Stefano Cesare

The VLTI fringe sensors: FINITO and PRIMA FSU

FINITO is the first generation VLTI fringe sensor, optimised for three beam observations, recently installed at Paranal and currently used for VLTI optimisation. The PRIMA FSU is the second generation, optimised for astrometry in dual-feed mode, currently in construction. We discuss the constraints of fringe tracking at VLTI, the basic functions required for stabilised interferometric observations, and their different implementation in the two instruments, with remarks on the most critical technical aspects. We provide an estimate of the expected performance and describe some of their possible observing and calibration modes, with reference to the current scientific combiners.

Location accuracy limitations for CCD cameras

The accurate measurement of the position of celestial objects is a fundamental step for several astrophysical investigations. For ground based instruments, the atmosphere is considered the basic limiting factor; in space, the knowledge of the instrumental parameters and/or of their stability define the performance limits, but CCD cameras operated in time delay integration may take advantage of their operating mode to reduce significantly the calibration problem. We implemented a low-cost laboratory experiment aimed at assessing the precision achievable in the location determination with a CCD camera, by evaluating the measurement repeatability throughout a set of images of a simulated stellar field. Our experiment provides an initial location dispersion of the order of 1/100 of the CCD pixel, with clear evidence of dominant common mode effects. After removing such terms with straightforward numerical procedures, we achieve a final location precision of 1/700 pixel on individual images, or 1/1300 pixel on co-added images. The scaling of precision with target magnitude is in quite good agreement with theoretical expectations. The initial common mode systematics appear to be induced by the thermal control of the CCD camera head, which degrades the structural stability. In actual implementations, such problems can be greatly reduced by proper design. Finally, our results show that residual effects, which could hamper the final astrometric accuracy, can be calibrated out with simple procedures.

Satellite Formation for a Next Generation Gravimetry Mission

The technique called “Low-Low Satellite-to-Satellite Tracking” makes use of two-satellite loose formations for detecting the Earth’s gravity field and its space/time variations: the effect of the geopotential shows up as a variation of the inter-satellite distance, which is measured by a suitable metrology. Accelerometers are utilized on each satellite for measuring and separating the effect of the non-gravitational forces. Thales Alenia Space Italia (TAS-I) has studied for the European Space Agency a gravimetric mission of this kind, in which the inter-satellite distance variation is measured by a laser interferometer. The reference mission scenario that has been defined and studied consists of two satellites flying along the same circular orbit at 10 km relative distance and 325 km altitude. The formation control for this mission shall be designed to work in synergy with the drag-free control (necessary for providing quiet operational environment to the accelerometers), to not interfere with the scientific measurement and to minimize the use of the thrusters. Another control system is in charge of maintaining the fine pointing of the interferometer laser beam from one satellite to the other. This chapter summarizes the main results of the studies performed by TAS-I and its team on these subjects 1 2 3.

The Future of the Satellite Gravimetry After the GOCE Mission

Launched on March 17th 2009 from the Plesetsk Cosmodrome (Northern Russia), GOCE maps the Earth’s gravity field with unprecedented accuracy and resolution and will be of benefit for many branches of Earth science. This paper gives an overview of the European Space Agency’s (ESA) recent technical developments and activities going beyond the GOCE mission and its technology. It describes the outcome of the recent Laser SST concept studies, the Laser metrology concept and the objectives of the ongoing parallel Next Generation Gravimetry Mission studies, together with an overview on the latest technology development studies on atomic clocks and atom interferometry for possible future gravity sensing.

Next Generation Gravity Mission

After the successful experience of the gravity missions GRACE and GOCE, several activities are on going in preparation of a “Next Generation Gravity Mission” (NGGM) aimed at measuring the temporal variations of the Earth’s gravity field over a long time span (up to ~11 years) with high spatial resolution (comparable to that provided by GOCE) and high temporal resolution (weekly or better). Its data will find wide application in geodesy, geophysics, hydrology, ocean circulation and many other disciplines. The most appropriate measurement technique identified for such mission is Low-Low Satellite-Satellite Tracking in which two (or more) satellites flying in “loose” formation in a low Earth orbit act as proof masses immersed in the Earth gravity field. The distance variation between the satellites (measured by a laser interferometer) and the non-gravitational accelerations of each satellite (measured by ultra-sensitive accelerometers) are the fundamental observables from which the gravity field is obtained. Suitable satellite formations for this mission include the “In-line” (the simplest one), the “Cartwheel” and the “Pendulum” (more complex but also scientifically more fruitful), with an inter-satellite distance up to 100 km. Polar, circular orbits with altitudes between ~340 and ~420 km are suitable candidates for the NGGM, providing all-latitude coverage, short repeat cycles/sub-cycles and a still excellent gravity signal compatibly with a long lifetime. Each satellite shall be endowed with a complex control system capable of carrying out several tasks in close coordination: orbit maintenance, formation keeping, provision of a “drag-free” environment to the accelerometers, laser beam pointing and attitude control.

Sub-Nanometric Optics Stabilization In View Of Gaia Astrometric Mission

Abstract The Global Astrometric Interferometer for Astrophysics (GAIA) mission of the European Space Agency aims to determine the position of one billion stars with an accuracy better than 10 micro-arcseconds. This implies stabilizing the most critical mirror movements of the GAIA telescope at the picometer level (10 -12 m), an unprecedented target requiring the development of appropriate sub-nanometric technology in the field of dimensional metrology and active mirror mechanisms. A team lead by Alenia Aerospazio has produced a design of the actively stabilized GAIA telescope that meets the stabilization target and has developed and demonstrated the relevant sub-nanometric technology through a laboratory prototype. The paper focuses on the design and on the simulated results of the GAIA active optics control system, employing the metrology and the mirror mechanisms tested by on-ground experiments.

METIS, the Multi Element Telescope for Imaging and Spectroscopy: An instrument proposed for the solar orbiter mission

METIS, the Multi Element Telescope for Imaging and Spectroscopy, is an instrument proposed to the European Space Agency to be part of the payload of the Solar Orbiter mission. The instrument design has been conceived for performing extreme ultraviolet (EUV) spectroscopy both on the solar disk and off-limb, and near-Sun coronagraphy and spectroscopy. The proposed instrument suite consists of three different interconnected elements, COR, EUS and SOCS, sharing the same optical bench, electronics, and S/C heat shield aperture. COR is a visible-EUV multiband coronagraph based on a classical externally occulted design. EUS is the component of the METIS EUV disk spectrometer which includes the telescope and all the related mechanisms. Finally, SOCS is the METIS spectroscopic component including the dispersive system and the detectors. The capability of inserting a small telescope collecting coronal light has been added to perform also EUV coronal spectroscopy. METIS can simultaneously image the visible and ultraviolet emission of the solar corona and diagnose, with unprecedented temporal coverage and space resolution the structure and dynamics of the full corona in the range from 1.2 to 3.0 (1.6 to 4.1) solar radii (R⊙, measured from Sun centre) at minimum (maximum) perihelion during the nominal mission. It can also perform spectroscopic observations of the solar disk and out to 1.4 R⊙ within the 50-150 nm spectral region, and of the geo-effective coronal region 1.7-2.7 R⊙ within the 30-125 nm spectral band.

Active distance stabilization of large bodies with picometer repeatability

The paper presents control design and results of a leading experiment in the distance stabilization of large bodies, emulating optical mirrors, with picometer repeatability. The experiment, called COSI (Control Optics Structure Interaction), was funded by the European Space Agency (ESA) in view of future space telescopes needing picoradian precision over time scales >1 s. Distance stabilization is achieved by actively controlling the optical length of Fabry-Perot cavities in the vacuum. The first experiments stabilized three 0.5 m distances between two 7 kg plates with a residual control error better than 3 pm (1/spl sigma/), in presence of severe environment noise and artificial micrometer distance variations, thus fully demonstrating feasibility of COSI concept and technology.

Illumination system in visible light with variable solar-divergence for the solar orbiter METIS coronagraph

The measurement of the stray-rejection capabilities of METIS is part of the acceptance package of the instrument. The Illumination System in Visible Light (ISVL) has been developed to allow the stray-light rejection measurement down to 1x10-9 and under different operating conditions. The main characteristics of ISVL are outlined and discussed; the activities for the integration and verification of ISVL included the absolute radiometric characterization of the facility, including radiance measurement and radiance spatial and angular distribution. The procedures used to measure the performances of the facility are discussed and the obtained results illustrated.

Preliminary performance analysis and design for a distributed P-band synthetic aperture radar

This paper focuses on a new concept for spaceborne P-band radar implementation, that is distributed SAR based on formation flying. This approach can in principle allow to overcome physical constraints that limit the performance of monolithic SARs, leading in the P-band case to huge antennas and hard swath/resolution trade-offs. The proposed SAR is based on a larger transmitting satellite and a set of light-weight receiving-only platforms. This architecture also allows for multi-mission capabilities. In particular, in the P-band case forests observation and biomass estimation can be in theory combined with interferometric ice sounding. Payload concept is clarified, and a preliminary performance analysis in terms of ambiguity and coverage is proposed.