Thin fused silica optics for a few arcsec angular resolution and large collecting area x-ray telescope

Abstract

The implementation of a X-ray mission with high imaging capabilities, similar to those achieved with Chandra (< 1 arcsec Half Energy Width, HEW), but with a much larger throughput is a very attractive perspective, even if challenging. For such a mission the scientific opportunities, in particular for the study of the early Universe, would remain at the state of the art for the next decades. At the beginning of the new millennium the XEUS mission has been proposed, with an effective area of several m 2 and an angular resolution better than 2 arcsec HEW. Unfortunately, after the initial study, this mission was not implemented, mainly due to the costs and the low level of technology readiness. Currently the most advanced proposal for such a kind of mission is the SMART-X project, led by CfA and involving several other US Institutes. This project is based on adjustable segments of thin foil mirrors with piezo-electric actuators, aiming to achieve an effective area > 2 m 2 at 1 keV and an angular resolution better than 1 arcsec HEW. Another attractive technology to realize an X-ray telescope with similar characteristics is being developed at NASA/Goddard. In this case the mirrors are based on Si substrates that are super-polished and figured starting from a bulky Si ingot, from which they are properly cut. Here we propose an alternative method based on precise direct grinding, figuring and polishing of thin (a few mm) glass shells with innovative deterministic polishing methods. This is followed by a final correction via ion figuring to obtain the desired accuracy in order to achieve the 1 arc sec HEW requirement. For this purpose, a temporary stiffening structure is used to support the shell from the polishing operations up to its integration in the telescope supporting structure. We will present the technological process under development, the results achieved so far and some mission scenarios based on this kind of optics, aiming to achieve an effective area more than 10 times larger than Chandra and an angular resolution of 1 arcsec HEW on axis and of a few arcsec off-axis across a large field of view (1 deg in diameter).

A quantitative analysis of the indentation fracture of fused silica

“…Fused silica has been widely used to make precision lenses and optics because of its low thermal expansion and excellent optical transmission in a broad spectrum from ultraviolet to near infrared. 1,2,4 For example, it has been extensively used to fabricate large-aperture (~0.5 to 1.0 m) optics in the National Ignition Facility (NIF) 2 and the Chinese high-power laser system. 3 Reducing power loss and increasing the laser-induced damage threshold, require the manufacture of these optics with an ultra-low form error (RMS ≤60/λ), an ultra-smooth surface (Ra ≤5 nm) and a damage-free surface/subsurface.…”

“…Fused silica has been widely used to make precision lenses and optics because of its low thermal expansion and excellent optical transmission in a broad spectrum from ultraviolet to near infrared . For example, it has been extensively used to fabricate large‐aperture (~0.5 to 1.0 m) optics in the National Ignition Facility (NIF) and the Chinese high‐power laser system .…”

Active figure control effects on mounting strategy for x-ray optics

“…At MSFC, we are involved in several technology development efforts for higher resolution, larger area, and lighter weight x-ray optics to support future replacement of the 15-year-old Chandra X-ray Observatory, [1][2][3][4] with the next mission to attain sub-arcsecond x-ray imaging. All of these technologies [5][6][7][8][9][10][11][12][13][14][15] share the need to be mounted, assembled, and aligned into modular units for testing or for flight. Because in our recent experience with moderate-resolution optics [16][17][18][19][20] we have found that without considerable development effort, it is difficult to prevent structure induced errors from contributing significantly, i.e.…”