1,721,376 research outputs found
Astrometric positions of comet P/Halley
No full textOne hundred astrometric positions of comet P/Halley (mostly in the pre-perihelion phase) have been derived from images taken at the Asiago and Merate Observatories. These positions have been used to determine a set of orbital elements for the comet that, although in good agreement with those of IHW Orbit Nr. 45, give smaller residuals
EFFECTS OF THERMAL DEFORMATION ON OPTICAL INSTRUMENTS FORSPACE APPLICATION
No full textOptical instruments for space missions work in hostile environment, it’s thus necessary to accurately study the
effects of ambient parameters variations on the equipment.
In particular optical instruments are very sensitive to ambient conditions, especially temperature. This variable
can cause dilatations and misalignments of the optical elements, and can also lead to rise of dangerous stresses
in the optics. Their displacements and the deformations degrade the quality of the sampled images.
In this work a method for studying the effects of the temperature variations on the performance of imaging
instrument is presented. The optics and their mountings are modeled and processed by a thermo-mechanical
Finite Element Model (FEM) analysis, then the output data, which describe the deformations of the optical
element surfaces, are elaborated using an ad hoc MATLAB routine: a non-linear least square optimization
algorithm is adopted to determine the surface equations (plane, spherical, nth polynomial) which best fit the data.
The obtained mathematical surface representations are then directly imported into ZEMAX for sequential
raytracing analysis. The results are the variations of the Spot Diagrams, of the MTF curves and of the
Diffraction Ensquared Energy due to simulated thermal loads.
This method has been successfully applied to the Stereo Camera for the BepiColombo mission reproducing
expected operative conditions.
The results help to design and compare different optical housing systems for a feasible solution and show that
it is preferable to use kinematic constraints on prisms and lenses to minimize the variation of the optical
performance of the Stereo Camera
Phobos grooves and impact craters: A stereographic analysis
No full textPhobos parallel grooves were first observed on Viking images 38. years ago and since then they have been greatly debated leading to several formation hypotheses. Nevertheless, none of them have been favoured and widely accepted. In this work, we provide a different approach, assuming that Phobos grooves can be the expression of fracture planes, and deriving their spatial distribution and orientation on 3D reconstructions, we point out that any origin related only to craters at Phobos surface should be ruled out, since the majority of the grooves is unrelated to any craters now present at its surface. This raises the intriguing possibility that such grooves, if expression of fracture planes, are remnant features of an ancient parent body from which Phobos could have originated. Such scenario has never been considered for Phobos, though this origin was already proposed for the formation of 433 Eros grooves (Buczkowski, D.L., Barnouin-Jha, O.S., Prockter, L.M. [2008]. Icarus 193, 39). If this idea holds true, the observed groove distribution could be explained as the result of possible major impacts on the larger parent body, which were inherited by the "Phobos shard"
Optical design and optimization of a hyperspectral-stereo camera for CubeSat
No full textThe Hyperspectral Stereo-Camera for CubeSat (HSCC) is a novel instrument for remote sensing able to simultaneously extract 4D information, three spatial and one spectral, using the two channels of a pushbroom stereo camera coupled to a suitable spectral filter. HSCC is designed to be a very compact instrument, compatible with CubeSat applications, to be suitable for planetary exploration as well for terrestrial environmental monitoring. In this paper we describe the optical design of HSCC, show its nominal optical performance, and give indications about the performed optimizations. The system consists of a very compact innovative layout (1-Unit CubeSat) aimed at optimizing overall dimensions and masses, in which the two stereo channels, looking forward and backward along track with respect to nadir, share both a three-mirror anastigmat telescope and a bidimensional detector. The spectral analysis of the observed target is realized by the combination of the pushbroom scan and a linear variable filter put in front of the sensor: in fact, each linear field of view projected on the sensor moves along the filter dispersion direction following the satellite orbit and is spectrally selected by the local filter bandpass. The spectral selection coupled to the three-dimensional scene obtained by the stereoscopic view provides a global 4D result, allowing to get simultaneously from a single instrument a complete set of information of the observed surface usually attainable by at least two different instruments, so avoiding the typically very cumbersome cross-calibration and co-registration issues
Distortion definition and correction in off-Axis systems
Full text linkOff-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
Micrometeoroids flux on the Moon
Full text linkContext. The Moon has a tenuous exosphere consisting of atoms that are ejected from the surface by energetic processes, including hypervelocity micrometeoritic impacts, photon-stimulated desorption by UV radiation, and ion sputtering.
Aims: We calculate the vapor and neutral Na production rates on the Moon caused by impacts of meteoroids in the radius range of 5-100 μm. We considered a previously published dynamical model to compute the flux of meteoroids at the heliocentric distance of the Moon.
Methods: The orbital evolution of dust particles of different sizes is computed with an N-body numerical code. It includes the effects of Poynting-Robertson drag, solar wind drag, and planetary perturbations. The vapor production rate and the number of neutral atoms released in the exosphere of the Moon are computed with a well-established formulation.
Results: The result shows that the neutral Na production rate computed following our model is higher than previous estimates. This difference can be due to the dynamical evolution model that we used to compute the flux and also to the mean velocity, which is 15.3 km s-1 instead of 12.75 km s-1 as reported in literature.
Conclusions: Until now, the micrometeoritic impacts have been considered a negligible source for the release of neutral sodium atoms into the exosphere compared to other mechanisms, but according to our calculations, the contribution may be 8% of the photo-stimulated desorption at the subsolar point, becoming similar in the dawn and dusk regions and dominant on the night side
Hyperspectral Data Compression Using Fully Convolutional Autoencoder
No full textIn space science and satellite imagery, better resolution of the data information obtained makes images clearer and interpretation more accurate. However, the huge data volume gained by the complex on-board satellite instruments becomes a problem that needs to be managed carefully. To reduce the data volume to be stored and transmitted on-ground, the signals received should be compressed, allowing a good original source representation in the reconstruction step. Image compression covers a key role in space science and satellite imagery and, recently, deep learning models have achieved remarkable results in computer vision. In this paper, we propose a spectral signals compressor network based on deep convolutional autoencoder (SSCNet) and we conduct experiments over multi/hyperspectral and RGB datasets reporting improvements over all baselines used as benchmarks and than the JPEG family algorithm. Experimental results demonstrate the effectiveness in the compression ratio and spectral signal reconstruction and the robustness with a data type greater than 8 bits, clearly exhibiting better results using the PSNR, SSIM, and MS-SSIM evaluation criteria
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