Correction of high spatial frequency errors on optical surfaces by means of Ion Beam Figuring

The Ion Beam Figuring is a well known technique able to correct shape errors on optical surfaces with high accuracy. The size of the ion beam dictates strongly the higher spatial frequencies that can be corrected on the optical surface. The correction of small optics of some cm in diameter or containing high spatial frequencies can be very time consuming or impossible. A system that permits the Ion Beam Figuring of small optical components has been developed in the Astronomical Observatory of Brera (INAF-OAB). It has a small ion beam size and large removal rate. The system employs a concentrator able to force the broader beam emitted from an ion source into a smaller spot having large removal rate. The concentrator is placed between the ion source and the optical surface to be figured and doesn't influence the long term stability of the source. It consists of a conical cavity in which is injected the beam extracted from the grids of the source. The grazing incidence angle of impact of the ions with the walls of the cone ensure a very low level of sputtering of the cone material and meanwhile permits the creation of a very small spot removal function having large removal rate. To demonstrate its functionality a number of test optics has been figured using this system with very good results.