The Central Calibration Facilities workpackage of the Cherenkov Telescope Array (CTA) observatory for very high energy gamma ray astronomy defines the overall calibration strategy of the array, develops dedicated hardware and software for the overall array calibration and coordinates the calibration efforts of the different telescopes. The latter include LED-based light pulsers, and various methods and instruments to achieve a calibration of the overall optical throughput. On the array level, methods for the inter-telescope calibration and the absolute calibration of the entire observatory are being developed. Additionally, the atmosphere above the telescopes, used as a calorimeter, will be monitored constantly with state-of-the-art instruments to obtain a full molecular and aerosol profile up to the stratosphere. The aim is to provide a maximal uncertainty of 10% on the reconstructed energy-scale, obtained through various independent methods. Different types of LIDAR in combination with all-sky-cameras will provide the observatory with an online, intelligent scheduling system, which, if the sky is partially covered by clouds, gives preference to sources observable under good atmospheric conditions. Wide-field optical telescopes and Raman Lidars will provide online information about the height-resolved atmospheric extinction, throughout the field-of-view of the cameras, allowing for the correction of the reconstructed energy of each gamma-ray event. The aim is to maximize the duty cycle of the observatory, in terms of usable data, while reducing the dead time introduced by calibration activities to an absolute minimum.
The Cherenkov Telescope Array (CTA) is the next generation very high-energy gamma-ray observatory, with at least 10
times higher sensitivity than current instruments. CTA will comprise several tens of Imaging Atmospheric Cherenkov
Telescopes (IACTs) operated in array-mode and divided into three size classes: large, medium and small telescopes. The
total reflective surface could be up to 10,000 m<sup>2</sup> requiring unprecedented technological efforts. The properties of the
reflector directly influence the telescope performance and thus constitute a fundamental ingredient to improve and
maintain the sensitivity. The R&D status of lightweight, reliable and cost-effective mirror facets for the CTA telescope
reflectors for the different classes of telescopes is reviewed in this paper.
The Cherenkov Telescope Array (CTA), currently in its early design phase, is a proposed new project for groundbased
gamma-ray astronomy with at least 10 times higher sensitivity than current instruments. CTA is planned
to consist of several tens of large Imaging Atmospheric Cherenkov Telescopes (IACTs) with a combined reflective
surface of up to 10,000 m<sup>2</sup>. The challenge for the future CTA array is to develop lightweight and cost efficient
mirrors with high production rates, good longterm durability and adequate optical properties. The technologies
currently under investigation comprise different methods of carbon fibre/epoxy based substrates, sandwich
concepts with cold-slumped surfaces made of thin float glass and different structural materials like aluminum
honeycomb, glass foam or PU foam inside, and aluminum sandwich structures with either diamond milled surfaces
or reflective foils. The current status of the mirror development for CTA will be summarized together with
investigations on the improvement of the reflective surfaces and their protection against degradation.