This PDF file contains the front matter associated with SPIE Proceedings Volume 11119, including the title page, copyright information, table of contents, and author and conference committee lists.

Expected to launch in 2021 Spring, the Imaging X-ray Polarimetry Explorer (IXPE) is a NASA Astrophysics Small Explorer Mission with significant contributions from the Italian space agency (ASI). The IXPE observatory features three identical x-ray telescopes, each comprised of a 4-m-focal length mirror module assembly (MMA, provided by MSFC) that focuses x-rays onto a polarization-sensitive, imaging detector (contributed by ASI-funded institutions). This paper summarizes the MMA’s design, fabrication, alignment and assembly, expected performance, and calibration plans.

The eXTP (enhanced X-Ray Timing and Polarimetry) mission is a Chinese science space mission developed in collaboration with many international countries. Devoted to observations in the X-ray band, with imaging, spectroscopic, timing and polarimetry capabilities, is now entering phase B. The payload includes 9 Spectroscopic Focusing Array (SFA) and 4 Polarimetry Focusing Array (PFA) telescopes. The SFA telescopes, equipped with SDDs, have a spatial resolution of 1 arcmin, while the PFA telescopes, equipped with imaging gas pixel photoelectric polarimeters, have a spatial resolution of 30 arcsec. Both optics work in the 0.5-10 keV range with a focal length of 5.25 m and a field of view of 12 arcmin. The technology used for the optics production is Nickel electroforming from super-polished mandrels, like many previous successful X-ray missions. The reflecting coating is a double layer Au+C, which ensures optimal response at high and low energies. The PFA and SFA have the same optical design, in order to minimize the number of mandrels to be produced. In this paper, we present the optical design of these telescopes assisted with raytracing and a preliminary concept for the mechanical design supported by FEM simulation.

The capability of an X-ray telescope depends on the quality of its mirror, which can be characterized by four quantities: point-spread-function, photon-collecting area, field of view, and energy bandwidth. In this paper, we report on our effort of developing an X-ray mirror technology that advances all of those four quantities for future X-ray astronomical missions. In addition, we have adopted a modular approach, capable of making mirror assemblies for missions of all sizes, from large missions like Lynx, to medium-sized Probes like AXIS, TAP, and HEX-P, to Explorers like STAR-X and FORCE, and to small sub-orbital missions like OGRE. This approach takes into account that all X-ray telescopes must be spaceborne and therefore require their mirror assemblies be lightweight. It is designed to make use of modern mass production techniques and commercial off-the-shelf equipment and materials to maximize production throughput and thereby to minimize implementation schedule and costs.

Progress within the field of x-ray astronomy depends on astronomical x-ray observations of ever-increasing quality and speed. Fast and high-resolution x-ray observations over a broad spectral range promise amazing new discoveries. These observations, however, require a spaceborne x-ray telescope of unprecedented imaging power. Of the numerous technological concerns associated with the design and construction of such a telescope, the x-ray focusing optics present a particularly complex and arduous set of challenges. An x-ray optical assembly comprises many thousands of x-ray mirrors, a most critical element. Our group at NASA Goddard Space Flight Center (GSFC) pursues the development of an x-ray mirror manufacturing process capable of meeting the stringent quality, production time, and cost requirements of the next-generation of x-ray telescopes. The manufacturing process employs monocrystalline silicon: a lightweight, stiff, thermally conductive, and readily available material which is free of internal stress; it is a nearly ideal material for a thin mirror substrate. The process involves various traditional optical fabrication techniques adapted to x-ray mirror geometry. Presently, our process is capable of fabricating sub-arcsecond half-powerdiameter (HPD) resolution mirror pairs (primary and secondary) at a mirror thickness of 0.5 mm and of virtually any x-ray optical design (e.g. Wolter-I, Wolter-Schwarzschild, etc.). The mirror substrate surface quality is comparable to, and sometimes exceeding, that of the mirrors on the Chandra X-ray Observatory. This paper describes the various manufacturing steps involved in the production of x-ray mirror substrates and a present status report.

The thermal oxide patterning method has been proven effective for compensating coating stress on flat silicon wafers. In this paper we report progress of applying this method on thin X-ray telescope silicon mirrors, produced by GSFC, which are 100 mm long, 0.5 mm thick, 30 degree in azimuthal span and 312 mm in radius of curvature (Wolter-I geometry). The mirrors’ front sides are sputter coated by 20 nm iridium layers which have ~-70 N/m compressive stress. Due to the difference in geometries, we developed a new FEA model to calculate the duty cycle in thermal oxide hexagon patterns to compensate the coating stress. In addition, a new fabrication process with customized photo lithographic tools has been developed to achieve optimum patterning-precision on curved surfaces. Measurement results show that the developed method can mitigate coating-induced distortion by a factor of ~4, corresponding to ~0.5 arc-second in RMS slope error. The residual surface error is dominated by mid-frequency ripples produced by the annealing process, which will be resolved in the future. The developed thermal oxide patterning method could be inexpensive and precise, which could be a candidate to resolve coating stress issues for the Lynx telescope in the future.

Recent advances in the fabrication of segmented silicon mirrors make it possible to build large-area, lightweight, high-resolution x-ray telescopes with arc-second angular resolution. To build such a telescope, we fabricate accurate silicon mirrors and develop alignment and bonding techniques to precisely align and integrate these silicon mirror segments into modular units. In this way, the processes of mirror fabrication, mirror alignment and bonding, and subsequent integration into units of successive larger scale are completely independent, and their technologies can be developed independently. In this paper, we present recent improvement in the precision of optical alignment and mirror bonding. We discuss the measurement of the mirror’s focusing in a parallel optical beam and address the practical challenges in bonding these mirrors into modules as an intermediate step to build up a full-scale telescope for space missions.

The fundamental developmental issue facing the next generation of X-ray astronomical telescopes is the manufacturability, assembly, and structural robustness of the grazing incidence optics. Combining the high angular resolution requirements with large effective areas and physical launch vehicle restrictions leads to very thin shelled optics that must remain very stable. Meeting these stability requirements while also surviving launch and space environments presents a significant engineering challenge. Over the last few years, the Next Generation X-ray Optics (NGXO) team at NASA Goddard has been developing thin segmented silicon optics that are assembled into both modules and meta-shells, which show great promise in meeting these challenges. This paper summarizes the analytical approaches, as well as the environmental tests, used to assess such assemblies. Many parameters in the design space of the assembly have been assessed and optimized using Finite Element (FE) models and ray trace algorithms. The results of these analyses have helped shape reasonable and justifiable error budgets, as well as guide the team’s decision making in both near and long term processes. The structural integrity of an assembly has been assessed both with testing and FE models. Preliminary strength testing has been conducted on the basic components used in the assembly.

Astronomical observations of distant and faint X-ray sources will expand our understanding of the evolving universe. These challenging science goals require X-ray optical elements that are manufactured, measured, coated, aligned, assembled, and tested at scale. The Next Generation X-ray Optics (NGXO) group at NASA Goddard Space Flight Center is developing solutions to the challenges faced in planning, constructing, and integrating X-ray optics for future telescopes such as the Lynx Large Mission concept for the Astro2020 Decadal Survey on Astronomy and Astrophysics, Probe Mission concepts AXIS, TAP, and HEX-P, the Explorer Mission concepts STAR-X and FORCE and the sub-orbital mission OGRE. The lightweight mirror segments, efficiently manufactured from blocks of commercially available monocrystalline silicon, are coated, aligned, and fixed in modular form. This paper discusses our first attempt to encapsulate our technology experience and knowledge into a model to meet the challenge of engineering and production of the many modules required for a spaceflight mission. Through parallel lines of fabrication, assembly, and testing, as well as the use of existing high throughput industrial technologies, ∼10⁴ coated X-ray mirror segments can be integrated into ∼10³ modules adhering to a set budget and schedule that survive environmental testing and approach the diffraction limit.

Mission studies and technology preparation for the ATHENA (Advanced Telescope for High ENergy Astrophysics) [1- 5] mission are continuing to progress. The X-ray optics of this future space observatory are based on the Silicon Pore Optics (SPO) technology [6-58], and is being evolved in a joint effort by industry, research institutions and ESA. The SPO technology benefits from substantial investments made by the semiconductor industry, and spins-in materials, processes and equipment into the development of novel X-ray optics. A comprehensive Technology Development Plan (TDP) is being implemented, funded by ESA and involving a large number of experts in key areas ranging from micro machining of Silicon, over sophisticated automation and robotic systems, to hybrid manufacturing. The performance, environmental compatibility and serial automated production and testing are being addressed in parallel, aiming at the demonstration of the required technology readiness for the ATHENA Mission Adoption Review (MAR) expected by the end of 2021. A formal Technology Readiness Assessment is in place and is being currently exercised in preparation of the ATHENA Mission Formulation review (MFR). The programmatics for the flight model implementation is being defined in detail, and preparations are starting for the design and implementation of the necessary facilities. An overview of the ATHENA optics technology preparation, the technology readiness assessment and the related activities is provided.

The Silicon Pore Optics (SPO) technology has been established as a new type of X-ray optics enabling future X-ray observatories such as ATHENA. SPO is being developed at cosine together with the European Space Agency (ESA) and academic as well as industrial partners. The SPO modules are lightweight, yet stiff, high-resolution X-ray optics, allowing missions to reach a large effective area of several square meters. These properties of the optics are mainly linked to the mirror plates consisting of mono-crystalline silicon. Silicon is rigid, has a relatively low density, a very good thermal conductivity and excellent surface finish, both in terms of figure and surface roughness. For Athena, a large number of mirror plates is required, around 100,000 for the nominal configuration. With the technology spin-in from the semiconductor industry, mass production processes can be employed to manufacture rectangular shapes SPO mirror plates in high quality, large quantity and at low cost. Within the last years, several aspects of the SPO mirror plate have been reviewed and undergone further developments in terms of effective area, intrinsic behavior of the mirror plates and mass production capability. In view of flight model production, a second source of mirror plates has been added in addition to the first plate supplier. The paper will provide an overview of most recent plate design, metrology and production developments.

We present the latest progress on the industrial scale coating facility for the Advanced Telescope for High-ENergy Astrophysics (ATHENA) mission. The facility has been successfully commissioned and tested, completing an important milestone in preparation of the Silicon Pore Optics (SPO) production capability. We qualified the coating facility by depositing iridium and boron carbide thin films in different configurations under various process conditions including pre-coating in-system plasma cleaning. The thin films were characterized with X-Ray Reectometry (XRR) using laboratory X-ray sources Cu K-α at 8.048 keV and PTB's four-crystal monochromator beamline at the synchrotron radiation facility BESSY II in the energy range from 3.6 keV to 10.0 keV. Additional X-ray Photoelectron Spectroscopy (XPS) measurements were performed with Al K-α radiation to analyze the composition of the deposited thin films.

Excellent X-ray reflective mirror coatings are key in order to meet the performance requirements of the ATHENA telescope. The baseline coating design of ATHENA was initially formed by Ir/B4C but extensive studies have identified critical issues with the stability of the B4C top layer which shows strong evolution over time and appears incompatible with the industrialization processes required for the production of mirror modules. Motivated by the need for a compatible top layer material to improve the telescope performance at low energies and based on simulated performance, a SiC top layer has been selected as the best substitute to B4C. We report the latest development of Ir/SiC bilayer coatings optimized for ATHENA and the characterization of coating performance and stability.

Silicon Pore Optics (SPO) is an enabling technology for future large-area space-based X-ray observatories, such as ESA's Athena mission and NASA's Arcus candidate mission. SPO consist of stacks of thin silicon mirrors, which together provide a large effective area with a relatively low mass. Stacks are produced by custom robots that bend the mirrors into the design shape and stack them. We discuss the latest developments in improving the stacking process. The main challenge is to minimize shape deviations in order to optimize the imaging resolution of the optic. During the stacking process, the shape of each plate is measured directly after it is added to a stack. This metrology allows us to quickly quantify the effect of different stacking recipes, which streamlines the development. We discuss recent improvements in reducing excess meridional curvature. Furthermore, we prepare for mass-production by optimizing the robotics for performance and reproducibility.

Silicon Pore Optics is the X-ray mirror technology selected for the European Space Agency's Athena X-ray observatory. We describe the X-ray testing and characterization cycle that the optics are subjected to at the PTB's X-ray Pencil/Paraller Beam Facility (XPBF) 1 and 2 beamlines at the synchrotron radiation facility BESSY II. Individual stacks are measured with a pencil beam to determine their optical quality and the orientation of the optical axis. Using metrics based on X-ray and manufacturing metrology, stacks are then paired in primary-secondary Wolter-I-like systems, that are in turn characterized to determine their optical performance. Finally, four stacks, two primaries and two secondaries, are assembled into a mirror module, that is also characterized, with pencil and wide X-ray beams. At each step models, metrology, and software are combined to arrive at the relevant parameters. We describe the methods used, and illustrate how the performance of imaging pairs can be described in terms of stack-level parameters.

Silicon Pore Optic (SPO) is the X-ray mirror technology selected for the Athena X-ray observatory. The optic is modular; in the current design, it is made of about 700 co-aligned mirror modules. SPO is produced as stacks of 35 mirror plates, which are then paired to form X-ray Optics Units (XOUs) following a modified Wolter I geometry. A mirror module is composed of two confocal XOUs bonded in between a pair of brackets allowing interfacing to the mirror structure. Mirror modules are assembled using the XPBF 2.0 beamline of PTB at the synchrotron radiation facility BESSY II, using pencil beam and dedicated jigs. In this paper we present the challenges and solutions related to making confocal mirror modules.

The European Space Agency (ESA) is developing the Athena (Advanced Telescope for High ENergy Astrophysics) X-ray telescope, an L-class mission in their current Cosmic Vision cycle for long-term planning of space science missions. Silicon Pore Optics (SPO) are a new type of X-ray optics enabling future X-ray observatories such as Athena and are being developed at cosine with ESA as well as academic and industrial partners. These high-performance, modular, lightweight yet stiff, high-resolution X-ray optics shall allow missions to reach an unprecedentedly large effective area of several square meters, operating in the 0.2 to 12 keV band with an angular resolution better than 5 arc seconds. As the development of Athena mission progresses, it is necessary to validate the SPO technology under launch conditions. To this end, ruggedisation and environmental testing studies are being conducted to ensure mechanical stability and optical performance of the optics before, during and after launch. At cosine, a facility with shock, vibration, tensile strength, long time storage and thermal testing equipment has been set up to test SPO mirror module components for compliance with the upcoming Ariane launch vehicle and the mission requirements. In this paper, we report on the progress of our ongoing investigations regarding tests on mechanical and thermal stability of mirror module components such as single SPO stacks complete mirror modules of inner (R = 250 mm), middle (R = 737 mm) and outer (R = 1500 mm) radii.

Silicon Pore Optics (SPO) uses commercially available monocrystalline double-sided super-polished silicon wafers as a basis to produce mirrors that form lightweight and stiff high-resolution x-ray optics. The technology has been invented by cosine and the European Space Agency (ESA) and developed together with scientific and industrial partners to mass production levels. SPO is an enabling element for large space-based x-ray telescopes such as Athena and ARCUS, operating in the 0.2 to 12 keV band, with angular resolution requirements up to 5 arc seconds. SPO has also shown to be a versatile technology that can be further developed for gamma-ray optics, medical applications and for material research. This paper will summarise the status of the technology and of the mass production capabilities, show latest performance results and discuss the next steps in the development.

The optics of ATHENA (Advanced Telescope for High-ENergy Astrophysics) – the next high-energy astrophysical mission of the European Space Agency – consists of 678 Silicon Pore Optics mirror modules integrated and co-aligned onto a common supporting structure. The integration process, already proved, exploits an optical bench to capture the focal plane image of each mirror module when illuminated by an ultra-violet plane wave at 218 nm. Each mirror module focuses the collimated beam onto a CCD camera placed at the 12 m focal position of the ATHENA telescope and the acquired point spread function is processed in real time to calculate the centroid position and intensity parameters. This information is used to guide the robot-assisted alignment sequence of the mirror modules. To implement the above process for the entire ATHENA optics, a dedicated vertical optical bench is being designed. The facility consists of a paraboloid mirror that collects the light from an ultraviolet point source and generates a single reference plane wave large enough to illuminate the 2.6 m aperture of the X-ray telescope; at 12 m from the ATHENA optics (focal plane position) a tower will support the CCD camera, where the light from each mirror module is focused. The facility must also allow an alignment accuracy of 1 arcsec for the integration of two mirror modules per day in any arbitrary integration sequence, including the option of removing, re-aligning, or replacing any mirror module. The detailed design of the optical bench and the status of the construction activities are presented.

The ATHENA X-ray telescope comprises an optical system with several hundreds of Silicon Pore Optics (SPO) Mirror Modules (MM) to be assembled. All the MMs have to be tested for acceptance before integration. INAF-Osservatorio Astronomico Brera is building in its premises of Merate (Italy) a unique pathfinder facility, BEaTriX, which is characterized by a broad (170 ×60 mm²), uniform and parallel X-ray beam (divergence ≤ 1.5 arcsec HEW) at the energies of 1.49 and 4.51 keV. BEaTriX prime goal is to prove that it is possible to perform the acceptance tests (PSF and Aeff) of the ATHENA SPO MM’s at the production rate of 3 MM/day. The system is very compact (9 × 18 m²) and it is designed with modular compartments where the vacuum can be broken independently to replace the optics under test. It works at a vacuum level of 10^-3 mbar, easily evacuated in a short time. The expanded and parallel beam is obtained with an X-ray microfocus source placed in the focus of a paraboloidal mirror, a monochromation stage with 4 symmetrically cut crystals, and an expansion stage where the beam is diffracted and expanded by an asymmetrically-cut crystal. The key axes of all the optical components are motorized in vacuum for a proper beam alignment. The expanded beam fully illuminates the aperture of the MMs, imaging the focused beam at 12 m distance on a CCD camera, with the sensor in vacuum and motorized in air for XYZ movements. A thermal box is also present to radiatively heat the MM and check its optical performances under different temperatures. The design of the facility started in 2012 and has been finalised under an ESA contract. After completing the design, the facility is now in the realization phase. This paper provides an overview of the current status of the facility realization.

Calibration of the ATHENA telescope is a critical aspect of the project and raises significant difficulties due to the unprecedented size, mass and focal length of the mirror assembly. The VERT-X project, financed by ESA and started in January 2019 by a Consortium led by INAF and which includes EIE, Media Lario Technologies, GPAP, and BCV Progetti, aims to design an innovative calibration facility. In the VERT-X design the parallel beam, needed for calibration, is produced placing a source in the focus of an X-ray collimator. This system is mounted on a raster-scan mechanism which covers the entire ATHENA optics. The compactness of the VERT-X design allows a vertical geometry for the ATHENA calibration facility, with several potential benefits with respect to the long horizontal tube calibration facilities.

The Athena Mirror comprises an array of Silicon Pore Optics (SPO) modules which conform to the Wolter- Schwartzschild (W-S) geometry. In common with all Wolter Type I telescope mirrors the SPO modules produce stray X-ray flux which does not form a focused image but nevertheless impacts on the focal plane and introduces an extra background component in the X-ray images. This stray X-ray flux has 3 components, single reflections from the 2ndWolter I surfaces (the hyperbolic surface), reflections from the backsides of the reflecting membranes and support ribs in the SPO structure and scattering associated with the 2-reection imaged X-rays. We present a detailed analytical model for predicting the distribution of the stray X-ray flux in the Athena Mirrors and assess the impact of this on the sensitivity and imaging capability of the Athena Observatory.

ATHENA is the 2nd ‘Large’ mission in the ESA Cosmic Vision programme, and currently in Phase A with a view to adoption in 2021. This paper presents an overview of the ATHENA mission architecture, instruments and spacecraft design, highlighting the main spacecraft-level challenges involved in realizing the mission. Firstly a consideration of the accommodation of the Silicon Pore Optics (SPO) Mirror Modules (MM) is presented, taking into account: The procedure for MM integration into the Mirror Assembly (MA) to achieve the required alignment; stiffness requirements and handling scheme required to constrain deformation under gravity during x-ray testing; temperature control to constrain thermo-elastic deformation during flight; the capability to focus using the Instrument Switching Mechanism (ISM), and strategies to minimize mechanical loading of the MMs during launch. Then the accommodation of the X-IFU and WFI instruments onto the Science Instrument Module (SIM) is presented, considering in particular: The very large thermal dissipation (~4 kW at a variety of interfaces and temperatures) necessitating a challenging thermal design for the SIM; the location of the SIM at the top of the launch stack, necessitating combination of a stiff SIM design with maximized instrument support, and mechanical-damping measures to maintain the loads seen by the instruments to feasible levels. The paper concludes with a programmatic outlook to the adoption.

Low density overcoatings (mainly based on materials containing Carbon) onto usual high-density coatings (based i.e. on materials like e.g. like Ir, Au or Pt) have been proposed since many years ago in order to enhance the X- ray reflectivity at low energy (between 0.5 and 4 keV) of X-ray astronomical optics. The trick is to make use of the total reflection from the thin low-density material (which does not suffer much the photoelectric absorption) at low X-ray energies; the reflection of photons at higher energies (< 4 keV) occurs thanks to the much denser material under the overcoating. For several future projects, like e.g. ATHENA, LYNX and eXTP, it is foreseen the use of low-density overcoatings that will importantly increase the effective area at low X-ray energies. In this paper we will introduce the use of overcoatings based on materials different from the usual ones considered so far like C, B4C and SiC. In particular, we will discuss about a novel approach based on the use of thin layer of a Carbon-like materials deposited using a dip coating method. A possible combination with an intermediate thin layer of Chromium deposited e.g. via sputtering onto the usual high density material (Ir, Au or Pt) before the application of the Carbon-like material is also considered in the study, because it can further greatly enhance the soft X-ray effective area of future X-ray telescopes.

Lynx is one of the four large strategic missions considered in the Astro2020 decadal survey. On the basis of Chandra heritage, Lynx is an X-ray Observatory with superb imaging capabilities (< 1 arcsec Half Energy Width, HEW) but with a larger throughput (2 m² effective area @ 1 keV). One of the technologies considered for the mirrors realization is based on monolithic glass (fused silica) shells, figured with direct polishing technique. The simplicity of the full shell concept is quite attractive. The complete optics module could be composed of few hundreds of parts, instead of several thousands of pieces. As a drawback, to be compliant with the mass budget, the shell thickness should be maintained very small even for larger mirror shell (<4 mm for mirror shells up of 3 m diameter). Starting from the opto-mechanical design of the Lynx mirror assembly based on this approach, a technology development roadmap is going to funded by ASI and realized by INAF/OAB. In this paper we discuss the work plan and the advancements obtained in the realization of a new breadboard, based on a fused silica semi-shell.

In an effort to manufacture high-angular-resolution, grazing-incidence, x-ray optics, Marshall Space Flight Center (MSFC) is taking measures to improve its electroformed replicated optics. A key development is the use of computer-numerical control (CNC) polishing to deterministically improve the surface of electroless nickel mandrels used to replicate grazing-incidence optics. Metrology, control software and polishing parameters must function together seamlessly to reach the specifications required to replicate sub-arcsecond optics. Each change in polishing parameters effects the wear pattern of the polishing head. Using Richardson-Lucy deconvolution, the controller software fits the wear pattern to metrology data to calculate the changing feedrates across the mandrel. Here we present an overview of our process, and early results showing the effectiveness of deterministic polishing for replicated optics.

Cold Slumping Glass Optics is a cost-effective technique to realize segmented X-ray optics. It is based on the replica approach: very thin flat glass foils are shaped to theoretical form by means of integration molds and fixed into stacks by means of ribs. As the theoretical shape of each of the layer depends on the radius of curvature inside the assembly, in principle different integration molds are needed to realize a stack. A worthwhile solution would be the usage of a single adjustable mold with and ‘optical surface’ to be shaped according to the position of the plate in the stack. The selection of the optical surface material and the bending strategy are the most relevant points. Glass would give major benefits in terms of thermal coupling with the glass foils, scratch resistance during operations and costs. On the other hand, given the reduced bending capability of the material coupled with its intrinsic fragility, experimental tests are necessary to assess the feasibility of the solution. In this paper we present the design and the implementation study as realized by the PoliMI-DAER. An experimental test campaign has been carried out beside the theoretical studies, to validate numerical models and obtain feedbacks from an early implementation. The test campaign is expounded along with the analysis of the results. A preliminary comparison with the numerical simulations is given.

The cold shaping of thin glass foils is a cost-effective method for x-ray optical units realization. This technique allows the manufacturing of cheap and lightweight optics with good angular resolution. The flat glass foils are shaped on curved integration molds and glued ribs freeze their shapes in the assembly. The simplest strategy for the coating deposition would be with the coating process operated on flats, before the bending and the integration steps. This approach can be considered only if the coating on the glass foils does not degrade with the integration process. Given that super-mirror coating is widely used in hard X-ray telescopes, this part of the process need to be carefully evaluated. INAF-OAB collaborates with Tongji University to verify the feasibility of this approach. In this paper, we present the experimental results achieved on glass samples, integrated with the Cold Slumping technique by means of integration mold with different radius of curvature and material. Two flat samples, coated with W/Si multilayer, are characterized before and after the integration. The measurement campaign includes micro-roughness on different spatial frequency and grazing incidence reflection measurements to return the microstructure of the multilayer.

X-ray focusing optics is one of the most important technologies for X-ray astronomy with capabilities of direct imaging, low background and high sensitivity. The enhanced X-ray Timing and Polarimetry (eXTP) and hot universe Baryon surveyor (HUBS), are proposed in China as two major X-ray missions for the next decade. The thermally slumping glass optics (SGO) is one of the approaches to satisfy the requirement of large collecting areas and high angular resolution. In the last years, we made a great progress in the research of SGO X-ray focusing telescope and established the whole fabrication platform. This paper will give a review of recent developments in each step, including lens fabrication, multilayer coating, telescope assembly and optical performance testing. We established an in-situ multi-probe platform based on the contact, trigger and non-contact probes. Thus, the radius deviation and the straightness of graphite strip, the surface shape and the radius of the inner surface of each layer can be measured during the telescope assembly process. We also developed a three-dimensional ray-tracing method to calculate the performance degradation induced by the deformed surface appearing on the forming process and optical assembly. Then the result could be feed back into the fabrication process for improving upon the later mounted mirrors in the real time. We have successfully fabricated two telescope prototypes with the focal length of 2000mm in the last years. The optical performance of the two 3-layer and 21-layer prototypes, which was numbered as No.6 and No.7 prototype respectively, were tested in PANTER test facility of MPE in September 2018. The measured angular resolution t of section A of No.6 prototype is 67±2 arc-second at 8keV, and the measured angular resolution in full aperture of No.7 telescope prototype is 96±3 arc-second at 8keV, which is much better than the previous prototypes.

We have fabricated a blazed X-ray reflection grating with a period of 160 nm using thermally activated selective topography equilibration (TASTE). The grating was tested for diffraction efficiency using the soft X-ray reflectometer at Lawrence Berkeley National Laboratory's Advanced Light Source. Preliminary results show total absolute diffraction efficiency ≥ 40% at lower energies, with maximum single order diffraction efficiency ranging from 20-40%. Total diffraction efficiency was ≥ 30% across the entire measured band pass of 180 eV to 1300 eV.

Historically, ultraviolet (UV) gratings used in astronomical spectrographs have been made using two different techniques. Mechanically-ruled gratings have been produced for more than an hundred years. A diamond tool is used to create parallel grooves onto a fine metallic coating layer that has been previously deposited on a polished substrate. The edge-on profile of these grooves is a sawtooth tilted at a specific angle called the "blaze" angle. The blaze angle is the deciding factor that will allow for maximum efficiency at a specific wavelength called the "blaze" wavelength. Holographic gratings are produced using interference lithography. A layer of photoresist deposited on a substrate is exposed to fringes usually formed by exposure to two coherent laser beams. After development, the resulting pattern is sinusoidal by nature. Blazing one side of the sinusoidal profile of holographic gratings is possible through ion etching, but is only commonly used for a limited subset of grating parameters. Both types of gratings have different advantages: - Ruled gratings have a sharp, blazed profile that ensures good efficiency. However, they suffer from groove spacing inconsistencies that lead to stray light and ghosting. - Holographic gratings do not suffer from groove spacing errors which minimizes stray light. However, their sinusoidal profile (if not ion etched) leads to efficiency losses. At Penn State University, we are studying new processes that allow to fabricate blazed, high efficiency and high spectral resolution UV gratings for astronomical purposes. These processes are derived from the ones we used in creating X-ray gratings and consist of: - writing an electron-beam pattern consisting of parallel grooves on a layer of positive resist that has been deposited on a Si substrate. - dry etching this profile into a hard mask made of silicon nitride. - wet etching with KOH that will create the sawtooth profile at a specific blaze angle thanks to the properties of the different crystallographic planes of silicon. Gratings fabricated using these techniques show an excellent behavior all around, combining the best qualities from both ruled and holographic gratings. Indeed, they display a sharp sawtooth profile, they do not suffer from periodicity errors, and grating facets show low roughness. Recent testing show efficiencies that are close to the maximum, theoretical limit. Combined with the possibility to create custom blaze angles through the use of custom cut Si wafers, this opens the way to new applications in the field of astronomical UV spectroscopy.

We report on a concept for curved diffractive X-ray optics (CDXO). CDXO would enable potential new optical designs for X-ray instruments, such as a two element spectrometer, in which the secondary element of an X- ray mirror pair is patterned with an X-ray grating. A two-element spectrometer design eliminates the grating assembly, resulting in decreased instrument mass and reduced power requirements. In addition, a two-element spectrometer may realize cost/schedule savings by eliminating a separate grating alignment effort, and yield increased effective area over three element designs. We present a raytrace concept study of a two-element spectrometer compact enough to fit within a sounding rocket payload. We also review the progress made in electron-beam lithography (EBL) techniques that would enable curved diffractive X-ray optics to be patterned, and outline a procedure by which the accuracy of the EBL patterning process can be measured interferometrically.

The Off-plane Grating Rocket Experiment (OGRE) is a soft X-ray spectroscopy suborbital rocket payload designed to obtain the highest-resolution soft X-ray spectrum of Capella to date. With a spectral resolution goal of R(λ/▵λ) < 2000 at select wavelengths in its 10-55 Å bandpass of interest, the payload will be able to study the line-dominated spectrum of Capella in unprecedented detail. To achieve this performance goal, the payload will employ three key technologies: mono-crystalline silicon X-ray mirrors developed at NASA Goddard Space Flight Center, reflection gratings manufactured at The Pennsylvania State University, and electron-multiplying CCDs developed by The Open University and XCAM Ltd. In this document, an updated optical design that can achieve the performance goal of the OGRE spectrometer and a new grating alignment concept to realize this optical design are described.

Future astronomical X-ray spectrometer missions call for high spectral resolution in conjunction with high throughput. To achieve both of these requirements simultaneously, many grating elements must be aligned such that their diffraction arcs overlap at the focal plane. Methods for the alignment of reflection gratings operated in the extreme off-plane mount are being developed at The Pennsylvania State University in support of the Off-plane Grating Rocket Experiment. We report on the alignment methodology and performance tests of an aligned reflection grating module.

High resolution absorption and emission line spectroscopy in the soft x-ray band are promising techniques to measure hot baryon distributions in extended galaxy halos, galaxy clusters, and the connecting filaments of the cosmic web. It can characterize outflows from supermassive black holes and help understand their impact on the structure and evolution of the interstellar medium and beyond. Stellar magnetic activity, accretion, coronal emission and flares are additional targets for soft x-ray spectroscopy. Higher resolving power and larger effective area than those provided by current missions are required in order to make progress. Recent breakthroughs in x-ray diffraction grating and mirror technologies promise order-of-magnitude improved performance if new missions were built today, and even greater improvements beyond the next decade. Here we describe recent progress in critical-angle transmission (CAT) grating technology. CAT gratings combine the advantages of traditional transmission gratings (relaxed alignment tolerances, low mass, transparency at higher energies) and blazed reflection gratings (high diffraction efficiency, high resolving power R due to blazing into high orders). CAT gratings have demonstrated resolving power greater than 10,000 and absolute diffraction efficiency above 30% in the soft x-ray band. They are an enabling technology for the Arcus grating spectrometer Explorer (R < 2500, effective area < 250 cm²) and a candidate for the Lynx x-ray grating spectrometer (R < 5000, effective area < 4,000 cm²).

BeppoSAX was an Italian-Dutch satellite designed for X-ray observations, launched in 1996 and deactivated in 2002. It was the first satellite to precisely localize the Gamma Ray Bursts and to detect the X-ray emission due to the afterglow, a breakthrough discovery for high energy astrophysics that eventually placed such events at extra-galactic distances after many debates in the previous years. On board of the satellite there were four identical X-ray imaging telescopes based on grazing incidence mirrors, ranging between 1-10 keV in energy and with a spatial resolution <1 arcmin HEW. In order to limit the costs of the telescopes, the optics were assembled using nickel nested mirrors replicated via electroforming using masters. During the development of the instruments of BeppoSAX, two qualification models of the grazing mirrors were assembled and used for tests at various X-ray facilities; they are now in storage in Milano (Italy) at the INAF-IASF Institute and Brera Observatory. In this paper, we describe the setup we have built in order to use these models for an outreach program. The aim of this effort is to illustrate to students and teachers of secondary schools how a grazing mirror for X-ray astronomy works. A first, basic experiment shows the reflection of light at normal incidence: using two small face-to-face parabolic reflectors, a powerful lamp positioned in the focus of the first parabola can light a match fixed in the focus of the other reflector. A second experiment uses the nested mirror to show how to deflect photons using grazing incidence. In this case a plane wave, created with a light source and a Fresnel lens, enters in the front of the Beppo SAX optics made of several nested mirror shells and is focused exactly as for X-rays.

The capability of future x-ray telescopes depends on the quality of their Point Spread Function (PSF) and the size of their field of view. Traditional designs, such as Wolter, and Wolter-Schwarzschild telescopes are stigmatic on the optical axis but their PSF degrades rapidly off-axis. At the optimal focal surface, their PSFs can be significantly improved. We present a simple optimization process for Wolter (W), Wolter-Schwarzschild (WS) and Hyperboloid- Hyperboloid (HH) telescopes that substantially improves the off-axis PSF for either narrow or wide field of view applications. In this paper, we will compare the optical performance of conventional and optimized W-, WS-, and HH-telescopes for a wide range of telescope diameters that can be used to build up future x-ray telescopes.

The PANTER X-ray test facility of the Max Planck Institute for Extraterrestrial Physics (MPE) has over 40 years of heritage in testing and calibrating x-ray optics. Having contributed to missions such as XMM-Newton, Chandra, and eROSITA, the facility measures the performance of x-ray optic technologies that will enable future x-ray telescopes to be realised. Over the last year, PANTER has been testing the latest developments in silicon pore optics for ESA’s ATHENA mission, as well as full-shell eROSITA-like optics for the CAS/ESA/MPE Einstein Probe mission. For ATHENA, complete mirror modules for the outer radius of the telescope have been tested. The latest developments in the optics for the mid-radius of the telescope, including the first confocal mirror module, have been measured for performance. The paper will provide an overview of the most recent testing carried out at PANTER, and the alignment and measurement techniques used.

Recently NASA Marshall Space Flight Center has made good progress in employing computer numerical control (CNC) polishing techniques on electroless nickel mandrels, as part of our replicated grazing incidence optics program. CNC polishing has afforded the ability to deterministically refine mandrel figure, therefore improving mirror performance. The Marshall Grazing Incidence X-ray Spectrometer (MaGIXS) is a MSFC-led sounding rocket instrument that comprises some of the first mirrors produced at MSFC using this polishing technique. Here we present the predicted mirror performance obtained from metrology, after completion of CNC polishing, as well as the results of X-ray tests performed on the MaGIXS telescope mirror before, and after mounting.

Thin-walled X-ray optics are going to be required to meet the demands of large collecting area versus volume and mass for the next generation X-ray astronomy Flagship Mission. We report here our progress on our concept of meeting the challenge of producing these mirrors. The case we address is the one where the initial fabrication process requires post-fabrication figure correction. Our technology can be applied prior to launch and also enable in-flight figure corrections. Our process is to coat a film of magnetic smart material onto the backside of the thin-walled X-ray mirrors. Then, an electromagnet is used to produce an in-plane stress and thus reshape the mirror. We show in this paper that 500 μm thick Si wafers can be coated and after coating remains significantly flat, i.e. they have a radius of curvature of about 30 m. We have carried out deflection measurements as a function of the external magnetic field of about 0.1 to 0.3 T and found a nearly linear relationship. We also revisited the stability of induced deflections for up to nearly 70 hours and also demonstrated that the process can produce deflections for fine-scale figure adjustments of order (10 nm) range deflections.

Next-generation soft X-ray missions, such as the Lynx X-ray observatory, are being proposed to increase the effective area of Chandra while keeping sub-arcsecond imaging resolution. Low-voltage thin-film actuators based on electroactive polymers can address the need for adjustable mirror control in future X-ray missions such as Lynx. Polymers can produce high strains at low voltages and correct the deformations that submillimeter-thick mirror shells will experience in future X-ray missions. Fabrication of polymer-based thin films is a low-cost, scalable technology that can easily be translated to production by industrial partners. With processing temperatures below 140^oC, electroactive polymer films can be deposited on glass mirror substrates without risk of introducing additional slumping errors. We will present metrology results on prototype actuators based on micrometer-scale PVDF-TrFE polymer films deposited on glass substrates.

Future space X-ray telescopes, for example, the Lynx X-ray Observatory under study in the 2020 Astrophysics Decadal Survey, require high angular resolution, large field of view and large effective area X-ray mirrors. Various scientific, engineering and economic considerations make the manufacturing of the telescope optics challenging. In spite of many major improvements in current methods, including slumping (glass shaping), silicon pore optics, and monocrystalline silicon polishing, etc., the required resolution and stability in thin optics have not yet been demonstrated. Furthermore, the high reflective coating films on the mirrors can stress and distort the mirror figure. Therefore, additional steps to correct the mirrors are needed to achieve the stringent requirements for the next generation high-performance X-ray mirrors. In this paper, we demonstrate a novel X-ray mirror figure correction method with the use of femtosecond lasers. Over the last two decades, rapid developments of ultrafast laser technologies have triggered wide applications in the processing of both transparent and opaque materials, from material micromachining to nano-surgeries. We apply this technology in a novel stress-based figure correction technique for X-ray telescope mirrors. We use femtosecond laser beams to micromachine thermal oxide layers on the back side of silicon mirrors, from which regions of intrinsic compressive stress are removed. We pattern laser micromachined spots over the full mirror to compensate the undesired stress introduced from mirror manufacturing processes and reflective coatings. We built a new optics setup using an infrared laser of 220 fs pulse duration and 1 kHz repetition rate, and we designed a procedure for imaging, correcting and measuring mirror substrates with this setup. In this paper, we present the experimental results on the stress manipulation in at silicon substrates, showing the laser induced integrated stress increases almost linearly with the fraction of area removal in the micromachining. This indicates great potential for correcting thin silicon optics by using appropriate machining parameters for future X-ray telescopes.

Hard X-ray astronomy is a crucial field for understanding physical processes occurring in several celestial objects, characterized by extreme physical conditions and strong gravity regime, but still poorly explored. Current hard-X ray instrumentation suffers from a poor angular resolution, compared with other wavelengths, and insufficient continuum sensitivity to resolve hard X-ray spectra, especially above few hundreds keV. Laue lenses made with bent crystals represent a viable solution to overcome both limitations, providing good angular resolution ( better than 30 arcsec) combined with excellent sensitivity (orders of magnitude better than that achievable with the current non focusing instrumentation). Supported by the Italian Space Agency, we test a modular method for building Laue lenses. This consists in the realization of portions of Laue lenses that are successively tuned and aligned under the control of a gamma-ray source for focusing the radiation on the common Laue lens focal point. Each module, composed with few tens of crystals, can be realized with an accuracy better than 20 arcsecs with the goal of achieving an overall alignment better than 30 arcsec. In addition, we present a significant improvement in the realization of a large number of bent crystals with the required curvature.

Focusing optics for hard x-/soft gamma-rays (above 100 keV) are in a development phase. One promising method is represented by the Laue lens technology that has already been validated through on-ground prototypes and balloon tests. Laue lens optics will be an outstanding tool for observing weak sources in a notably short integration time thanks to the excellent sensitivity they can provide. Such performances has been further increased with the employ of cylindrical bent crystals that are capable to dramatically narrow the Laue lens Point Spread Function (PSF). One aspect that is under investigation is the image aberrations for off-axis sources. This fact limits the Field of View (FoV) of a Laue lens to few arcmin. The employ of bent crystals in double diffraction configuration would reduce the mentioned aberration, increasing the FoV of the resulting Laue lens. Double diffraction crystals would represent an extension to hundreds of keV of the Lobster Eye (LE) principles that is well tested for focusing < 10 keV photons. We investigate pros and cons of the double diffraction configuration with respect to the single diffraction through Monte Carlo simulations and we compare their performances in terms of efficiency, PSF, pass-band and effective area. We also present preliminary tests performed at the LARIX facility to evaluate the technical feasibility of crystals with the aforementioned characteristics.

ASTRI-Horn is a small-sized Cherenkov telescope (SST) developed by the Italian National Institute of Astrophysics (INAF) for very-high energy gamma-ray astronomy. It is characterized by a dual-mirror optical system and a curved focal surface covered by SiPM sensors managed by a non-conventional fast front-end electronics. Installed in Italy at the INAF "M.C. Fracastoro" observing station (Mt. Etna, Sicily), ASTRI-Horn has been developed implementing a complete end-to-end approach that includes all aspects from the design, construction and implementation of the entire hardware and software system to the final scientific products. ASTRI-Horn is the prototype for nine SSTs that will form the ASTRI mini-array, considered by INAF as pathfinder for the Cherenkov Telescope Array (CTA). This phase will permit to exploit previous experimented technologies for the realization of the telescope structure and mirrors in a large production scale. After a short description of the major results obtained by the prototype in Serra La Nave, the proposed design of the new telescope structure and the status of the production of the optics are described. Moreover a review about the integration and alignment strategies to reach the required performance of the mini-array is reported.

Qualification of coating performance at the low-energy range of the Advanced Telescope for High Energy Astrophysics (ATHENA) is important to ensure that the mirror coatings satisfy the performance criteria required to meet ATHENA's science objectives. We report on the design, implementation, and expected performance of a state-of-the-art Low-Energy X-ray Reflectometer (LEXR) acquired with the purpose of qualifying the soft energy X-ray performance of mirror coatings for ATHENA. The reflectometer components are housed in a vacuum chamber and utilizes a microfocus Al source with custom made Kirkpatrick-Baez mirrors and W/Si monochromator to produce a collimated beam of 1.487 keV photons. The system has been designed with source interchangeability, allowing reconfiguration to an 8.048 keV reflectometer using a Cu source or other energies with sources such as Fe, Mg, etc. Several mirror samples can be mounted on a motorized stage, and a 2D CCD camera is used to obtain spatially resolved detection.

The BEaTriX facility is currently in the realization phase at INAF – Osservatorio Astronomico di Brera in Merate (Italy) and it will be used as demonstrator for the acceptance test (PSF and Aeff) of the ATHENA Mirror Modules (MMs), with a rate of 3 MMs/day. The BEaTriX facility is a compact X-ray test facility (13m x 6 m x 2m) working in low vacuum at the energies of 1.5 keV and 4.5 keV. The testing optic is placed into a small chamber (MM chamber), that allows a fast MMs replacing time. All the MMs will be tested at room temperature (20°C), but the possibility of X-ray characterization at different temperature (-10°C / +50°C) is also foreseen. For this task a modular thermal box is designed and will be installed inside the MM chamber. The thermal box radiatively heats or cools the MM, by means of heat pipes that transfer the desired heat from dedicated chillers. It is designed to be able to impose a uniform temperature across the MM or impose also some gradient in order to characterize the optical performance of the MM in different thermal condition. In this paper, we present simulations of the temperature distribution at the MM level, as function of the thermal box design, which has been optimized considering the space envelope and result of the FEM simulations.

The BEaTriX (Beam Expander Testing X-ray) facility is now under construction at INAF-Brera Astronomical Observatory with the support of ESA. We aim to use it as a pathfinder to demonstrate the feasibility of the acceptance tests of the ATHENA’s Silicon pore optics Mirror Modules (MM), i.e., point spread function and effective area measurements at two energies for all MMs. A microfocus X-ray source placed in the focus of a paraboloid mirror will provide a collimated X-ray beam to the next stages featuring the facility, i.e., the monochromator and the beam expansion units, which will finally enable the full illumination of the mirror modules under test. The collimating mirror has to satisfy severe surface specifications to allow the divergence of the X-ray beam reaching the requirements. Simulations, based on physical optics, have shown that the optical quality of the mirror surface has to reach 0.5 arcsec Half-Energy Width (HEW) at 4.51 keV. We procured a ground and lapped mirror substrate in HOQ 310 fused quartz material. The first step of the process is based on the bonnet polishing technology with our CNC IRP1200 Zeeko machine, and it is guided by the analysis of the metrological data obtained by the optical profilometer present at Media Lario S.r.l. laboratories. The spatial frequency errors not correctable by bonnet polishing can be then removed by smoothing and superpolishing processes, followed by ion beam figuring to correct the remaining errors in the low frequency range. In this paper, we report on the analysis performed to correlate the requirement on beam divergence to the manufacturing tolerances on the surface. We present the manufacturing process and the results so far achieved.

X-ray reflectivity (XRR) characterization of X-ray mirrors is an essential step for designing space telescopes and instruments. We report on production and characterization of platinum thin films coated onto a at thick glass substrate for evaluating measurement results obtained using several XRR systems. The main objective of this study is to compare the XRR results measured using facilities at the Technical University of Denmark, DTU Space, and BESSY II for the Advanced Telescope for High-ENergy Astrophysics (ATHENA) mission funded by the European Space Agency, ESA. This sample will be used as a reference sample for testing and calibrating similar measurements at relevant X-ray facilities. This information demonstrates the stable performance of the platinum mirror as a reference sample. Also, the overlayer effect on mirror performance is investigated.

Studying astronomical objects in the X-ray regime, iridium-based layer systems are highly effective reflective materials for telescopes mirrors. Aschaffenburg University and the Czech Technical University in Prague jointly developed stress compensated chromium-iridium coatings. To overcome the disturbing reflectivity reduction of the iridium absorption edge around 2 keV photon energy and improve general reflectivity at lower incident energies, thin overcoat layers of chromium have been applied in addition. Corresponding measurements at several X-ray lines have been performed on these samples at the PANTER test facility of the Max-Planck Institute for extraterrestrial Physics. A part of the experimental results and their comparison with theoretical simulations are presented in this contribution.

A direct current (DC) magnetron sputtering system has been developed at NASA Marshall Space Flight Center (MSFC) for depositing broadband X-ray optic multilayer coatings. The system has the capability to house up to four, 2-inch diameter direct-cooled DC magnetron cathodes. The relatively small dimensions of the cathodes facilitate low-cost, early stage research and development of various types of optical coatings while producing good coating uniformity across substrates that are twice the cathode diameter. Sources of non-repeatability in the deposition of depth-graded multilayer coatings were identified and compensated for, resulting in the ability to implement complex multilayer coating designs.

We are developing Wolter-I X-ray optics for use in SmallSat missions. These optics are being designed for telescope focal lengths on the order of 0.5 - 1 m, much shorter than typical Astrophysics missions. The various parameters of the optics module: diameter, length, number nested shells, coatings, etc., depend partly on the spacecraft bus but the final design is driven by the science and instrument requirements of the mission (effective area, resolution, and energy band of interest). Ray trace software was developed and used to project the performance of several optics configurations, which, designed for SmallSat missions, meet the instrument requirements for the SmallSat Exosphere Explorer of hot Jupiters (SEEJ) . Results of this modeling is presented.

We present a ray tracing simulation of the Soft X-ray Imager (SXI) optics for the THESEUS observatory using the open-source McXtrace software package. THESEUS is a candidate mission within the Cosmic Vision programme of the European Space Agency and is designed to monitor high-energy transient events in the entire sky, therefore relying on fast slewing of the spacecraft. This capability will however impose large temperature variations and gradients across the optics which can cause thermoelastic deformation of the Soft X-ray Imager microchannel optics. The purpose of this study is to evaluate the impact on the point spread function (PSF) due to in-orbit temperature variation. Based on thermal analysis of the Soft X-ray Imager, perturbations to the ideal microchannel geometry are derived and provided as input to the ray tracer.

The Imaging X-Ray Polarimetry Explorer (IXPE) is an X-ray telescope mission currently being built at NASA Marshall Space Flight Center (MSFC). IXPE will measure polarized X-rays in the 2-8 keV range and will reveal new information about astrophysical sources that produce polarized X-rays such as pulsars, AGN, magnetars, and supernovae remnants. Telescope calibration at MSFC's Stray Light Test Facility 100-m beamline will require a bright, highly polarized X-ray source. The design of the IXPE polarized sources for end-to-end telescope calibration will allow for testing at three energies (Rh Lα₁ at 2.622 keV, Ti Kα₁ at 4.511 keV, and Fe Kα₁ at 6.404 keV). We present the design of the polarized sources and results based on laboratory operation.

A stress relief technique is presented to reduce the distortion of X-ray optics from adhesive shrinkage at bonding points. The bonding distortions are particularly relevant for thin-shell mirror optics and need to be minimized to meet subarcsecond angular resolution goals. Thin-shell optics are critical to achieving large collecting areas for X-ray telescopes, such as the Lynx mission concept. A localized topography modification of the optic surrounding the adhesive point has been explored to reduce global distortion. Previous efforts utilized finite element analysis (FEA) to study removal of material around the adhesive joint in an annulus pattern, creating a trench. Current work has improved the finite element model for the trench design as well as shown an experimental verification. These experiments involve using silicon wafers as test mirrors and silicon oxide to mimic stress induced by adhesive shrinkage. The silicon oxide is encompassed by the trench etch, which is used to demonstrate a reduction in global deformation. We present the recent progress made with this stress relief technique.

The Cherenkov Telescope Array (CTA) will be the next generation ground-based observatory for gamma-ray astronomy at very-high energies. It will consist of over a hundred telescopes of different sizes (small, medium, and large) located in the northern and southern hemispheres. The Italian National Institute of Astrophysics (INAF) contributes to CTA through the ASTRI project (Astrofisica con Specchi a Tecnologia Replicante Italiana), whose main aim is to provide a series of dual-mirror small-sized telescopes (SST-2M ASTRI) and the mirrors for the single-mirror design of the medium-sized telescopes (MST). Both the primary mirror of the SST-2M ASTRI and the mirror of the MST are segmented, and such segments are realized with cold-slumping technology already used for the mirror facets of MAGIC, a system of two Cherenkov telescopes operating on the Canary Island of La Palma. On the other hand, the secondary mirror of the SST-2M ASTRI is monolithic and is realized with hot-slumping technology. Currently, we have completed the mirror production for nine SST-2M ASTRI telescopes, which will form the so-called ASTRI Mini-Array. Moreover, we have almost completed also the production of mirrors for two MSTs. In this paper, we present the mirror designs and describe the qualification activities that were performed to assess and consolidate the production process. Moreover, we report on the quality assurance approach we adopted to monitor and verify the production reliability. Finally, we present the performance of the produced mirrors and discuss their compliance with the CTA requirements.

FlashCam is an innovative camera designed for the focal plane instrumentation of Cherenkov telescopes. The concept of the FlashCam trigger and readout system is based on the continuous digitization and digital processing of the photo detector signals on FPGAs, and on a high performance, Ethernet-based front-end readout. Because of the modular design, the electronics that have been developed can serve either photomultiplier tubes or silicon-based photon detectors. In the framework of the CTA (Cherenkov telescope array) project, the FlashCam team has developed a PMT-based camera that is suitable for the medium-sized telescopes. With over 100 Cherenkov telescopes, the CTA observatory will run the most sensitive ground-based telescope systems for TeV gamma-ray astronomy when the two arrays in the Northern and Southern Hemisphere (with three different telescope sizes) will go into operation in the upcoming years.

The measurement of the photomultiplier gain is a crucial step for Imaging Atmospheric Cherenkov Telescopes in order to translate the measured charge to a number of photons. For that, one of the possibility is to perform measurements with a flashing light source at a low intensity level, in order to reconstruct the charge distribution of a single photon. In this contribution, we will present a method to reconstruct the single photo electron spectrum using low intensity charge distribution. We will show that the usually adopted Gaussian assumption does not reproduce well this data, and will present a new model which explains the data behavior. The impact of this measurement on the gain determination will be presented, and the associated uncertainty will be discussed.