arXiv subjects

Roberto Ragazzoni

Publications and source records attributed to Roberto Ragazzoni.

At least 55 records · Page 3Linked to original sources

arXiv2018

A Holographic Diffuser Generalised Optical Differentiation Wavefront Sensor

The wavefront sensors used today at the biggest World's telescopes have either a high dynamic range or a high sensitivity, and they are subject to a linear trade off between these two parameters. A new class of wavefront sensors, the Generalised Optical Differentiation Wavefront Sensors, has been devised, in a way not to undergo this linear trade off and to decouple the dynamic range from the sensitivity. This new class of WFSs is based on the light filtering in the focal plane from a dedicated amplitude filter, which is a hybrid between a linear filter, whose physical dimension is related to the dynamic range, and a step in the amplitude, whose size is related to the sensitivity. We propose here a possible technical implementation of this kind of WFS, making use of a simple holographic diffuser to diffract part of the light in a ring shape around the pin of a pyramid wavefront sensor. In this way, the undiffracted light reaches the pin of the pyramid, contributing to the high sensitivity regime of the WFS, while the diffused light is giving a sort of static modulation of the pyramid, allowing to have some signal even in high turbulence conditions. The holographic diffuser zeroth order efficiency is strictly related to the sensitivity of the WFS, while the diffusing angle of the diffracted light gives the amount of modulation and thus the dynamic range. By properly choosing these two parameters it is possible to build a WFS with high sensitivity and high dynamic range in a static fashion. Introducing dynamic parts in the setup allows to have a set of different diffuser that can be alternated in front of the pyramid, if the change in the seeing conditions requires it.

astro-ph.IM↗

arXiv2018

GMCAO simulation tool development

Global MCAO aims to exploit a very wide technical field of view to find AO-suitable NGSs, with the goal to increase the overall sky coverage. The concept foresees the use of numerical entities, called Virtual Deformable Mirrors, to deal with the nominally thin depth of focus reduction, due to the field of view size. These objects act together as a best fit of the atmospheric layers behavior, in a limited number of conjugation altitudes, so to become the starting point for a further optimization of the real deformable mirrors shapes for the correction of the -smaller- scientific field. We developed a simulator, which numerically combines, in a Layer-Oriented fashion, the measurements of a given number of wavefront sensors, each dedicated to one reference star, to optimize the performance in the NGSs directions. Here we describe some details of the numerical code employed in the simulator, along with the philosophy behind some of the algorithms involved, listing the main goals and assumptions. Several details, including, for instance, how the number and conjugation heights of the VDMs are chosen in the simulation code, are briefly given. Furthermore, we also discuss the possible approaches to define a merit function to optimize the best solution. Finally, after an overview of the remaining issues and limitations of the method, numerical results obtained studying the influence of Cn2 profiles on the reconstruction quality and the delivered SR in a number of fields in the sky are given.

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arXiv2018

SHARK-NIR, the coronagraphic camera for LBT, moving toward construction

SHARK-NIR is one of the two coronagraphic instruments proposed for the Large Binocular Telescope. Together with SHARK-VIS (performing coronagraphic imaging in the visible domain), it will offer the possibility to do binocular observations combining direct imaging, coronagraphic imaging and coronagraphic low resolution spectroscopy in a wide wavelength domain, going from 0.5μm to 1.7μm. Additionally, the contemporary usage of LMIRCam, the coronagraphic LBTI NIR camera, working from K to L band, will extend even more the covered wavelength range. In January 2017 SHARK-NIR underwent a successful final design review, which endorsed the instrument for construction and future implementation at LBT. We report here the final design of the instrument, which foresees two intermediate pupil planes and three focal planes to accomodate a certain number of coronagraphic techniques, selected to maximize the instrument contrast at various distances from the star. Exo-Planets search and characterization has been the science case driving the instrument design, but the SOUL upgrade of the LBT AO will increase the instrument performance in the faint end regime, allowing to do galactic (jets and disks) and extra-galactic (AGN and QSO) science on a relatively wide sample of targets, normally not reachable in other similar facilities.

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arXiv2018

Multiple spatial frequencies wavefront sensing

We describe the concept of splitting spatial frequency perturbations into some kind of pupil planes wavefront sensors. Further to the existing approach of dropping higher spatial frequency to suppress aliasing effects (the so-called spatial filtered Shack-Hartmann), we point out that spatial frequencies splitting and mixing of these in a proper manner, could be handled in order to exhibit some practical or fundamental advantages. In this framework we describe the idea behind such class of concepts and we derive the relationship useful to determine if, by which extent, and under what kind of merit function, these devices can overperform existing conventional sensors.

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arXiv2018

Ingot Laser Guide Stars Wavefront Sensing

We revisit one class of z-invariant WaveFront sensor where the LGS is fired aside of the telescope aperture. In this way there is a spatial dependence on the focal plane with respect to the height where the resonant scattering occurs. We revise the basic parameters involving the geometry and we propose various merit functions to define how much improvement can be attained by a z-invariant approach. We show that refractive approaches are not viable and we discuss several solutions involving reflective ones in what has been nicknamed "ingot wavefront sensor" discussing the degrees of freedom required to keep tracking and the basic recipe for the optical design.

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arXiv2018

Hierarchichal-segmented AO in order to attain wide field compensation in the visible on an 8m class telescope

We describe the preliminary optimized layout for a partially optimized concept of an optical-8m class VLT-like 2x2 segmented camera where each channel is assisted by an equivalent of an MCAO system where the ground layer correction is commonly employed while the high altitude ones is performed in an open-loop fashion. While we derive the basic relationships among the Field of View and attainable correction with a pre-defined choice for the hardware, we discuss sky coverage and wavefront sensing issues employing natural and artificial references, involving the latest stateof-the-art in the development of wavefront sensing. We show that a flexible approach allow for a compensated Field of View that is variable and can be properly tuned matching the current turbulence situation and the requirement in term of quality of the compensation. A preliminary description of the overall optomechanical package is given as well along with a rough estimates of the efforts required to translates such a concept into reality.

astro-ph.IM↗

arXiv2018

The MICADO first light imager for the ELT: overview, operation, simulation

MICADO will enable the ELT to perform diffraction limited near-infrared observations at first light. The instrument's capabilities focus on imaging (including astrometric and high contrast) as well as single object spectroscopy. This contribution looks at how requirements from the observing modes have driven the instrument design and functionality. Using examples from specific science cases, and making use of the data simulation tool, an outline is presented of what we can expect the instrument to achieve.

astro-ph.IM↗

arXiv2018

The calibration procedure of the LINC-NIRVANA ground and high layer WFS

LINC--NIRVANA (LN) is an MCAO module currently mounted on the Rear Bent Gregorian focus of the Large Binocular Telescope (LBT). It mounts a camera originally designed to realize the interferometric imaging focal station of the telescopes. LN follows the LBT binocular strategy having two twin channels: a double Layer Oriented Multi-Conjugate Adaptive Optics system assisting the two arms, supplies high order wave-front correction. In order to counterbalance the field rotation, a mechanical derotation is applied for the two ground wave-front sensors, and an optical (K-mirror) one for the two high layers sensors, fixing the positions of the focal planes with respect to the pyramids aboard the wavefront sensors. The derotation introduces a pupil images rotation on the wavefront sensors, changing the projection of the deformable mirrors on the sensor consequently.

astro-ph.IM↗

arXiv2017

The Chandra Deep Field South as a test case for Global Multi Conjugate Adaptive Optics

The era of the next generation of giant telescopes requires not only the advent of new technologies but also the development of novel methods, in order to exploit fully the extraordinary potential they are built for. Global Multi Conjugate Adaptive Optics (GMCAO) pursues this approach, with the goal of achieving good performance over a field of view of a few arcmin and an increase in sky coverage. In this article, we show the gain offered by this technique to an astrophysical application, such as the photometric survey strategy applied to the Chandra Deep Field South as a case study. We simulated a close-to-real observation of a 500 x 500 arcsec^2 extragalactic deep field with a 40-m class telescope that implements GMCAO. We analysed mock K-band images of 6000 high-redshift (up to z = 2.75) galaxies therein as if they were real to recover the initial input parameters. We attained 94.5 per cent completeness for source detection with SEXTRACTOR. We also measured the morphological parameters of all the sources with the two-dimensional fitting tools GALFIT. The agreement we found between recovered and intrinsic parameters demonstrates GMCAO as a reliable approach to assist extremely large telescope (ELT) observations of extragalactic interest.

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arXiv2016

Dark tip-tilt sensing

Dark wavefront sensing in its simplest and more crude form is a quad-cell with a round spot of dark ink acting as occulting disk at the center. This sensor exhibits fainter limiting magnitude than a conventional quad-cell, providing that the size of the occulting disk is slightly smaller than the size of the spot and smaller than the residual jitter movement in closed loop. We present simulations focusing a generic Adaptive Optics system using Natural Guide Stars to provide the tip-tilt signal. We consider a jitter spectrum of the residual correction including amplitudes exceeding the dark disk size.

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arXiv2016

High order dark wavefront sensing simulations

Dark wavefront sensing takes shape following quantum mechanics concepts in which one is able to "see" an object in one path of a two-arm interferometer using an as low as desired amount of light actually "hitting" the occulting object. A theoretical way to achieve such a goal, but in the realm of wavefront sensing, is represented by a combination of two unequal beams interferometer sharing the same incoming light, and whose difference in path length is continuously adjusted in order to show different signals for different signs of the incoming perturbation. Furthermore, in order to obtain this in white light, the path difference should be properly adjusted vs the wavelength used. While we incidentally describe how this could be achieved in a true optomechanical setup, we focus our attention to the simulation of a hypothetical "perfect" dark wavefront sensor of this kind in which white light compensation is accomplished in a perfect manner and the gain is selectable in a numerical fashion. Although this would represent a sort of idealized dark wavefront sensor that would probably be hard to match in the real glass and metal, it would also give a firm indication of the maximum achievable gain or, in other words, of the prize for achieving such device. Details of how the simulation code works and first numerical results are outlined along with the perspective for an in-depth analysis of the performances and its extension to more realistic situations, including various sources of additional noise.

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arXiv2014

A high resolution image of the inner-shell of the PCygni nebula in the infra-red [Fe II] line

We have obtained with the LBT Telescope AO system Near-Infrared camera PISCES images of the inner-shell of the nebula around the luminous blue variable star P Cygni in the [Fe II] emission line at 1.6435 μm. We have combined the images in order to cover a field of view of about 20" around P Cygni thus providing the high resolution (0".08) 2-D spatial distribution of the inner-shell of the P Cygni nebula in [Fe II]. We have identified several nebular emission regions which are characterized by an S/N>3. A comparison of our results with those available in the literature shows full consistency with the finding by Smith & Hartigan (2006) which are based on radial velocity measurements and their relatively good agreement with the extension of emission nebula in [NII] λ6584 found by Barlow et al. (1994). We have clearly detected extended emission also inside the radial distance R=7".8 and outside R=9".7 which are the nebular boundaries proposed by Smith & Hartigan (2006). New complementary spectroscopic observations to measure radial velocities and to derive the 3-D distribution of P Cygni nebula are planned.

astro-ph.SR↗

arXiv2014

Pathfinder first light: alignment, calibration, and commissioning of the LINC-NIRVANA ground-layer adaptive optics subsystem

We present descriptions of the alignment and calibration tests of the Pathfinder, which achieved first light during our 2013 commissioning campaign at the LBT. The full LINC-NIRVANA instrument is a Fizeau interferometric imager with fringe tracking and 2-layer natural guide star multi-conjugate adaptive optics (MCAO) systems on each eye of the LBT. The MCAO correction for each side is achieved using a ground layer wavefront sensor that drives the LBT adaptive secondary mirror and a mid-high layer wavefront sensor that drives a Xinetics 349 actuator DM conjugated to an altitude of 7.1 km. When the LINC-NIRVANA MCAO system is commissioned, it will be one of only two such systems on an 8-meter telescope and the only such system in the northern hemisphere. In order to mitigate risk, we take a modular approach to commissioning by decoupling and testing the LINC-NIRVANA subsystems individually. The Pathfinder is the ground-layer wavefront sensor for the DX eye of the LBT. It uses 12 pyramid wavefront sensors to optically co-add light from natural guide stars in order to make four pupil images that sense ground layer turbulence. Pathfinder is now the first LINC-NIRVANA subsystem to be fully integrated with the telescope and commissioned on sky. Our 2013 commissioning campaign consisted of 7 runs at the LBT with the tasks of assembly, integration and communication with the LBT telescope control system, alignment to the telescope optical axis, off-sky closed loop AO calibration, and finally closed loop on-sky AO. We present the programmatics of this campaign, along with the novel designs of our alignment scheme and our off-sky calibration test, which lead to the Pathfinder's first on-sky closed loop images.

astro-ph.IM↗

arXiv2014

Laboratory Results and Status Update for Pathfinder at LBT, The LINC-NIRVANA NGS Ground- Layer AO Subsystem

The full LINC-NIRVANA instrument will be one of the most complex ground-based astronomical systems ever built. It will consist of multiple subsystems, including two multi-conjugate ground layer AO systems (MCAO) that drive the LBT adaptive secondaries, two mid-high layer AO systems with their own Xynetics 349 actuator DM's , a fringe tracker, a beam combiner, and the NIR science camera. In order to mitigate risk, we take a modular approach to instrument testing and commissioning by decoupling these subsystems individually. The first subsystem tested on-sky will be one of the ground-layer AO systems, part of a test-bed known as the Pathfinder. The Pathfinder consists of a 12-star pyramid wavefront sensor (PWFS) that drives one of the LBT's adaptive secondaries, a support structure known as "The Foot," and the infrared test camera (IRTC), which is used for acquisition and alignment. The 12 natural guide stars are acquired by moveable arms called "star enlargers," each of which contains its own optical path. The Pathfinder was shipped from MPIA in Heidelberg, Germany to the LBT mountain lab on Mt. Graham, Arizona in February 2013. The system was unpacked, assembled in the LBT clean room, and internally optically aligned. We present the results of our system tests, including star enlarger alignment and system alignment. We also present our immediate plans for on-sky closed loop tests on the LBT scheduled for late Fall. Because plans for all ELTs call for ground layer correction, the Pathfinder provides valuable preliminary information not only for the full LINC-NIRVANA system, but also for future advanced MCAO systems.

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arXiv2012

MarcoPolo-R narrow angle camera: a three-mirror anastigmat design proposal with a smart finite conjugates refocusing optical system

MarcoPolo-R is a medium-class space mission proposed for the 2015-2025 ESA Cosmic Vision Program with primary goal to return to Earth an unaltered sample from a primitive near-Earth asteroid (NEA). Among the proposed instruments on board, its narrow-angle camera (NAC) should be able to image the candidate object with spatial resolution of 3 mm per pixel at 200 m from its surface. The camera should also be able to support the lander descent operations by imaging the target from several distances in order to locate a suitable place for the landing. Hence a refocusing system is requested to accomplish this task, extending its imaging capabilities. Here we present a three-mirror anastigmat (TMA) common-axis optical design, providing high-quality imaging performances by selecting as entrance pupil the system aperture stop and exploiting the motion of a single mirror inside the instrument to allow the wide image refocusing requested, from infinity up to 200 m above the NEA surface. Such proposal matches with the NAC technical specifications and can be easily implemented with present day technology.

astro-ph.IM↗

arXiv2012

Star Cluster Populations in the Outer Disks of Nearby Galaxies

We present a Large Binocular Telescope (LBT) imaging study that characterizes the star cluster component of nearby galaxy outer disks (beyond the optical radius R_25). Expanding on the pilot project of Herbert-Fort et al. (2009), we present deep (~ 27.5 mag V-band point-source limiting magnitude) U- and V-band imaging of six galaxies: IC 4182, NGC 3351, NGC 4736, NGC 4826, NGC 5474, and NGC 6503. We find that the outer disk of each galaxy is populated with marginally-resolved star clusters with masses ~10^3 M_sun and ages up to ~ 1 Gyr (masses and ages are limited by the depth of our imaging and uncertainties are large given how photometry can be strongly affected by the presence or absence of a few stars in such low mass systems), and that they are typically found out to at least 2 R_25 but sometimes as far as 3 to 4 R_25- even beyond the apparent HI disk. The mean rate of cluster formation for 1 R_25<= R <= 1.5R_25 is at least one every ~2.5 Myr and the clusters are spatially correlated with the HI, most strongly with higher density gas near the periphery of the optical disk and with lower density neutral gas at the HI disk periphery. We hypothesize that the clusters near the edge of the optical disk are formed in the extension of spiral structure from the inner disk and are a fairly consistent phenomenon and that the clusters formed at the periphery of the HI disk are the result of accretion episodes.

astro-ph.GA↗

arXiv2011

Hide and seek between Andromeda's halo, disk, and giant stream

Photometry in B, V (down to V ~ 26 mag) is presented for two 23' x 23' fields of the Andromeda galaxy (M31) that were observed with the blue channel camera of the Large Binocular Telescope during the Science Demonstration Time. Each field covers an area of about 5.1kpc x 5.1kpc at the distance of M31 ((m-M)o ~ 24.4 mag), sampling, respectively, a northeast region close to the M31 giant stream (field S2), and an eastern portion of the halo in the direction of the galaxy minor axis (field H1). The stream field spans a region that includes Andromeda's disk and the giant stream, and this is reflected in the complexity of the color magnitude diagram of the field. One corner of the halo field also includes a portion of the giant stream. Even though these demonstration time data were obtained under non-optimal observing conditions the B photometry, acquired in time-series mode, allowed us to identify 274 variable stars (among which 96 are bona fide and 31 are candidate RR Lyrae stars, 71 are Cepheids, and 16 are binary systems) by applying the image subtraction technique to selected portions of the observed fields. Differential flux light curves were obtained for the vast majority of these variables. Our sample includes mainly pulsating stars which populate the instability strip from the Classical Cepheids down to the RR Lyrae stars, thus tracing the different stellar generations in these regions of M31 down to the horizontal branch of the oldest (t ~ 10 Gyr) component.

astro-ph.CO↗

arXiv2011

Astrometry with the MCAO instrument MAD - An analysis of single-epoch data obtained in the layer-oriented mode

Context: Current instrument developments at the largest telescopes worldwide have provisions for Multi-Conjugated Adaptive Optics (MCAO) modules. The large field of view and more uniform correction provided by these systems is not only highly beneficial for photometric studies but also for astrometric analysis of, e.g., large dense clusters and exoplanet detection and characterization. The Multi-conjugated Adaptive optics Demonstrator (MAD) is the first such instrument and was temporarily installed and tested at the ESO/VLT in 2007. We analyzed two globular cluster data sets in terms of achievable astrometric precision. Data were obtained in the layer-oriented correction mode, one in full MCAO correction mode with two layers corrected (NGC 6388) and the other applying ground-layer correction only (47 Tuc). Aims: We aim at analyzing the first available MCAO imaging data in the layer-oriented mode obtained with the MAD instrument in terms of astrometric precision and stability. Methods: We calculated Strehl maps for each frame in both data sets. Distortion corrections were performed and the astrometric precision was analyzed by calculating mean stellar positions over all frames and by investigation of the positional residuals present in each frame after transformation to a master-coordinate-frame. Results: The mean positional precision for stars between K = 14-18 mag is ~1.2 mas in the full MCAO correction mode data of the cluster NGC 6388. The precision measured in the GLAO data (47 Tuc) reaches ~1.0 mas for stars corresponding to 2MASS K magnitudes between 9 and 12. The observations were such that stars in these magnitude ranges correspond to the same detector flux range. The jitter movement used to scan a larger field of view introduced additional distortions in the frames, leading to a degradation of the achievable precision.

astro-ph.IM↗