Giampiero Naletto

First Results from the SOHO Ultraviolet Coronagraph Spectrometer

The SOHO Ultraviolet Coronagraph Spectrometer (UYCS/SOHO) is being used to observe the extended solar corona from 1.25 to 10 R from Sun center. Initial observations of polar coronal holes and equatorial streamers are described. The observations include measurements of spectral line profiles for H I Lα and Lβ, O VI 1032 A and 1037 A, Mg × 625 A, Fe XII 1242 A and several others. Intensities for Mg × 610 A, Si XII 499 A, and 520 A, S × 1196 A, and 22 others have been observed. Preliminary results for derived H0, O5+, Mg9+, and Fe11+ velocity distributions and initial indications of outflow velocities for O5+ are described. In streamers, the H0 velocity distribution along the line of sight (specified by the value at e-1, along the line of sight) decreases from a maximum value of about 180 km s-1 at 2 R to about 140 km s-1 at 8 R. The value for O5+ increases with height reaching a value of 150 km s-1 at 4.7 R. In polar coronal holes, the O5+ velocity at e-1 is atout equal to that of H0 at 1.7 R and significantly larger at 2.1 R. The O5+ in both streamers and coronal holes were found to have amsotropic velocity distributions with the smaller values in the radial direction.

E-type Asteroid (2867) Steins as Imaged by OSIRIS on Board Rosetta

Smooth Space Pebble In September 2008, on its way to meet comet 67P/Churyumov-Gerasimenko, the Rosetta spacecraft flew by asteroid Steins, a member of a very rare class of asteroids that had never been observed closely by spacecraft. Keller et al. (p. 190) analyzed the images to generate a reconstruction of the asteroids shape. Steins is oblate with an effective spherical diameter of 5.3 kilometers, and it lacks small craters, which may have been erased by surface reshaping. Indeed, Steinss shape resembles that of a body that was spun-up by the YORP effect—a torque produced by incident sunlight, which can alter the rotation rate of a small body—that causes material to slide toward the equator. This effect may have produced Steinss distinctive diamond-like shape. Incident sunlight probably caused this asteroid to spin, which redistributed its mass and smoothed its surface. The European Space Agency’s Rosetta mission encountered the main-belt asteroid (2867) Steins while on its way to rendezvous with comet 67P/Churyumov-Gerasimenko. Images taken with the OSIRIS (optical, spectroscopic, and infrared remote imaging system) cameras on board Rosetta show that Steins is an oblate body with an effective spherical diameter of 5.3 kilometers. Its surface does not show color variations. The morphology of Steins is dominated by linear faults and a large 2.1-kilometer-diameter crater near its south pole. Crater counts reveal a distinct lack of small craters. Steins is not solid rock but a rubble pile and has a conical appearance that is probably the result of reshaping due to Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) spin-up. The OSIRIS images constitute direct evidence for the YORP effect on a main-belt asteroid.

Amorphous silicon/silicon carbide photodiodes with excellent sensitivity and selectivity in the vacuum ultraviolet spectrum

An innovative family of thin‐film photodetectors optimized for the ultraviolet (UV) spectrum is presented here. The devices are made of hydrogenated amorphous silicon (a‐Si:H) and silicon carbide (a‐SiC:H) on glass substrates. At room temperature, the photodetectors exhibit values of quantum efficiency of 21% in the vacuum UV and 0.08% at 750 nm, without external voltage. The great advantage of this technology lies in the possibility to produce low‐cost, large‐area arrays of photodetectors on glass or flexible substrates. All these features candidate the a‐Si/SiC:H photodetectors as possible, concurrent to specialized commercial devices.

Detection of exposed H2O ice on the nucleus of comet 67P/Churyumov-Gerasimenko

Since the orbital insertion of the Rosetta spacecraft, comet 67P/Churyumov-Gerasimenko (67P/C-G) has been mapped by OSIRIS camera and VIRTIS spectro-imager, producing a huge quantity of images and spectra of the comet’s nucleus. The aim of this work is to search for the presence of H 2 O on the nucleus which, in general, appears very dark and rich in dehydrated organic material. After selecting images of the bright spots which could be good candidates to search for H 2 O ice, taken at high resolution by OSIRIS, we check for spectral cubes of the selected coordinates to identify these spots observed by VIRTIS. Methods. The selected OSIRIS images were processed with the OSIRIS standard pipeline and corrected for the illumination condi- tions for each pixel using the Lommel-Seeliger disk law. The spots with higher I/F were selected and then analysed spectrophotomet- rically and compared with the surrounding area. We selected 13 spots as good targets to be analysed by VIRTIS to search for the 2 μm absorption band of water ice in the VIRTIS spectral cubes. Results. Out of the 13 selected bright spots, eight of them present positive H 2 O ice detection on the VIRTIS data. A spectral analysis was performed and the approximate temperature of each spot was computed. The H 2 O ice content was confirmed by modeling the spectra with mixing (areal and intimate) of H 2 O ice and dark terrain, using Hapke’s radiative transfer modeling. We also present a detailed analysis of the detected spots.

Monochromator for the synchrotron radiation beamline X-MOSS at ELETTRA

The optical configuration of the monochromator for the new beamline X-MOSS at the ELETTRA synchrotron ring is described. The requirements for this instrument are to collect a 3 mrad X 2 mrad aperture beam produced by a bending magnet in the 3 - 1400 eV energy range; the energy resolution has to be 3000 or better over the whole range, with a focused beam of the order of 10 - 50 micrometer. The designed monochromator, presently under construction, is a slitless four grazing incidence optical elements: the first element is a one-meter paraboloidal mirror in sagittal focusing, then there is a plane mirror-plane grating dispersion system and finally a second one-meter paraboloidal mirror, also used in sagittal focusing. The latter focuses the radiation on the monochromator exit slit. This monochromator design is not limited by a defined working curve: in this way it is possible to select the preferred operational parameters, to optimize either the flux or the resolution or the high order rejection. The monochromatic beam is finally sent on the sample under examination by an ellipsoidal refocusing mirror.

Multi Element Telescope for Imaging and Spectroscopy (METIS) coronagraph for the Solar Orbiter mission

METIS, the “Multi Element Telescope for Imaging and Spectroscopy”, is a coronagraph selected by the European Space Agency to be part of the payload of the Solar Orbiter mission to be launched in 2017. The unique profile of this mission will allow 1) a close approach to the Sun (up to 0.28 A.U.) thus leading to a significant improvement in spatial resolution; 2) quasi co-rotation with the Sun, resulting in observations that nearly freeze for several days the large-scale outer corona in the plane of the sky and 3) unprecedented out-of-ecliptic view of the solar corona. This paper describes the experiment concept and the observational tools required to achieve the science drivers of METIS. METIS will be capable of obtaining for the first time: • simultaneous imaging of the full corona in polarized visible-light (590-650 nm) and narrow-band ultraviolet HI Lyman α (121.6 nm); • monochromatic imaging of the full corona in the extreme ultraviolet He II Lyman α (30.4 nm); • spectrographic observations of the HI and He II Ly α in corona. These measurements will allow a complete characterization of the three most important plasma components of the corona and the solar wind, that is, electrons, hydrogen, and helium. This presentation gives an overview of the METIS imaging and spectroscopic observational capabilities to carry out such measurements.

Fluorescence of metachrome in the far- and vacuum-ultraviolet spectral region

Some measurements have been performed in order to fully characterize metachrome as down- converter fluorescent layer in the far and vacuum ultraviolet, from 46 up to 254 nm. In particular, a couple of samples having different thickness provided by Photometrics Inc. have been tested and their performances have been compared with the ones obtained by tetraphenyl butadiene (TPB), another scintillator widely used in this spectral region. The emission angular distribution, the fluorescent spectrum and the absolute conversion efficiencies have been measured for all the phosphors. The results show that TPB offers higher efficiency than metachrome, but also that the latter is well efficient in all the investigated spectral region. This is interesting in the case of using this phosphor as down-converter with silicon detectors, so permitting for example to have CCD sensitive also in the extreme ultraviolet region.

Performance evaluation of DTM area-based matching reconstruction of Moon and Mars

High resolution DTMs, suitable for geomorphological studies of planets and asteroids, are today among the main scientific goals of space missions. In the framework of the BepiColombo mission, we are experimenting the use of different matching algorithms as well as the use of different geometric transformation models between stereo pairs, assessing their performances in terms of accuracy and computational efforts. Results obtained with our matching software are compared with those of established software. The comparison of the performance of image matching being the main objective of this work, all other steps of the DTM generation procedure have been made independent of the matching software by using a common framework. Tests with different transformation models have been performed using computer generated images as well as real HiRISE and LROC NAC images. The matching accuracy for real images has been checked in terms of reconstruction error against DTMs of Mars and the Moon published online and produced by the University of Arizona.

Optical performances of the Ultraviolet Coronagraph Spectrometer of the Solar Heliospheric Observatory.

The optical performances of the spectrometer assembly for the Ultraviolet Coronagraph Spectrometer of the Solar and Heliospheric Observatory mission have been tested. The flight unit of the spectrometer assembly, consisting of the structure equipped with the entrance slits assembly, the grating drive mechanisms mounting two toroidal gratings, and the photon-counting detectors, has been integrated and aligned; also the flight unit of the White Light Channel has been integrated and aligned in the spectrometer assembly. Tests with both visible and UV radiation have been performed. Aberration and stray-light measurements have shown that the instrument performs satisfactorily, almost in compliance with the scientific requirements; also some measurements of the polarimeter modulation curve and the relative error have shown performances within the specified requirements.

The solar orbiter METIS coronagraph data signal processing chain

METIS, the Multi Element Telescope for Imaging and Spectroscopy, is one of the instruments selected in 2009 by ESA to be part of the payload of the Solar Orbiter mission. The instrument design has been conceived to perform both multiband imaging and UV spectroscopy of the solar corona. The two sensors of the detecting system will produce images in visible light and in two narrow UV bands, at 121.6 and 30.4 nm. The instrument is constituted by several subunits that have to be properly controlled and synchronized in order to provide the expected performances. Moreover, the large amount of data collected by METIS has to be processed by the on board electronics to reduce the data volume to be delivered to ground by telemetry. These functionalities will be realized by a dedicated electronics, the Main Power and Processing Unit (MPPU). This paper will provide an overview of the METIS data handling system and the expected on board data processing.