Kill two birds with one stone - measure shape and reflectivity at the same time

The Cherenkov Telescope Array (CTA) collaboration will construct an array of telescopes to perform astronomical observations in the very high energy range. The about 100 telescopes will consist of a reflective and light focusing dish of all in all 10.000 mirror tiles. To reduce costs and weight of the telescope, these mirrors will be constructed as composite mirrors with an underlying structure and a coated glass surface on top. Two important features of the mirrors have to be tested: first the shape must be accurate in order to concentrate photons within a certain spot. Second, the local reflectivity must be sufficient within a wavelength range from 300 nm to 600 nm. An obvious geometry to measure both features is by concentric illumination and observation. This method requires a big lab (~30m), so we developed a compact deflectomeric device (PMD) to measure the shape (e.g., within a climate chamber). We will discuss reflectivity measurements with different geometries and will specifically deal with convenient measurement options, to be integrated in the PMD device.