El Roque de Los Muchachos site characteristics

Abstract

Context. It is known that the Canary Islands are normally affected by dominant winds flowing from north-northeast, that in some meteorological conditions, can transport sand from the Sahara desert at high altitude. The dust may affect the efficiency of the telescopes and decrease the transparency of the sky. Aims. To maximize the scientific return of the telescopes located at the Observatorio del Roque de Los Muchachos (ORM), we present an analysis of the atmospheric dust content and its effects on astronomical observations. Than B, V and I dust aerosol astronomical extinction are derived. Methods. Using a 5-year series database of data taken from a dust monitor located inside the Telescopio Nazionale Galileo (TNG) dome, we computed mean hourly and daily values of the dust content as measured with a four-channel dust monitor. Results. We detected particles of 0.3, 0.5, 1.0, and 5.0 µm. Furthermore, using a power law we derived the content of 10.0 µm particles. We found a typical local dust concentration ranging from 3 × 10 6 particles per cubic metre at 0.3 µm, to 10 3 at 5.0 µm and 10 at 10.0 µm, increasing up to 3 orders of magnitudes during the dust storms, with a relatively higher increase of 1.0, 5.0, and 10.0 µm particles. The number of local dust storm events is the same in the local winter and summer, but the average background and storm-related increases in the dust concentration in summer are significantly higher than in winter. In a uniform approximation, during the dust storms, an average height of the dust layer of 2.5 km above the telescope is inferred. Conclusions. During the sand storms, La Palma Island is affected by an almost uniform layer extending up to 5 km above the sea level. The visible extinction is dominated by particles at 1.0, 5.0 and 10.0 µm. In agreement with the results from Carlsberg Automatic Meridian Circle (CAMC), we find a typical extinction of about 0.2 mag airmass −1 during dust storms.

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

“…Most of our selected clear nights (88 per cent) present an astronomical extinction of less than 0.13 mag. The extinction value of 0.2 mag airmass −1 on clear nights was found to be discriminant for calima events by Lombardi et al (2008).…”

Clear sky fraction above Indonesia: an analysis for astronomical site selection

“…In North Africa, optical seeing at the Oukaïmeden has been monitored for a long time (Siher & Benkhaldoun ; Benkhaldoun et al ). Not far from it, in Spain, these factors have been continuously monitored for the largest European astronomical sites in the northern hemisphere, in La Palma, (Lombardi et al , , ), and also in Calar Alto (Sánchez et al ). In addition, (Varela et al ) analysed atmospheric aerosol intensively above the Canary Islands based on both satellite data and in situ measurements.…”

On the Estimation of Atmospheric Aerosol Extinction Parameters from Measurements in the Johnson–Cousins Photometric System

“…The evaluation of the atmospheric extinction at a particular site is of great importance in determination of the quality of the astronomical observations, especially in photometric measurements. For this reason, many studies devoted to this subject have been carried out in recent decades (Hayes & Latham 1975;Gutiérrez-Moreno et al 1982;Rufener 1986;Reimann et al 1992;Sterken & Manfroid 1992;Burki et al 1995;Schuster & Parrao 2001;Lombardi et al 2008). Atmospheric extinction in the optical domain is usually described in terms of three main components: scattering by molecules (Rayleigh scattering), scattering by particles of dust or droplets (aerosol scattering) and molecular absorption (mainly due to ozone, water vapour and other gases).…”