Improvement of the angular resolution of glass/plastic thin-foil mirrors for large area X-ray telescopes via active control

The improvement of the angular resolution of focusing optics for X-rays to levels of 1 arcsec or better is one of the frontiers of the instrumentation development for future X-ray telescopes in space. However, such a level can be reached only with thick mirrors (like Chandra’s) in order to preserve their accurate figure. The next generation of X-ray telescopes will also require a high effective area, i.e., a number of thin mirrors to be assembled to maintain the optic mass within acceptable limits. For the same reason, the mirrors have to be made of a lightweight material like thermally formed glass foils. However, this makes the mirrors prone to deform, at the expense of the achievable angular resolution. Even though studies based on the hot slumping of thin glass (at INAF/OAB) or plastic (UNIPA and INAF/OAPA) foils are on-going to solve this problem a true leap forward can be achieved by adopting thin mirrors with adjustable profile via actuators, in a fashion similar to the concept of adaptive optics in on-ground optical telescopes. There is no atmospheric seeing to correct, because X-ray optics operate in space. Rather, it is the unavoidable fabrication errors that have to be corrected by applying appropriate voltages to a matrix of actuators. Active X-ray mirrors are in use at Synchrotron beam-lines, where the focal spot has to be on the order of 0.1 arcsec. In this project, we are proposing the study and the realization of prototypes of thin X-ray mirrors with adjustable shape via different concepts of actuators. Other research groups are investigating this approach (noticeably at the Center for Astrophysics, Boston) on thin glasses, with encouraging results. However, they essentially rely on profile measurement to assess the mirror shape correction, and in real cases the optical surface is seldom accessible to off-situ metrology because of the dense mirror nesting. In addition, no metrology is possible for optics already operated in orbit; the sole available information to correct possible unpredictable deformation caused by, e.g., the telescope launch would be optical. For this reason, in this project we will study the shape optimization of X-ray mirrors, made of glass/plastic formed foils, via an piezoelectric actuation system, mostly driven by X-ray images taken intra-focus; in this X-ray setup, the mirror shape can be reconstructed and the needed correction can be imparted to the mirrors via a dedicated electronic providing the appropriate voltages to the actuators, an approach so far unexplored by the other research groups. A team of skilled institutes will carry out this research, complementing the existing facilities and skills in an effective synergy: • INAF/OAB will provide the facilities and the skills in glass foil forming and integration, surface and profile metrology, piezoelectric actuators, thin film characterization, Finite Element Analysis, X-ray data interpretation, and imaging quality prediction. • UNIPA/DiFC will provide the facilities and the skills in plastic foils forming, thin film deposition and characterization, control electronic development. • INAF/OAPA will provide the 35 m long X-ray testing facility (XACT) to perform the intrafocus measurements, and the computation skills to elaborate the voltage convergence algorithm. The project foresees to manufacture some active optic prototypes, made of single or multiple shells, to serve as demonstrators of the effectiveness of the approach. If financed and successful, this project will expectedly improve the imaging quality of future, large X-ray telescopes by more than a factor of ten, leading to unexpected discoveries in high-energy Astronomy. Also, emergent researchers will be involved, providing them new opportunities of career. Finally, this study will continue and advance the 50-year Italian tradition in the field of X-ray optics.