Marco Dima

The NIR arm of SHARK: System for coronagraphy with High-order Adaptive optics from R to K bands

SHARK is a proposal aimed at investigating the technical feasibility and the scientific capabilities of high-contrast cameras to be implemented at the Large Binocular Telescope (LBT). SHARK foresees two separated channels: near-infrared (NIR) channel and visible, both providing imaging and coronagraphic modes. We describe here the SHARK instrument concept, with particular emphasis on the NIR channel at the level of a conceptual study, performed in the framework of the call for proposals for new LBT instruments. The search for giant extra-Solar planets is the main science case, as we will outline in the paper.

The Multiple Field of View Layer Oriented wavefront sensing system of LINC-NIRVANA: two arcminutes of corrected field using solely Natural Guide Stars

LINC-NIRVANA is an infrared camera that will work in Fizeau interferometric way at the Large Binocular Telescope (LBT). It will take advantage of a field corrected from two MCAO systems, one for each arm, based on the Layer Oriented Technique and using solely Natural Guide Stars. For each arm, there will be two wavefront sensors, one conjugated to the Ground and one conjugated to a selectable altitude, ranging from 4 to 15 Km. They will explore different fields of view for the wavefront sensing operations, accordingly to the Multiple Field of View concept, and particularly the inner 2 arcminutes FoV will be used to select the references for the high layer wavefront sensor while the ground one will explore a wider anular field, going from 2 to 6 arcminutes in diameter. The wavefront sensors are under INAF responsibility, and their construction is ongoing in different italian observatories. Here we report on progress, and particularly on the test ongoing in Padova observatory on the Ground Layer Wavefront Sensor.

Adaptive optics with solely natural guide stars for an extremely large telescope

In the past decade the ingredients for making real an Extremely Large Telescope with an Adaptive Optics system driven solely by Natural Guide Stars have been conceived, developed, built and proven on the sky. Still, the straightforward merging of these concepts is not enough to fulfill such an ambitious goal. We show here that a combination of the layeroriented approach, the virtual deformable mirrors concept, and a combined use of different kind of wavefront sensors, some taking advantage of working in Closed Loop and some other characterized by an extremely high dynamic range, make the goal a reachable one. It is remarkable that such an approach requires, on a telescope of ELT class, including a common Deformable Mirror conjugated to the entrance pupil or close-by, a minimum impact on the guide probe units. The last involves the adoption of small Closed Loop AO system with an extremely high dynamic range wavefront sensor looking at the detailed shape of a small Deformable Mirror that allows the use of sensors taking advantage of the Closed Loop conditions. A pyramid wavefront sensor, fed by the Natural Guide Stars light and closing the loop with the mirror, and a YAW wavefront sensor looking at the mirror itself, allow for a natural and efficient combination of the data. The limits in the Field of View covered by such an approach are given by pure meta-pupils superimposition rather than to the spatial frequency of the achievable correction, breaking the limits previously thought for this kind of systems. The overall combination leads to a significant sky coverage, with performances comparable to the ones under discussion for some Laser Guide Stars approaches, without the related hurdle. The small technical impact on the telescope makes this approach not directly in-conflict with a Laser Guide Stars one allowing the designer to keep all the options on the table up to a very late stage.

PLATO: a multiple telescope spacecraft for exo-planets hunting

PLATO stands for PLAnetary Transits and Oscillation of stars and is a Medium sized mission selected as M3 by the European Space Agency as part of the Cosmic Vision program. The strategy behind is to scrutinize a large fraction of the sky collecting lightcurves of a large number of stars and detecting transits of exo-planets whose apparent orbit allow for the transit to be visible from the Earth. Furthermore, as the transit is basically able to provide the ratio of the size of the transiting planet to the host star, the latter is being characterized by asteroseismology, allowing to provide accurate masses, radii and hence density of a large sample of extra solar bodies. In order to be able to then follow up from the ground via spectroscopy radial velocity measurements these candidates the search must be confined to rather bright stars. To comply with the statistical rate of the occurrence of such transits around these kind of stars one needs a telescope with a moderate aperture of the order of one meter but with a Field of View that is of the order of 50 degrees in diameter. This is achieved by splitting the optical aperture into a few dozens identical telescopes with partially overlapping Field of View to build up a mixed ensemble of differently covered area of the sky to comply with various classes of magnitude stars. The single telescopes are refractive optical systems with an internally located pupil defined by a CaF2 lens, and comprising an aspheric front lens and a strong field flattener optical element close to the detectors mosaic. In order to continuously monitor for a few years with the aim to detect planetary transits similar to an hypothetical twin of the Earth, with the same revolution period, the spacecraft is going to be operated while orbiting around the L2 Lagrangian point of the Earth-Sun system so that the Earth disk is no longer a constraints potentially interfering with such a wide field continuous uninterrupted survey.

Ground layer correction: the heart of LINC-NIRVANA

The delivered image quality of ground-based telescopes depends greatly on atmospheric turbulence. At every observatory, the majority of the turbulence (up to 60-80% of the total) occurs in the ground layer of the atmosphere, that is, the first few hundred meters above the telescope pupil. Correction of these perturbations can, therefore, greatly increase the quality of the image. We use Ground-layer Wavefront Sensors (GWSs) to sense the ground layer turbulence for the LINC-NIRVANA (LN) instrument, which is in its final integration phase before shipment to the Large Binocular Telescope (LBT) on Mt. Graham in Arizona.19 LN is an infrared Fizeau interferometer, equipped with an advanced Multi-Conjugate Adaptive Optics (MCAO) module, capable of delivering images with a spatial resolution equivalent to that of a ~23m diameter telescope. It exploits the Layer-Oriented, Multiple Field of View, MCAO approach3 and uses only natural guide stars for the correction. The GWS has more than 100 degrees of freedom. There are opto-mechanical complexities at the level of sub- systems, the GWS as a whole, and at the interface with the telescope. Also, there is a very stringent requirement on the superposition of the pupils on the detector. All these conditions make the alignment of the GWS very demanding and crucial. In this paper, we discuss the alignment and integration of the left-eye GWS of LN and detail the various tests done in the lab at INAF-Padova to verify proper system operation and performance.

PLATO: detailed design of the telescope optical units

The project PLAnetary Transits and Oscillations of stars (PLATO) is one of the three medium class (M class) missions selected in 2010 for definition study in the framework of the ESA Cosmic Vision 2015-2025 program. The main scientific goals of PLATO are the i) discovery and study of extra-solar planetary systems, (including those hosting Earth-like planets in their habitable zone) by means of planetary transits detection from space and radial velocity follow-up from ground, and ii) the characterization of the hosting stars through seismic analysis, in order to determine with high accuracy planetary masses and ages. According to the study made by the PLATO Payload Consortium (PPLC) during the PLATO assessment phase, the scientific payload consists of 34 all refractive telescopes having small aperture (120 mm) and wide field of view (greater than 1000 degree2) observing over 0.5-1 micron wavelength band. The telescopes are mounted on a common optical bench and are divided in four families with an overlapping line-of-sight in order to maximize the science return. In this paper, we will describe the detailed design of the Telescope Optical Units (TOUs) focusing on the selected optical configuration and the expected performances.

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.

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.

SHARK (System for coronagraphy with High order Adaptive optics from R to K band): A proposal for the LBT 2nd generation instrumentation

This article presents a proposal aimed at investigating the technical feasibility and the scientific capabilities of high contrast cameras to be implemented at LBT. Such an instrument will fully exploit the unique LBT capabilities in Adaptive Optics (AO) as demonstrated by the First Light Adaptive Optics (FLAO) system, which is obtaining excellent results in terms of performance and reliability. The aim of this proposal is to show the scientific interest of such a project, together with a conceptual opto-mechanical study which shows its technical feasibility, taking advantage of the already existing AO systems, which are delivering the highest Strehl experienced in nowadays existing telescopes. Two channels are foreseen for SHARK, a near infrared channel (2.5-0.9 um) and a visible one (0.9 – 0.6 um), both providing imaging and coronagraphic modes. The visible channel is equipped with a very fast and low noise detector running at 1.0 kfps and an IFU spectroscopic port to provide low and medium resolution spectra of 1.5 x 1.5 arcsec fields. The search of extra solar giant planets is the main science case and the driver for the technical choices of SHARK, but leaving room for several other interesting scientific topics, which will be briefly depicted here.

Modelling global multi-conjugated adaptive optics

The recently proposed concept of Global MCAO (GMCAO) aims to look for Natural Guide Stars in a very wide technical Field of View (FoV), to increase the overall sky coverage, and deals with the consequent depth of focus reduction introducing numerically a quite-high number of Virtual Deformable Mirrors (VDMs), which are then the starting point for an optimization of the real DMs shapes for the correction of the -smaller- scientific FoV. To translate the GMCAO concept into a real system, a number of parameters requires to be analyzed and optimized, like the number of references and VDMs to be used, the technical FoV size, the spatial samplings, the sensing wavelength. These and some other major choices, like the open loop WFSs concept and design, will then drive the requirements and the performance of the system (e.g. limiting magnitude, linear response, and sensitivity). This paper collects some major results of the on-going study on the feasibility of an Adaptive Optics system for the E-ELT, based on GMCAO, with a particular emphasis on the sky coverage issue. Besides the sensitivity analysis of the optimization of the already mentioned parameters, such a topic involves the implementation of an IDL code simulation tool to estimate the system performance in terms of Strehl Ratio in a 2×2 arcmin FoV, when a variable number of NGSs and VDMs are used. Different technical FoV diameters for the references selection and various constellations can be also compared. This study could be the starting point for a dedicated laboratory testing and, in the future, an on-sky experiment at an 8m telescope with a “scaled down” demonstrator.