Atmosphere Extinction at the ORM on La Palma: A 20 yr Statistical Database Gathered at the Carlsberg Meridian Telescope

Abstract

The Observatorio del Roque de los Muchachos (ORM), in the Canary Islands (Spain), was one of the candidates to host the future European Extremely Large Telescope (E-ELT) and is the site of the Gran Telescopio Canarias (GTC), the largest optical infrared facility to date. Sky transparency is a key parameter as it defines the quality of the photometry to be acquired in the astronomical observations. We present a study of the atmosphere extinction at the ORM, carried out after analysis of a database spanning more than 20 yr, 1 to our knowledge, the longest and most complete and homogeneous in situ database available for any observatory. It is based on photometric measurements in the V band and r 0 band (transformed to the V -band extinction coefficient k V ) using the Carlsberg Meridian Telescope (CMT). Clear seasonal variations that repeat yearly are observed. The median value of k V is 0:13 mag airmass À1 ; the mean value has a maximum in the summer months (June-September), corresponding to the season with maximum frequency of nights affected by dust or cirrus (∼29% in summer, but only ∼13% during the rest of the year). Two volcanic eruptions took place during the database baseline, which has enabled the study of the impact of volcanoes on the global atmosphere extinction. For the 5 yr of available information, we have estimated the average monthly weather downtime from the CMT data log, obtaining a result (20.7%) in reasonable agreement with earlier studies. The main conclusion of our study is that there is no significant evidence from the CMT data for any secular changes in k V over the 20 yr database baseline.

Site Characterization of the Northern Site of the Cherenkov Telescope Array

“…This leads to typical Ångström exponents of 1.5-2.0 [26,30] and a single scattering albedo of 0.91 [27] and LIDAR ratios around 30 sr [31]. These findings are compatible with the median value for the V-band extinction of 0.13 mag airmass −1 found by [8], translating into an AOT of about 0.02.…”

“…Aerosols have been characterized at the ORM with the help of elastic LIDARs [10,23], dust counters [6] and extinction measurements by optical telescopes [8]. Moreover, several types of particulate analyzers and nephelometers are employed at the Izaña station on Tenerife [25][26][27] and on airplanes [28].…”

“…Such events occur during about 30% of the Summer nights [8], and rarely during the rest of the year. Typical LIDAR ratios are then ∼60 sr [31], and AOTs can reach values in excess of 0.5 [6].…”

“…in Winter (∼1000 m a.s.l. in Summer) 8 [13], which can be usually well identified by the "sea of stratocumulus" at its base. This layer separates two well-defined regimes: below it, the moist marine boundary layer (MBL) and above it, the dry free troposphere (FT).…”

Atmospheric Extinction Coefficients in the Ic Band for Several Major International Observatories: Results from the BiSON Telescopes, 1984–2016

“…There is also a hint of the tail-end of effects from the El Chichón eruption in Mexico in 1982 April where the start of the data in 1984 show extinction values around 0.1, double the typical value expected outside of a dust intrusion. Both Guerrero et al (1998) and García-Gil et al (2010) have previously observed these features from telescopes based at the Canary Islands.…”