Carlo Pelliciari

The Palermo XACT facility: a new 35 m long soft x-ray beam-line for the development and calibration of next-generation x-ray observatories

The X-ray Astronomy Calibration and Testing (XACT) facility of the Instituto Nazionale di Astrofisica (INAF) at Osservatorio Astronomico di Palermo has recently undergone a major upgrade with the design and construction of a 35 meter long vacuum beam-line operating in the soft X-rays (0.1-20 keV) and the addition of new hardware to meet the requirements for testing and calibration of next generation X-ray missions. We report on the present configuration of the facility and briefly survey the range of its applications.

The ASTRI SST-2M prototype for the Cherenkov Telescope Array: manufacturing of the structure and the mirrors

The Cherenkov Telescope Array (CTA) observatory will represent the next generation of Imaging Atmospheric Cherenkov Telescopes. Using a combination of large-, medium-, and small-scale telescopes (LST, MST, SST, respectively), it will explore the Very High Energy domain from a few tens of GeV up to few hundreds of TeV with unprecedented sensitivity, angular resolution and imaging quality. In this framework, the Italian ASTRI program, led by the Italian National Institute of Astrophysics (INAF), is currently developing a scientific and technological SST prototype named ASTRI SST-2M; a 4-meter class telescope, it will adopt an aplanatic, wide-field, double-reflection optical layout in a Schwarzschild-Couder configuration. In this contribution we give an overview of the technological solutions adopted for the ASTRI SST-2M prototype. In particular we focus on the manufacturing of the telescope structure and mirrors. We will also describe early results from tests.

Active shape correction of a thin glass/plastic x-ray mirror

Optics for future X-ray telescopes will be characterized by very large aperture and focal length, and will be made of lightweight materials like glass or plastic in order to keep the total mass within acceptable limits. Optics based on thin slumped glass foils are currently in use in the NuSTAR telescope and are being developed at various institutes like INAF/OAB, aiming at improving the angular resolution to a few arcsec HEW. Another possibility would be the use of thin plastic foils, being developed at SAO and the Palermo University. Even if relevant progresses in the achieved angular resolution were recently made, a viable possibility to further improve the mirror figure would be the application of piezoelectric actuators onto the non-optical side of the mirrors. In fact, thin mirrors are prone to deform, so they require a careful integration to avoid deformations and even correct forming errors. This however offers the possibility to actively correct the residual deformation. Even if other groups are already at work on this idea, we are pursuing the concept of active integration of thin glass or plastic foils with piezoelectric patches, fed by voltages driven by the feedback provided by X-rays, in intra-focal setup at the XACT facility at INAF/OAPA. In this work, we show the preliminary simulations and the first steps taken in this project.

Deflectometry for optics evaluation: free form segments of polynomial mirror

Deflectometry is a well-known method for astronomical mirror metrology. This paper describes the method we developed for the characterization of free-form concave mirrors. Our technique is based on the synergy between deflectometry and ray-tracing. The deflectometrical test is performed by illuminating the reflecting surface with a known light pattern in a Ronchi – like configuration and retrieving the slope errors by the observed rays deflection. The ray-tracing code allows us to measure the slopes and to evaluate the mirror optical performance. This technique has two main advantages: it is fast and it is applicable on-site, as an intermediate step in the manufacturing process, preventing that out-of-specification mirrors may proceed towards further production steps. Thus, we obtain a considerable time and cost reduction. As an example, we describe the results obtained measuring the primary mirror segments of the Cherenkov prototypal telescope manufactured by the Italian National Institute for Astrophysics in the context of the ASTRI Project. This specific case is challenging because the segmentation of the polynomial primary mirror lead to individual mirrors with deviations from the spherical optical design up to a few millimeters.

Manufacturing an active X-ray mirror prototype in thin glass

Adjustable mirrors equipped with piezo actuators are commonly used at synchrotron and free-electron laser (FEL) beamlines, in order to optimize their focusing properties and sometimes to shape the intensity distribution of the focal spot with the desired profile. Unlike them, X-ray mirrors for astronomy are much thinner in order to enable nesting and reduce the areal mass, and the application of piezo actuators acting normally to the surface appears much more difficult. There remains the possibility to correct the deformations using thin patches that exert a tangential strain on the rear side of the mirror: some research groups are already at work on this approach. The technique reported here relies on actively integrating thin glass foils with commercial piezoceramic patches, fed by voltages driven by the feedback provided by X-rays, while the tension signals are carried by electrodes on the back of the mirror, obtained by photolithography. Finally, the shape detection and the consequent voltage signal to be provided to the piezoelectric array will be determined by X-ray illumination in an intra-focal setup at the XACT facility. In this work, the manufacturing steps for obtaining a first active mirror prototype are described.

Functional tests of modular elements of segmented optics for X-ray telescopes via an expanded beam facility

Future large X-ray observatories will be equipped with very large optics obtained by assembling modular optical elements. The final quality of the modular optic is determined by the accuracy in the assembly alignment, but also by the compliance of the focusing elements to the nominal shape and the roughness tolerance in order to avoid excessive levels of X-ray scattering. Because of the large number of modules, quality tests need to be routinely performed to assess the technology readiness, and, in a later phase, to select the most performing stacked blocks to be integrated into the final optic. Besides the usual metrology based on profile and roughness measurements, a direct, at-wavelength, focusing measurement in X-rays would be the most reliable test. Synchrotron light beams are in general not sufficiently broad to cover the aperture of a block without scanning it, which requires a focal spot reconstruction. To this end, we designed a 12 m long X-ray facility to be realized at INAF/ OAB, devoted to the functional tests of the focusing elements. A grazing incidence parabolic mirror and an asymmetric Silicon crystal will produce a wide, parallel, and uniform beam of X-rays to illuminate the entire aperture of the focusing elements. A X-ray camera at the focal distance from the mirrors directly records the image. The tests will be performed at 4.5 keV, with the components operating in gaseous Helium to minimize the absorption.

An expanded x-ray beam facility (BEaTriX) to test the modular elements of the ATHENA optics

Future large X-ray observatories like ATHENA will be equipped with very large optics, obtained by assembling modular optical elements, named X-ray Optical Units (XOU) based on the technology of either Silicon Pore Optics or Slumped Glass Optics. In both cases, the final quality of the modular optic (a 5 arcsec HEW requirement for ATHENA) is determined by the accuracy alignment of the XOUs within the assembly, but also by the angular resolution of the individual XOU. This is affected by the mirror shape accuracy, its surface roughness, and the mutual alignment of the mirrors within the XOU itself. Because of the large number of XOUs to be produced, quality tests need to be routinely done to select the most performing stacked blocks, to be integrated into the final optic. In addition to the usual metrology based on profile and roughness measurements, a direct measurement with a broad, parallel, collimated and uniform Xray beam would be the most reliable test, without the need of a focal spot reconstruction as usually done in synchrotron light. To this end, we designed the BEaTriX (Beam Expander Testing X-ray facility) to be realized at INAF-OAB, devoted to the functional tests of the XOUs. A grazing incidence parabolic mirror and an asymmetrically cut crystal will produce a parallel X-ray beam broad enough to illuminate the entire aperture of the focusing elements. An X-ray camera at the focal distance from the mirrors will directly record the image. The selection of different crystals will enable to test the XOUs in the 1 - 5 keV range, included in the X-ray energy band of ATHENA (0.2-12 keV). In this paper we discuss a possible BEaTriX facility implementation. We also show a preliminary performance simulation of the optical system.

Optimization of the MARS-XRD collimator using converging blades

Mars-XRD is an X-ray diffractometer developed for the in situ mineralogical analysis of the Martian soil. The main components of the Mars-XRD experiment are: a Fe55 radioactive source, a collimator and a CCD-based detector system. For spectroscopic requirements and quality of the machined micrograin of the sample, the beam section should not be larger then 1 × 10 mm2 at sample distance. The current collimator baseline is based on a two-windows system that uses about 20% of the total source emitting surface. To improve the X-ray flux, we are studying a collimator with converging blades which permits to use the entire source emission and tune the beam section. In order to better estimate the efficiency of this collimator and because of the high number of variables, a C++ program has been written that look for the best blades configuration among billion of combinations. In addition to the collimator configuration, this software simulator gives the sample photons distribution for different angles of the tilt of the source and for each couple of blades. The optimized collimator transmits a flux 30% higher than a system with blades with the same angular aperture and 5 times higher than a two windows collimator. Moreover the target photon distribution is a triangle function well focused on the sample surface instead of an irregular function obtained with the previous system. Higher performances arise with the source perpendicular to the source-sample direction. Thanks to this optimization we expect to strongly improve the resolution of the diffraction pattern which is the main goal of the current activities of the instrument development. This software simulator could be used also for the optimization of collimator system for the other wavelength and applications (e.g. radiotherapy).

AHEAD joint research activity on x-ray optics

The progress of X-ray Optics joint research activity of the European Union Horizon 2020 AHEAD project is presented here covering the X-ray optic technologies that are currently being worked on in Europe. These are the Kirkpatrick Baez, lobster eye micropore (SVOM, SMILE), slumped glass, and silicon pore (ATHENA, ARCUS) optics technologies. In this activity detailed comparisons of the measurements, of the different optics produced by the participating optics groups, obtained mainly at the MPEs PANTER X-ray test facility, are compared with simulations. In preparation for the ATHENA mission a study has been made to design the BEaTRiX X-ray test facility for testing individual silicon pore optics mirror modules, and the realization of the facility is now on going. A zone plate collimating optics developed for PANTER is being studied, optimized, and tested at PANTER. This zone plate will be used for characterising a high quality optics module in a parallel beam to verify the BEaTriX performance. Several of the measurements and selected results are presented here.

The glass cold-shaping technology for the mirrors of the Cherenkov Telescope Array

The next generation of imaging atmospheric Cherenkov telescopes will require the production of thousands of mirror segments; an unprecedented amount of optical surface. To accomplish this, the Italian Istituto Nazionale di AstroFisica (INAF) has recently developed a successful technique. This method, called glass cold-shaping, is mainly intended for the manufacturing of mirrors for optical systems with an angular resolution of a few arcminutes, intended to operate in extreme environments. Its principal mechanical features are very low weight and high rigidity of the resulting segments, and its cost and production time turn out to be very competitive as well. The process is based on the shaping of thin glass foils by means of forced bending at room temperature; a sandwich structure is then assembled for retaining the imposed shape. These mirrors are composted of commercial, off-the-shelf materials. In this contribution we give an overview of the latest results achieved in the manufacturing of the pre-production series of mirrors for the Medium Size and Small Size Telescopes of the Cherenkov Telescope Array observatory.