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) [1], [2]. 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), to be implemented beyond eRosita and proposed within the
NASA RFI call 2011, requires an angular resolution of less than 10 arcsec (with goal of 5”) constant across a wide field
of view (1 deg2). To achieve this requirement the design is based on nested modified grazing incidence Wolter-I mirrors with polynomial profiles. Our goals in terms of mass and stiffness can be meet with the use of fused silica glass, a wellknown material with good thermo-mechanical properties and polishability characteristics, together with an innovative polishing approach. Here we present the X-ray calibration results obtained for a prototypal shell tested in fullillumination mode at the Panter/MPE facility.
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.
The Wide Field X-ray Telescope (WFXT) is a medium class mission for X-ray surveys of the sky with an unprecedented
area and sensitivity. In order to meet the effective area requirement, the design of the optical system is based on very thin
mirror shells, with thicknesses in the 1-2 mm range. In order to get the desired angular resolution (10 arcsec requirement,
5 arcsec goal) across the entire 1x1 degree FOV (Field Of View), the design of the optical system is based on nested
modified grazing incidence Wolter-I mirrors realized with polynomial profiles, focal plane curvature and plate scale
corrections. This design guarantees an increased angular resolution at large off-axis angle with respect to the normally
used Wolter I configuration, making WFXT ideal for survey purposes. The WFXT X-ray Telescope Assembly is
composed by three identical mirror modules of 78 nested shells each, with diameter up to 1.1 m. The epoxy replication
process with SiC shells has already been proved to be a valuable technology to meet the angular resolution requirement
of 10 arcsec. To further mature the telescope manufacturing technology and to achieve the goal of 5 arcsec, a
deterministic direct polishing method is under investigation. The direct polishing method has already been used for past
missions (as Einstein, Rosat, Chandra): the technological challenge now is to apply it for almost ten times thinner shells.
Under investigation is quartz glass (fused silica), a well-known material with good thermo-mechanical and polishability
characteristics that could meet our goal in terms of mass and stiffness, with significant cost and time saving with respect
to SiC. Our approach is based on two main steps: first quartz glass tubes available on the market are grinded to conical
profiles, and second the obtained shells are polished to the required polynomial profiles by CNC (Computer Numerical
Control) polishing machine. In this paper, the first results of the direct grinding and polishing of prototypes shells made
by quartz glass with low thickness, representative of the WFXT optical design, are presented.
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