Mauro Ghigo

Characteristics of the flight model optics for the JET-X telescope onboard the Spectrum-X-Gamma satellite

The joint European x-ray telescope (JET-X) is one of the core scientific instruments of the RUssian SPECTRUM X-(gamma) astrophysics mission. JET-X is designed to study the emission from x-ray sources in the band of 0.3-10 keV; in particular to meet primary scientific goals in cosmology and extragalactic astronomy. JET-X consists of two identical, coaligned x-ray telescopes, each with a spatial resolution of better than 30 arcsec half energy width. Focal plane imaging is provided by cooled x-ray sensitive CCD detectors which combine high spatial resolution with good spectral resolution, including coverage of the iron line complex around 7 keV at a resolution of (Delta) E/E approximately 1.5 percent. Each telescope is composed of a nested array of 12 mirror shells with an aperture of 300 mm and focal length of 3500 mm; the total effective area is 330 cm2 at 1.5 keV and 145 cm2 at 8.1 keV. The mirror shells have a Wolter I geometry and are manufactured by an electroforming replica process. The paper presents the characteristic of the flight model x-ray optics.

Testing the pyramid wavefront sensor on the sky

The pyramid wavefront sensor is a novel concept device whose features are attractive for adaptive optics for several reasons. We show here the first loop closure of an AO system using this kind of sensor at the focal plane of a 4m-class telescope. One of the critical optical elements of our wavefront sensor is the pyramid that splits the light from the star used for the wavefront correction. This component is essentially a four faces prism having actually a full vertex angle of 7 degrees with specifications on its edges and roof of 4 - 5 microns or better. The best turned edges obtained on the prototypes already built have shown values of the order of 6 microns, with roofs of the same order, not far from the required tolerances. In this article we describe the techniques and the system used for the construction of this optical component and the improvements to the polishing procedure that we plan to adopt in order to increase the quality of its edges and optical surfaces. Pixel processing is suitable to fit with existing Shack-Hartmann systems, making this device an attractive add-on option for existing SH-based AO systems. The plans for future developments in order to firmly establish the performances of the pyramid wavefront sensor are briefed out.

Accurate integration of segmented x-ray optics using interfacing ribs

Abstract. Future lightweight and long-focal-length x-ray telescopes must guarantee a good angular resolution (e.g., 5 arc sec HEW) and reach an unprecedented large effective area. This goal can be reached with the slumping of borosilicate glass sheets that allow the fabrication of lightweight and low-cost x-ray optical units (XOU). These XOUs, based on mirror segments, have to be assembled together to form complete multishell Wolter-I optics. The technology for the fabrication and the integration of these XOUs is under development in Europe, funded by European Space Agency, and led by the Brera Observatory (INAF-OAB). While the achievement of the required surface accuracy on the glass segments by means of a hot slumping technique is a challenging aspect, adequate attention must be given to the correct integration and coalignment of the mirror segments into the XOUs. To this aim, an innovative assembly concept has been investigated, based on glass reinforcing ribs. The ribs connect pairs of consecutive foils, stacked into a XOU, with both structural and functional roles, providing robust monolithic stacks of mirror plates. Moreover, this integration concept allows the correction of residual low-frequency errors still present on the mirror foil profile after slumping. We present the integration concept, the related error budget, and the results achieved so far with a semi-robotic integration machine especially designed and realized to assemble slumped glass foils into XOUs.

Glass mirrors by cold slumping to cover 100 m2 of the MAGIC II Cherenkov telescope reflecting surface

We report on the production and implementation of 100 square panels 1 m x 1 m, based on the innovative approach of cold slumping of thin glass sheets. The more than 100 segments will cover around one half of the 240 m-square reflecting surface of the MAGIC II, a clone of the atmospheric Cherenkov telescope MAGIC I (with a single-dish 17 m diameter mirror) which is already operating since late 2003 at La Palma. The MAGIC II telescope will be completed by the end of 2008 and will operate in stereoscopic mode with MAGIC I. While the central part of the of the reflector is composed of by diamond milled Aluminum of 1m2 area panels (following a design similar to that already used for MAGIC I), the outer coronas will be made of sandwiched glass segments. The glass panel production foresees the following steps: a) a thin glass sheet (1-2mm) is elastically deformed so as to retain the shape imparted by a master with convex profile - the radius of curvature is large, the sheet can be pressed against the master using vacuum suction -; b) on the deformed glass sheet a honeycomb structure that provides the needed rigidity is glued ; c) then a second glass sheet is glued on the top in order to obtain a sandwich; d) after on the concave side a reflecting coating (Aluminum) and a thin protective coating (Quartz) are deposited. The typical weight of each panel is about 12 kg and its resolution is better than 1 mrad at a level of diameter that contains the 90% of the energy reflected by the mirror; the areal cost of glass panels is ~2 k per 1m2. The technology based on cold slumping is a good candidate for the production of the primary mirrors of the telescopes forming the Cherenkov Telescope Array (CTA), the future large TeV observatory currently being studied in Europe. Details on the realization of MAGIC II new mirrors based on cold slumping glass will be presented.

Final commissioning phase of the AdOpt@TNG module

The AdOptTNG module is an adaptive optics facility permanently mounted at the Nasmyth focus of the 4m-class Telescopio Nazionale Galileo (TNG). Its integration on the telescope started in late November 1998 and first-light of the speckle and tip-tilt modes took place shortly after. Both modes have been offered to the astronomical community and turned out to provide performances close to the expectations. Double stars with separation below 0.1 arcsec have been resolved by the speckle facility. Improvement of the Strehl ratio of a factor two and enhancement in the FWHM from 0.65 arcsec to 0.35 arcsec have been obtained on relatively faint reference stars. The high-speed low noise CCD, namely an 80 X 80 pixel read from the four corners, has been mounted and aligned with the Shack-Hartmann wavefront sensor. A Xinetics mirror with 96 actuators has been calibrated against the wavefront sensor with on-board alignment fibers. This has been done using a modal approach and using Singular Value Decomposition in order to get a reliable interaction matrix. Filtering can be modal too, using a default integrative filter coupled with a limited FIR-fashioned technique. Open loop measurements on the sky provide data to establish open loop transfer functions and realistic estimates of limiting magnitude. High-order wavefront correction loop has been successfully tested on the sky. In this paper we give a description of the overall functionality of the module and of the procedure required to acquire targets to be used as reference in the correction. A brief overview of the very first astronomical results obtained so far on angular size and shape measurements of a few asteroids and sub-arcsec imaging of Planetary Nebulae and Herbig Haro objects is also given.

Manufacturing of Wolter-I mirror segments with slumped glass

In our ongoing studies of high precision glass slumping we have successfully formed the first Wolter-I X-ray mirror segments with parabola and hyperbola in one piece. It could be demonstrated that the excellent surface roughness of the 0.55 mm thick display glass chosen is conserved during the slumping process. The influence of several parameters of the process, such as maximum temperature, heating and cooling rates etc. have to be measured and controlled with adequate metrology. Currently, we are optimizing the process to reduce the figure errors down to 1 micrometer what will be the starting point for further, final figure error corrections. We point out that metrology plays an important role in achieving a high precision optics, i.e. an angular resolution of a few arcsec. In this paper we report on the results of our studies and discuss them in the context of the requirements for future X-ray telescopes with large apertures.

X-ray optics for the WFXT telescope

Wide field x-ray telescope (WFXT) is the core instrument of the Wide Angle X-ray Survey (WAXS) mission, which is aimed at conducting a high angular resolution, high sensitivity x- ray survey over a large solid angle of the sky. The project has been developed as a feasibility study in the frame of the Agenzia Spaziale Italiana (ASI) program for small-medium satellite mission. WFXT uses grazing incidence optics based on a new design where the Wolter I profile is substituted by a five term polynomial profile. The coefficients of the polynomium are optimized to obtain high spatial resolution over a field of view of about 1 degree. The WFXT optics consist of 25 nested shells. In order to have both a large effective area at low energies and a meaningful area at higher energies, a design consisting of 9 large mirror shells and 16 smaller shells, contributing mainly at higher energies, has been developed. The outermost and innermost mirror shells have a diameter of 600 and 226 mm, respectively. The total length of the mirror shells is 120 + 120 mm, while the focal length of the optical system is 3000 mm. For the WFXT optics, in addition to the well proved manufacturing process by nickel electroforming, we considered a novel replication technique for the manufacture of the mirrors which make use of ceramic material like Silicon Carbide in order to meet the stringent requirements of high spatial resolution and low weight. In this paper we give the details of the optical design and report the rest of the x-ray measurements of the prototypes of the outermost mirror shell manufactured with nickel and SiC.

Making the ATHENA optics using Silicon Pore Optics

Silicon Pore Optics, after 10 years of development, forms now the basis for future large (L) class astrophysics Xray observatories, such as the ATHENA mission to study the hot and energetic universe, matching the L2 science theme recently selected by ESA for launch in 2028. The scientific requirements result in an optical design that demands high angular resolution (5“) and large effective area (2 m2 at a few keV) of an X-ray lens with a focal length of 12 to14 m. Silicon Pore Optics was initially based on long (25 to 50 m) focal length telescope designs, which could achieve several arc second angular resolution by curving the silicon mirror in only one direction (conical approximation). With the advent of shorter focal length missions we started to develop mirrors having a secondary curvature, allowing the production of Wolter-I type optics, which are on axis aberration-free. In this paper we will present the new manufacturing process, discuss the impact of the ATHENA optics design on the technology development and present the results of the latest X-ray test campaigns.

X-ray optics developments at ESA

Future high energy astrophysics missions will require high performance novel X-ray optics to explore the Universe beyond the limits of the currently operating Chandra and Newton observatories. Innovative optics technologies are therefore being developed and matured by the European Space Agency (ESA) in collaboration with research institutions and industry, enabling leading-edge future science missions. Silicon Pore Optics (SPO) [1 to 21] and Slumped Glass Optics (SGO) [22 to 29] are lightweight high performance X-ray optics technologies being developed in Europe, driven by applications in observatory class high energy astrophysics missions, aiming at angular resolutions of 5” and providing effective areas of one or more square meters at a few keV. This paper reports on the development activities led by ESA, and the status of the SPO and SGO technologies, including progress on high performance multilayer reflective coatings [30 to 35]. In addition, the progress with the X-ray test facilities and associated beam-lines is discussed [36].

Development of grazing-incidence multilayer mirrors by direct Ni electroforming replication: a status report

The Ni electroforming replication process has been used successfully by Beppo-SAX, JET-X/SWIFT, and XMM-Newton, to produce their gold-coated X-ray mirrors. The important feature of the technique is that, also with thin substrates, it is possible to achieve a good angular resolution, which is important for obtaining high signal-to-noise ratios in deep observations and imaging extended sources, while the assembly and integration of the monolithic shells is a relatively easy task. Two approaches can be used for the up grade of this technique also to the case of mirrors with multilayer coating, to be used in future hard X-ray missions: i) the direct replication of the mirror shell, after the deposition of the multilayer film on the master (mandrel) surface followed by the electroforming of the Ni walls, ii) the application of the multilayer film to the internal surface of Ni mirror shells, previously realized by replication. In this paper the last results achieved in Italy in the context of an activity aiming at the development of the former of the two methods will be presented and discussed.