Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA.

The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project. ?? 2013 Elsevier B.V. All rights reserved

Comparing the number of clear (cloud‐free) nights available for astronomical observations is a critical task, which should be based on homogeneous methodologies. Current data are mainly based on different judgements based on observer logbooks or on different instruments. In this paper, we present a new homogeneous methodology at very different astronomical sites for modern optical astronomy, in order to quantify the fraction of available nights. The data are extracted from night‐time Geostationary Operational Environmental Satellite 12 (GOES12) infrared images and compared with ground‐based conditions when available. In this analysis, we introduce a wider average matrix and three‐band correlation in order to reduce the noise and to distinguish between clear and stable nights. Temporal data are used for the classification. In the time interval 2007–2008, we have found that the percentages of satellite clear nights are 88 per cent at Paranal, 76 per cent at La Silla, 72.5 per cent at La Palma, 59 per cent at Mt Graham and 86.5 per cent at Tolonchar. A correlation analysis of the three GOES12 infrared bands B3, B4 and B6 indicates that the fraction of stable nights is lower by 2–20 per cent, depending on the site.

The analysis of night cloud cover is very important for astronomical observations in real time, considering a typical observation time of about 15 minutes, and to provide statistics. In this article, we use the Sky Quality Meter (SQM) for high-resolution temporal analysis of the La Silla and Asiago (Ekar Observatory) sky: 3 and 5 minutes respectively. We investigate the annual temporal evolution of the natural contributions of the sky at a site not influenced by artificial light at night (ALAN) and at one highly influenced. We also make a correlation between GOES and Aqua satellite data and ground-based SQM data to confirm the relationship between the SQM data and cloud cover. We develop an algorithm that allows the use of the SQM for night cloud detection and reach correlations with the nighttime cloud cover detected by the GOES and Aqua satellites of 97.2 per cent at La Silla and 94.6 per cent at Asiago. Our algorithm also classifies photometric (PN) and spectroscopic nights (SN). We measure 59.1 per cent PN and 21.7 per cent SN for a total percentage of clear nights of 80.8 per cent at La Silla in 2018. The respective Ekar Observatory values are 31.1 per cent PN, 24.0 per cent SN and 55.1 per cent of total clear night time. Application to the SQM network would involve the development of long-term statistics and large data forecasting models for site testing and real-time astronomical observation.

We present, for the first time, a new method to estimate seeing using remote sounding from the infrared night‐time data of the Geostationary Operational Environmental Satellite 12. We discuss the correlation found between the ground‐based data and the satellite‐derived values from the analysis of the sites located at Cerro Paranal (Chile) and Roque de los Muchachos (Canary Islands, Spain). We obtain a ground–satellite correlation percentage of about 90 per cent. Finally, by studying the correlation between the data from the afternoon and the following night, we are able to provide a forecast for the photometric night quality.