The Wide Field X-ray Telescope (WFXT) is a medium class mission proposed to address key questions about cosmic origins and physics of the cosmos through an unprecedented survey of the sky in the soft X-ray band (0.2-6 keV) , . In order to get the desired angular resolution of 10 arcsec (5 arcsec goal) on the entire 1 degrees Field Of View (FOV), the design of the optical system is based on nested grazing-incidence polynomial profiles mirrors, and assumes a focal plane curvature and plate scale corrections among the shells. This design guarantees an increased angular resolution also at large off-axis positions with respect to the usually adopted Wolter I configuration. In order to meet the requirements in terms of mass and effective area (less than 1200 kg, 6000 cm2 @ 1 keV), the nested shells are thin and made of quartz glass. The telescope assembly is composed by three identical modules of 78 nested shells each, with diameter up to 1.1 m, length in the range of 200-440 mm and thickness of less than 2.2 mm. At this regard, a deterministic direct polishing method is under investigation to manufacture the WFXT thin grazing-incidence mirrors made of quartz. The direct polishing method has already been used for past missions (as Einstein, Rosat, Chandra) but based on much thicker shells (10 mm ore more). The technological challenge for WFXT is to apply the same approach but for 510 times thinner shells. The proposed approach is based on two main steps: first, quartz glass tubes available on the market are ground to conical profiles; second the pre-shaped shells are polished to the required polynomial profiles using a CNC polishing machine. In this paper, preliminary results on the direct grinding and polishing of prototypes shells made by quartz glass with low thickness, representative of the WFXT optical design, are presented.
The next generation wide-field X-ray telescope (WFXT) will require an angular resolution of ~5-10 arcsec almost
constant across a wide field of view (~1 deg2 diameter). To achieve this goal, the design of the optical system has to be
based on mirrors characterized by short length and polynomial profiles, as well as focal plane curvature and plate scale
corrections. These concepts guarantee an improved angular resolution at large off-axis angle with respect to the normally
used Wolter-I configuration. These telescopes are therefore optimal for survey purposes. A significant increase of
effective area and grasp with respect to previous missions must also be achieved. This is possible with high precision but
at the same time thin (2-3 mm thickness for mirror diameters of 30-110 cm) glass mirror shells. To achieve the goal of 5
arcsec and improve further the technology, we are considering different materials. Fused silica, a well-known material
with good thermo-mechanical and polishability characteristics provide the best choice. To bring the mirror shells to the
needed accuracy, we are adopting a deterministic direct polishing method (already used for past missions as Einstein,
Rosat, Chandra). The technological challenge now is to apply it for almost ten times thinner shells.
In the last decade a new window for ground-based high energy astrophysics has been opened. It explores the energy band
from about 100 GeV to 10 TeV making use of Imaging Atmospheric Cherenkov Telescopes (IACTs). Research in Very
High Energy (VHE) gamma-ray astronomy is improving rapidly and thanks to the newest facilities as MAGIC, HESS
and VERITAS astronomers and particle physicists are obtaining surprising implications in the theoretical models.
New projects have been started as the European Cherenkov Telescope Array (CTA) and the U.S. Advanced Gamma-ray
Imaging System (AGIS). The aim is to enhance both the sensitivity and the energy band coverage to perform imaging,
photometry and spectroscopy of sources. In this framework, tens of thousands of optical mirror panels have to be
manufactured, tested and mounted into the telescopes. Because of this high number of mirrors it is mandatory to develop
a technique easily transferable to industrial mass production, but keeping the technical and cost-effectiveness
requirements of the next generation of TeV telescopes.
In this context the Astronomical Observatory of Brera (INAF-OAB) is investigating a technique for the manufacturing of
stiff and lightweight glass mirror panels with modest angular resolution. These panels have a composite sandwich-like
structure with two thin glass skins on both sides of a core material; the reflecting skin is optically shaped using an ad-hoc
slumping procedure. The technology here presented is particularly attractive for the mass production of cost-effective
mirror segments with long radius of curvature like those required in the primary mirrors of the next generation of
Cherenkov telescopes. In this paper we present and discuss some relevant results we have obtained from the latest panels
In the last decade Very High Energy (VHE) gamma-ray astronomy has improved rapidly opening a new window for
ground-based astronomy with surprising implications in the theoretical models. Nowadays, it is possible to make
imaging, photometry and spectroscopy of sources with good sensitivity and angular resolution using new facilities as
MAGIC, HESS and VERITAS. The latest results of astronomy in the TeV band obtained using such facilities
demonstrate the essential role of this window for high energy astrophysics. For this reason new projects (e.g. CTA and
AGIS) have been started with the aim to increase the sensitivity and expand the energy band coverage.
For such telescopes arrays probably tens of thousands of optical mirror panels must be manufactured with an adequate
industrial process, then tested and mounted into the telescopes. Because of the high number of mirrors it is mandatory to
perform feasibility studies to test various techniques to meet the technical and cost-effectiveness requirements for the
next generation TeV telescopes as CTA and AGIS.
In this context at the Astronomical Observatory of Brera (INAF-OAB) we have started the investigation of different
techniques for the manufacturing of stiff and lightweight optical glass mirror panels. These panels show a sandwich-like
structure with two thin glass skins on both sides, the reflective one being optically shaped using an ad-hoc slumping
procedure. The technologies here presented can be addressed both for primary or secondary mirrors for the next
generation of Cherenkov telescopes. In this paper we present and discuss the different techniques we are investigating
with some preliminary results obtained from test panels realized.