury planet. STC is part of the Spectrometers and Imagers for MPO BepiColombo Integrated Observatory SYStem (SIMBIO-SYS) suite. STC main scientific objective is the 3D global mapping of the entire surface of Mercury with a mean scale factor of 55 m per pixel at periherm.
To determine the design requirements and to model the on-ground and in-flight performance of STC, a radiometric model has been developed. In particular, STC optical characteristics have been used to define the instrument response function. As input for the model, different sources can be taken into account depending on the applications, i.e. to simulate the in-flight or on-ground performances. Mercury expected radiance, the measured Optical Ground Support Equipment (OGSE) integrating sphere radiance, or calibrated stellar fluxes can be considered.
Primary outputs of the model are the expected signal per pixel expressed in function of the integration time and its signal-to-noise ratio (SNR). These outputs allow then to calculate the most appropriate integration times to be used during the different phases of the mission; in particular for the images taken during the calibration campaign on-ground and for the in-flight ones, i.e. surface imaging along the orbit around Mercury and stellar calibration acquisitions.
This paper describes the radiometric model structure philosophy, the input and output parameters and presents the radiometric model derived for STC. The predictions of the model will be compared with some measurements obtained during the Flight Model (FM) ground calibration campaign. The results show that the model is valid, in fact the foreseen simulated values are in good agreement with the real measured ones.
The ESA-JAXA mission BepiColombo that will be launched in 2018 is devoted to the observation of Mercury, the innermost planet of the Solar System. SIMBIOSYS is its remote sensing suite, which consists of three instruments: the High Resolution Imaging Channel (HRIC), the Visible and Infrared Hyperspectral Imager (VIHI), and the Stereo Imaging Channel (STC). The latter will provide the global three dimensional reconstruction of the Mercury surface, and it represents the first push-frame stereo camera on board of a space satellite. Based on a new telescope design, STC combines the advantages of a compact single detector camera to the convenience of a double direction acquisition system; this solution allows to minimize mass and volume performing a push-frame imaging acquisition. The shared camera sensor is divided in six portions: four are covered with suitable filters; the others, one looking forward and one backwards with respect to nadir direction, are covered with a panchromatic filter supplying stereo image pairs of the planet surface. The main STC scientific requirements are to reconstruct in 3D the Mercury surface with a vertical accuracy better than 80 m and performing a global imaging with a grid size of 65 m along-track at the periherm. Scope of this work is to present the on-ground geometric calibration pipeline for this original instrument. The selected STC off-axis configuration forced to develop a new distortion map model. Additional considerations are connected to the detector, a Si-Pin hybrid CMOS, which is characterized by a high fixed pattern noise. This had a great impact in pre-calibration phases compelling to use a not common approach to the definition of the spot centroids in the distortion calibration process. This work presents the results obtained during the calibration of STC concerning the distortion analysis for three different temperatures. These results are then used to define the corresponding distortion model of the camera.
Off-axis optical configurations are becoming more and more used in a variety of applications, in particular they are the most preferred solution for cameras devoted to Solar System planets and small bodies (i.e. asteroids and comets) study. Off-axis designs, being devoid of central obstruction, are able to guarantee better PSF and MTF performance, and thus higher contrast imaging capabilities with respect to classical on-axis designs. In particular they are suitable for observing extended targets with intrinsic low contrast features, or scenes where a high dynamical signal range is present. Classical distortion theory is able to well describe the performance of the on-axis systems, but it has to be adapted for the off-axis case. A proper way to deal with off-axis distortion definition is thus needed together with dedicated techniques to accurately measure and hence remove the distortion effects present in the acquired images. In this paper, a review of the distortion definition for off-axis systems will be given. In particular the method adopted by the authors to deal with the distortion related issues (definition, measure, removal) in some off-axis instruments will be described in detail.
The research group with the responsibility of the STereo Camera (STC) for the ESA BepiColombo mission to Mercury, has realized an innovative and compact camera design in which the light collected independently by two optical channels at ±20° with respect to the nadir direction converges on unique bidimensional detector. STC will provide the 3Dmapping of Mercury surface, acquiring images from two different perspectives. A stereo validation setup has been developed in order to give a much greater confidence to the novel instrument design and to get an on ground verification of the actual accuracies in obtaining elevation information from stereo pairs. A series of stereo-pairs of an anorthosite stone sample (good analogue of the hermean surface) and of a modelled piece of concrete, acquired in calibration clean room by means of an auxiliary optical system, have been processed in the photogrammetric pipeline using image correlation for the 3D model generation. The stereo reconstruction validation has been performed by comparing the STC DTMs (Digital Terrain Models) to an high resolution laser scanning 3D model of the stone samples as reference data. The latter has a much higher precision (ca. 20 μm) of the expected in-lab STC DTM (190 μm). Processing parameters have been varied in order to test their influence on the DTM generation accuracy. The main aim is to define the best illumination conditions and the process settings in order to obtain the best DTMs in terms of accuracy and completeness, seeking the best match between the mission constraints and the specific matching aspects that could affect the mapping process.
In the context of a stereo-camera, measuring the image quality allows to define the accuracy of the 3D reconstruction. In fact, depending on the precision of the camera position data, on the kind of reconstruction algorithm, and on the adopted camera model, it determines the vertical accuracy of the reconstructed terrain model. Aim of this work is to describe the results and the method implemented to determine the Line Spread Function (LSF) of the Stereoscopic Channel (STC) of the SIMBIOSYS imaging system for the BepiColombo mission. BepiColombo is the cornerstone mission n.5 of the European Space Agency dedicated to the exploration of the innermost planet of the Solar System, Mercury, and it is expected to be launched in 2016. STC is a double push-frame single-detector camera composed by two identical sub-channels looking at ±21° wrt the nadir direction. STC has been designed so to have many optical elements common to both sub-channels. Also the image focal plane is common to the sub-channels and this permits the use of a single detector for the acquisition of the two images, i.e. one for each viewing direction. Considering the novelty of the design, conceived to sustain a harsh environment and to be as compact as possible, the STC unit is very complex. To obtain the most accurate 3D reconstruction of the Mercury surface, a camera model as precise as possible is needed, and an ad-hoc calibration set-up has been designed to calibrate the instrument both from the usual geometrical and radiometrical points of view and more specifically for the instrument stereo capability. In this context LSF estimation was performed with a new method applying a particular oversampling approach for the curve fitting to determine at first the entire calibration system transfer function and at the end the optical properties of the single instrument.
The Stereo Camera (STC) of the SIMBIO-SYS imaging suite of the BepiColombo ESA mission to Mercury is based on
an innovative and compact design in which the light independently collected by two optical channels at ±20° separation
with respect to nadir falls on a common bidimensional detector. STC adopts a novel stereo acquisition mode, based on
the push-frame concept, never used before on a space mission. To characterize this camera for obtaining the most
accurate data of the Mercury surface, standard calibration measurements have been performed. In addition, we also
wanted to demonstrate and characterize the capability of the instrument to reconstruct a 3D surface with the desired
accuracy by means of the stereo push-frame concept. To this end, a lab setup has been realized with an evaluation model
of STC, in which the problem of working at an essentially infinite object distance over hundred km baselines has been
overcome by means of a simple collimator and two precision rotators. The intrinsic and extrinsic parameters of the
camera have been obtained with standard stereo procedures, adapted to the specific case. The stereo validation has been
performed by comparing the shape of the target object accurately measured by laser scanning, with the shape
reconstructed by applying the adopted stereo algorithm to the acquired image pairs. The obtained results show the
goodness of this innovative validation technique, that will be applied also for validating the stereo capabilities of STC
We present the application of the shape-from-silhouette algorithm to reconstruct the 3D profile of handworks from a set
of X-ray absorption images taken at different angles around the object. The acquisition technique is similar to
tomography, but the number of images that are required to reconstruct the 3D appearance is very low compared to
tomography, therefore the acquisition time is substantially reduced. Some reference points are placed on a structure corotating
with the object and are acquired on the images for calibration and registration. The shape-from-silhouette
algorithm gives finally the 3D appearance of the object. We present the analysis of a tin pendant from the Venetic area,
VI century b.C., that was completely hidden by corrosion products and solid ground at the moment of the retrieval. The
3D reconstruction shows that the pendant is a very elaborated piece, with two embraced figures that were completely
invisible before restoration.
In the field of restoration of ancient handworks, X-ray tomography is a powerful method to reconstruct the internal
structure of the object in non-invasive way. In some cases, such as small objects fully realized with hard metals and
completely hidden by clay or products of oxidation, the tomography, although necessary to obtain the 3D appearance of
the object, does not give any additional information on its internal monolithic structure. We present here the application
of the shape-from-silhouette technique on X-ray images to reconstruct the 3D profile of handworks. The acquisition
technique is similar to tomography, since several X-ray images are taken while the object is rotated. Some reference
points are placed on a structure co-rotating with the object and are acquired on the images for calibration and
registration. The shape-from-silhouette algorithm gives finally the 3D appearance of the handwork. We present the
analysis of a tin pendant of VI-VIII century b.C. (Venetian area) completely hidden by solid ground. The 3D
reconstruction shows surprisingly that the pendant is a very elaborated piece, with two embraced figures that were
completely invisible before restoration.