V. Zitelli

Fraction of clear skies above astronomical sites: a new analysis from the GOES12 satellite

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.

Analysis of the fraction of clear sky at the La Palma and Mt Graham sites

The amount of available telescope time is one of the most important requirements when selecting astronomical sites, as it affects the performance of ground-based telescopes. We present a quantitative survey of cloud coverage at La Palma and Mt Graham using both ground- and satellite-based data. The aim of this work is to derive clear nights for the satellite infrared channels and to verify the results using ground-based observations. At La Palma, we found a mean percentage of clear nights of 62.6 per cent from ground-based data and 71.9 per cent from satellite-based data. Taking into account the fraction of common nights, we found a concordance of 80.7 per cent of clear nights for ground- and satellite-based data. At Mt Graham, we found a 97 per cent agreement between the Columbine heliograph and the night-time observing log. From the Columbine heliograph and the Total Ozone Mapping Spectrometer–Ozone Monitoring Instrument (TOMS–OMI) satellite, we found that about 45 per cent of nights were clear, while satellite data (GOES, TOMS) are much more dispersed than those of La Palma. Setting a statistical threshold, we retried a comparable seasonal trend between the heliograph and satellite.

Status of the Galileo National Telescope

The Project Telescopio Nazionale GALILEO (TNG) will provide a 3.5-m telescope for the Italian astronomical community. Its main features closely parallel those of the ESO New Technology Telescope (NNT). We describe here its characteristics and its most important differences with respect to the NTT. The figuring of the three mirrors was successfully completed, with results exceeding the specifications. The telescope structure has been assembled in the workshop, and alignment tests are under way. The control system (motors, encoders, VME and workstation environment) is also in an advanced stage of development. The TNG will be part of the Observatorio del Roque de los Muchachos, in the Canary Islands. Its location is on the West side of the mountain, some 400-m from the NOT; excavation works are already well advanced. Civil works should be finished in the course of the current year. It is expected to complete the rotating building, and the erection of the telescope before the end of 1995, to start regular operations in 1996.

Near‐infrared thermal emissivity from ground‐based atmospheric dust measurements at ORM★

We present an analysis of the atmospheric content of aerosols measured at Observatorio del Roque de los Muchachos (ORM; Canary Islands). Using a laser diode particle counter located at the Telescopio Nazionale Galileo (TNG) we have detected particles of 0.3, 0.5, 1.0, 3.0, 5.0 and 10.0 μm size. The seasonal behaviour of the dust content in the atmosphere is calculated. Spring has been found to be dustier than summer, but dusty conditions may also occur in winter. A method to estimate the contribution of the aerosols’ emissivity to sky brightness in the near-infrared (NIR) is presented. The contribution of dust emission to the sky background in the NIR has been found to be negligible compared to the airglow, with a maximum contribution of about 8–10 per cent in the KS band in the dusty days.

The astroclimatological comparison of the Paranal Observatory and El Roque de Los Muchachos Observatory

The new extremely large telescope projects need accurate evaluation of the candidate sites. In this paper, we present the astroclimatological comparison between the Paranal Observatory, located on the coast of the Atacama Desert (Chile), and the Observatorio del Roque de Los Muchachos (ORM), located in La Palma (Canary Islands). We apply a statistical analysis using long-term data bases from Paranal and Carlsberg Automatic Meridian Circle (CAMC) weather stations. The monthly, seasonal and annual averages of the main synoptical parameters in the two sites are computed. We compare the long-term trends in order to understand the main differences between the two sites. Significant differences between the two analysed sites have been found. Temperature has increasing trends in both observatories with somewhat higher evidence at the ORM. Seasonal variations of pressure at Paranal have been highly decreasing since 1989, and we do not see the same phenomenon at the ORM. The two sites are dominated by high pressure. In cold seasons, relative humidity (RH) is lower than 60 per cent at CAMC and 15 per cent at Paranal. In warm seasons, RH is lower than 40 per cent at CAMC and 20 per cent at Paranal. The analysis of the dew point has shown better conditions at Paranal with respect to CAMC in winter, autumn and spring before 2001, while the two sites are becoming similar afterwards. Winds at the ORM are subject to pronounced local variations.

Satellite characterization of four interesting sites for astronomical instrumentation

In this paper we have evaluated the amount of available telescope time at four interesting sites for astronomical instrumentation. We use the GOES 12 data for the years 2008 and 2009. We use a homogeneous methodology presented in several previous papers to classify the nights as clear (completely cloud-free), mixed (partially cloud-covered), and covered. Additionally, for the clear nights, we have evaluated the amount of satellite stable nights which correspond to the amount of ground based photometric nights, and the clear nights corresponding to the spectroscopic nights. We have applied this model to two sites in the Northern Hemisphere (San Pedro Martir (SPM), Mexico; Izana, Canary Islands) and to two sites in the Southern Hemisphere (El Leoncito, Argentine; San Antonio de Los Cobres (SAC), Argentine). We have obtained, from the two years considered, a mean amount of cloud free nights of 68.6% at Izana, 76.0% at SPM, 70.6% at Leoncito and 70.0% at SAC. We have evaluated, among the cloud free nights, an amount of stable nights of 62.6% at Izana, 69.6% at SPM, 64.9% at Leoncito, and 59.7% at SAC.

The new TNG-DIMM: calibrations and first data analysis

Since 2011 a new differential image motion monitor (DIMM) works at Roque de los Muchachos Observatory close to Telescopio Nazionale Galileo (TNG). The purpose of this instrument is to provide useful information about the optical turbulence for the astronomical observations at TNG. We present the instrument and the first statistical results including a relationship between the principal components analysis of local-ground atmospheric parameters and seeing.