Advancements in ion beam figuring of very thin glass plates

The high quality surface characteristics, required on the mirrors of a high angular resolution x-ray telescope, are challenging but well suited to a deterministic and non-contact process, like the ion beam figuring. This process has been recently proven to be compatible even with very thin (thickness around 0.4mm) sheet of D263 and Eagle glasses. In the last decade, these types of glass have been investigated as substrates for hot slumping, with residual figure errors of hundreds of nanometres. In this view, the mirrors segments fabrication could be envisaged as a simple two phases process: a first replica step based on hot slumping (direct/indirect) followed by an ion beam figuring which can be considered as a post-fabrication correction method. The first ion-beam-figuring trials, realized on flat samples, showed that the micro-roughness was not damaged but a deeper analysis was necessary to characterize and eventually control/compensate the glass shape variations. In this paper we present the advancements in the process definition, achieved on slumped glass samples.