Daniele Gardiol

Design, deposition, and characterization of multilayer coatings for the Ultraviolet and Visible-Light Coronagraphic Imager

The Ultraviolet and Visible-Light Coronagraphic Imager is the baseline coronagraph for the European Space Agencys payload Solar Orbiter, a solar mission whose launch in 2011 is expected. To prove the feasibility of its innovative design, a sounding rocket version of the same instrument has been approved by NASA. One of the main technological challenges of the instrument is the achievement of multilayer optical coatings with suitable properties. We describe the design, fabrication, and characterization of such coatings.

Chromaticity in all-reflective telescopes for astrometry

Aims. Chromatic effects are usually associated with refractive optics, so reflective telescopes are assumed to be free from them. We show that all-reflective optics still bears significant levels of such perturbations, which are especially critical to modern micro-arcsecond astrometric experiments. Methods. We analyze the image formation and measurement process to derive a precise definition of the chromatic variation of the image position, and we evaluate the key aspects of optical design with respect to chromaticity. Results. The fundamental requirement related to chromaticity is the symmetry of the optical design and of the wavefront errors. Finally, we address some optical engineering issues, such as manufacturing and alignment, providing recommendations to minimize the degradation that chromaticity introduces into astrometry.

Fizeau interferometer for global astrometry in space

We discuss the design and the performance of a Fizeau interferometer with a long focal length and a large field of view that is well suited for a global astrometry space mission. Our work focuses on the geometric optimization and minimization of aberration of such an astrometric interferometer, which is able to observe astronomical targets down to the visual magnitude (mag) mv = 20 mag, with an accuracy in the measurements of 10 micro-arcseconds at mv = 15 mag. We assume a mission profile similar to that of the Global Astrometric Interferometer for Astrophysics mission of the European Space Agency. In this framework, data acquisition is performed by an array of CCDs working in time-delay integration mode. Optical aberrations, particularly distortion and coma, play a crucial role in the efficiency of this technique. We present a design solution that meets the requirements for the best possible exploitation of the time-delay integration mode over a field of view of 0.7 degrees x 0.7 degrees.

Ultraviolet and Visible-light Coronagraphic Imager (UVCI)

The HERSCHEL (HElium Resonance Scattering in the Corona and HELiosphere) Sun-Earth Sub-Orbital Program is a proposed sounding-rocket payload designed to investigate helium coronal abundance and solar wind acceleration from a range of solar source structures by obtaining simultaneous observations of the electron, proton and helium solar coronae. HERSCHEL will provide the first measurements of the coronal helium abundance in source regions of the solar wind, thus bringing key elements to our understanding of the Sun-Earth connections. The HERSCHEL instrument package consists of the Extreme Ultraviolet Imaging Telescope (EIT) for on-disk coronal observations and the Ultraviolet and Visible-light Coronagraphic Imager (UVCI) for off-limb observations of the corona. The UVCI is an externally occulted, reflecting coronagraph with an off-axis Gregorian telescope. UVCI will be able to take coronal images at heliocentric heights comprised between 1.2 to 3.5 solar radii of a) K-corona polarized brightness (pB); b) H I Lyman-α, 121.6 nm, line-emission; c) He II Lyman-α, 30.4 nm, line. The key element in the UVCI instrument concept is that the mirrors with multilayer coatings optimized for 30.4 nm still have good reflectivity at 121.6 nm and in the visible. The optical design concept for the UVCI instrument will be discussed, together with its expected optical and throughput performances.

The acquisition camera system for SOXS at NTT

SOXS (Son of X-Shooter) will be the new medium resolution (R~4500 for a 1 arcsec slit), high-efficiency, wide band spectrograph for the ESO-NTT telescope on La Silla. It will be able to cover simultaneously optical and NIR bands (350-2000nm) using two different arms and a pre-slit Common Path feeding system. SOXS will provide an unique facility to follow up any kind of transient event with the best possible response time in addition to high efficiency and availability. Furthermore, a Calibration Unit and an Acquisition Camera System with all the necessary relay optics will be connected to the Common Path sub-system. The Acquisition Camera, working in optical regime, will be primarily focused on target acquisition and secondary guiding, but will also provide an imaging mode for scientific photometry. In this work we give an overview of the Acquisition Camera System for SOXS with all the different functionalities. The optical and mechanical design of the system are also presented together with the preliminary performances in terms of optical quality, throughput, magnitude limits and photometric properties.

An active telescope secondary mirror control system

One of the main constraints for a modern astronomical telescope is the active control of the secondary mirror, mainly for the correction of the decentering coma and defocus induced by thermomechanical distortions. The mirror movements should be smooth, as precise as the optical design requires, and restricted to within predefined limits. It should be possible to perform mirror alignment and focus corrections without pausing the exposure (online control). The Galileo telescope achieves all this by using a support structure driven via six actuator bars (a hexapod system or Stewart platform) and a real-time control system based on a transputer network that allows parallel control of each actuator. Both the hexapod secondary support and the control system have been built and tested at the telescope. The results show that the errors introduced during mirror positioning lead to optical aberrations well below the diffraction figure of the telescope, and the systems work smoothly enough to allow online control.

The new SOXS instrument for the ESO NTT

SOXS (Son Of X-Shooter) will be a unique spectroscopic facility for the ESO-NTT 3.5-m telescope in La Silla (Chile), able to cover the optical/NIR band (350-1750 nm). The design foresees a high-efficiency spectrograph with a resolutionslit product of ~4,500, capable of simultaneously observing the complete spectral range 350 - 1750 nm with a good sensitivity, with light imaging capabilities in the visible band. This paper outlines the status of the project.

Development of multilayer coatings for solar space experiments

Deposition and characterization results of multilayer coatings optimized for HeII 30.4 nm high reflectivity will be presented. Additional characterization of reflectivity at HI Ly-α and in the visible spectral range has been also accomplished in order to investigate the performances of such coatings in view of their application to the UVCI instrument on board of ESA payload SOLO and to HERSCHEL, a sounding rocket experiment.

SOXS: a wide band spectrograph to follow up transients

SOXS (Son Of X-Shooter) will be a spectrograph for the ESO NTT telescope capable to cover the optical and NIR bands, based on the heritage of the X-Shooter at the ESO-VLT. SOXS will be built and run by an international consortium, carrying out rapid and longer term Target of Opportunity requests on a variety of astronomical objects. SOXS will observe all kind of transient and variable sources from different surveys. These will be a mixture of fast alerts (e.g. gamma-ray bursts, gravitational waves, neutrino events), mid-term alerts (e.g. supernovae, X-ray transients), fixed time events (e.g. close-by passage of minor bodies). While the focus is on transients and variables, still there is a wide range of other astrophysical targets and science topics that will benefit from SOXS. The design foresees a spectrograph with a Resolution-Slit product ≈ 4500, capable of simultaneously observing over the entire band the complete spectral range from the U- to the H-band. The limiting magnitude of R~20 (1 hr at S/N~10) is suited to study transients identified from on-going imaging surveys. Light imaging capabilities in the optical band (grizy) are also envisaged to allow for multi-band photometry of the faintest transients. This paper outlines the status of the project, now in Final Design Phase.

Tests characterization and alignment for the optics of the ASTRI SST-2M telescope prototype for the Cherenkov Telescope Array

ASTRI SST-2M is a prototype dual mirror Small Size class of Telescope for the Cherenkov Telescope Array (CTA). Its innovative design based on a Schwarzschild-Couder configuration will permit use Silicon Photo-multipliers as focalplane detectors. The dual mirror configuration is a challenge for the realization of both the primary and secondary. Accurate tests and characterization are mandatory to understand the behavior of the optical configuration, its limits and the possibility of improvements of the full CTA array. Moreover, optical alignment requires solutions and procedures that have not been used so far on Cherenkov telescopes. The aim of paper is to provide an analysis of these topics in the context of the ASTRI SST-2M telescope.