Entity

Time filter

Source Type


Patru F.,Osservatorio Astrofisico di Arcetri | Antichi J.,Osservatorio Astrofisico di Arcetri | Mawet D.,European Southern Observatory | Jolissaint L.,Haute Ecole DIngenierie et de Gestion du Canton de Vaud | And 6 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2014

Discretized Aperture Mapping (DAM) appears as an original filtering technique easy to play with existing adaptive optics (AO) systems. In its essential DAM operates as an optical passive filter removing part of the phase residuals in the wavefront without introducing any difficult-to-align component in the Fourier conjugate of the entrance pupil plane. DAM reveals as a new interferometric technique combined with spatial filtering allowing direct imaging over a narrow field of view (FOV). In fact, the entrance pupil of a single telescope is divided into many sub-pupils so that the residual phase in each sub-pupil is filtered up to the DAM cut-off frequency. DAM enables to smooth the small scale wavefront defects which correspond to high spatial frequencies in the pupil plane and to low angular frequencies in the image plane. Close to the AO Nyquist frequency, such pupil plane spatial frequencies are not well measured by the wavefront sensor (WFS) due to aliasing. Once bigger than the AO Nyquist frequency, they are no more measured by the WFS due to the fitting limit responsible for the narrow AO FOV. The corresponding image plane angular frequencies are not transmitted by DAM and are useless to image small FOVs, as stated by interferometry. That is why AO and DAM are complementary assuming that the DAM cut-off frequency is equal to the AO Nyquist frequency. Here we describe the imaging capabilities when DAM is placed downstream an AO system, over a convenient pupil which precedes the scientific detector. We show firstly that the imaging properties are preserved on a narrow FOV allowing direct imaging throughout interferometry. Then we show how the residual pupil plane spatial frequencies bigger than the AO Nyquist one are filtered out, as well as the residual halo in the image is dimmed. © 2014 SPIE. Source


Patru F.,Osservatorio Astrofisico di Arcetri | Patru F.,European Southern Observatory | Antichi J.,Osservatorio Astrofisico di Arcetri | Rabou P.,Institute Of Planetologie Et Dastrophysique | And 6 more authors.
3rd AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes | Year: 2013

Discretized Aperture Mapping (DAM) appears as an original filtering technique easy to play with existing adaptive optics (AO) systems. In its essential DAM operates as an optical passive filter removing part of the phase residuals in the wavefront without introducing any difficult-to-align component in the Fourier conjugate of the entrance pupil plane. DAM reveals as a new interferometric technique combined with spatial filtering allowing direct imaging over a narrow field of view (FOV). In fact, the entrance pupil of a single telescope is divided into many sub-pupils so that the residual phase in each sub-pupil is filtered up to the DAM cut-off frequency. DAM enables to smooth the small scale wavefront defects which correspond to high spatial frequencies in the pupil plane and to low angular frequencies in the image plane. Close to the AO Nyquist frequency, such pupil plane spatial frequencies are not well measured by the wavefront sensor (WFS) due to aliasing. Once bigger than the AO Nyquist frequency, they are no more measured by the WFS due to the fitting limit responsible for the narrow AO FOV. The corresponding image plane angular frequencies are not transmitted by DAM and are useless to image small FOVs, as stated by interferometry. That is why AO and DAM are complementary assuming that the DAM cut-off frequency is equal to the AO Nyquist frequency. Here we describe the imaging capabilities when DAMis placed downstream an AO system, over a convenient pupil which precedes the scientific detector. We show firstly that the imaging properties are preserved on a narrow FOV allowing direct imaging throughout interferometry. Then we show how the residual pupil plane spatial frequencies bigger than the AO Nyquist one are filtered out, as well as the residual halo in the image is dimmed. Source


Mendillo M.,Boston University | Narvaez C.,Boston University | Withers P.,Boston University | Matta M.,Boston University | And 2 more authors.
Planetary and Space Science | Year: 2013

Abstract The Mars Express (MEX) mission includes a multi-purpose radio instrument called the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS). When used in its ionospheric-penetrating subsurface sounder (SS) radar mode, a by-product of the MARSIS observations is the ray-path-integral of electron densities, called the total electron content (TEC). We have used the initial TEC database of approximately 1.2 million TEC values spanning the period June 2005 to September 2007 to study the basic characteristics of TEC morphology and variability. We find quantitative agreement between the TEC values measured and those computed from model simulations of global diurnal behavior. With the basic photo-chemistry of the martian ionosphere a well understood process, it is the departures from average conditions that need specification and modeling. Here we use MARSIS TEC to do this quantitatively. We explore the specification of variability using different ways to define it: standard deviations from sample averages versus departures from control curves. For global studies, we computed the standard deviation (σ in %) of mean values of TEC (in TECU of 1015 e -/m2) sorted by latitude, longitude, solar zenith angle (SZA), local time, season, and locations with/without strong crustal magnetic fields (50 nT at 150 km). For daytime conditions (SZA<75), the global average 〈TEC⌠is ~6 TECU with σ=~20%, while for nighttime (SZA>105) 〈TECâŒ; is ~0.3 TECU with σ=~75%. Daytime variability is enhanced in the latitude region 0-30 S, a pattern that needs validation by later observations before its source can be identified. Nighttime variability is noticeably larger in regions of strong crustal magnetic fields (B) - an effect noted by previous authors. For regional studies, high resolution latitude patterns of variability in the southern hemisphere - within the longitude sector 150-210 of strong crustal-B values - were computed as percentage changes with respect to zonally-averaged patterns outside the region of interest. We present evidence for the first time of B-fields affecting the variability of the daytime ionosphere by small amounts (~±5%). Under nighttime conditions, the B-field associated variability is ~±20%. The results also reveal an anti-correlation between daytime and nighttime variability ordered by the inclination angle (I) of the B-fields. TEC variability is greater as I approaches vertical at night, but higher during the day (by smaller amounts) where I approaches horizontal patterns. © 2013 Elsevier Ltd. Source


Mendillo M.,Boston University | Narvaez C.,Boston University | Lawler G.,Boston University | Kofman W.,Institute Of Planetologie Et Dastrophysique | And 4 more authors.
Geophysical Research Letters | Year: 2015

The total electron content (TEC) of a planetary ionosphere is dominated by plasma near and above the height of maximum electron density (Nmax). The ratio TEC/Nmax represents the thickness (τ) of a TEC slab of uniform density (Nmax). For a photochemical ionosphere, τ relates to the scale height (H=kT/mg) of the ionized neutral gas as τ∼4×H. Derived temperatures refer to ∼160km in thermosphere height - below the asymptotic temperature of the exosphere. The MARSIS instrument on Mars Express has produced data sets of TEC and Nmax. We used them to form τ patterns versus solar zenith angle and solar cycle phase. For daytime (SZA<90°) conditions, <τ>day∼50km, decreasing rapidly for solar zenith angle (SZA)>90° to<τ>night∼25km. These correspond to Tn values of 250°K and 125°K. Using Mars Global Surveyor data, τ patterns show a mild dependence upon the solar EUV flux proxy F10.7, with ΔTn(°K)∼0.3° per unit change in F10.7 at Mars. Key Points The ratio TEC/Nmax of a planetary ionosphere represents the slab thickness Slab thickness relates to the neutral atmosphere's scale height and temperature Mars' ionosphere has a nighttime slab thickness of 25 km that doubles in daytime ©2015. American Geophysical Union. All Rights Reserved. Source


Sautter V.,IMPMC | Toplis M.J.,IRAP | Beck P.,Institute Of Planetologie Et Dastrophysique | Mangold N.,LPG Nantes | And 16 more authors.
Lithos | Year: 2016

Until recently, Mars was considered a basalt-covered world, but this vision is evolving thanks to new orbital, in situ and meteorite observations, in particular of rocks of the ancient Noachian period. In this contribution we summarise newly recognised compositional and mineralogical differences between older and more recent rocks, and explore the geodynamic implications of these new findings. For example the MSL rover has discovered abundant felsic rocks close to the landing site coming from the wall of Gale crater ranging from alkali basalt to trachyte. In addition, the recently discovered Martian regolith breccia NWA 7034 (and paired samples) contain many coarse-grained noritic-monzonitic clasts demonstrably Noachian in age, and even some clasts that plot in the mugearite field. Olivine is also conspicuously lacking in these ancient samples, in contrast to later Hesperian rocks. The alkali-suite requires low-degree melting of the Martian mantle at low pressure, whereas the later Hesperian magmatism would appear to be produced by higher mantle temperatures. Various scenarios are proposed to explain these observations, including different styles of magmatic activity (i.e. passive upwelling vs. hotspots). A second petrological suite of increasing interest involves quartzo-feldspathic materials that were first inferred from orbit, in local patches in the southern highlands and in the lower units of Valles Marineris. However, identification of felsic rocks from orbit is limited by the low detectability of feldspar in the near infrared. On the other hand, the MSL rover has described the texture, mineralogy and composition of felsic rocks in Gale crater that are granodiorite-like samples akin to terrestrial TTG (Tonalite-Trondhjemite-Granodiorite suites). These observations, and the low average density of the highlands crust, suggest the early formation of 'continental' crust on Mars, although the details of the geodynamic scenario and the importance of volatiles in their generation are aspects that require further work. © 2016 Elsevier B.V. Source

Discover hidden collaborations