Deflectometric Measurement of 10,000 Telescope Mirror Tiles

The Cherenkov Telescope Array (CTA) is an international project to design, build and operate Cherenkov-radiation-based observatories for high energetic cosmic rays. For these kinds of observations, a large area on the ground has to be covered with telescopes, requiring a huge amount (>10,000) of mirror tiles. Each single mirror (~1m diameter, ~15m focal length) has to be tested. We will discuss how this can be done in a fast and accurate way by a modified deflectometric method, avoiding a tedious alignment of the specimen under test. Our method satisfies several additional requirements: In spite of the big diameter and long focal length, the testing can be done within the very limited space of a climate chamber (4.5m x 4m). Furthermore, the quantity of interest is the Point Spread Function (PSF) for rays coming from nearly infinity. Since deflectometry intrinsically delivers local slope data, the PSF for any required configuration can easily be calculated by ray tracing - without a prior (noise prone!) numerical differentiation of surface data. First measurement results obtained at different mirror prototypes are presented.