Identification of Potential Sites for Astronomical Observations in Northern South America

Abstract

In this study we describe an innovative method to determine potential sites for optical and infrared astronomical observations in the Andes region of northern South America. The method computes the Clear sky fraction (CSF) from Geostationary Observational Environmental Satellite (GOES) data for the years 2008-12 through a comparison with temperatures obtained from long-term records of weather stations and atmospheric temperature profiles from radiosonde. Criteria for sky clearance were established for two infrared GOES channels in order to determine potential sites in the Andes region of northern South-America. The method was validated using the reported observed hours at Observatorio Nacional de Llano del Hato in Venezuela. Separate CSF percentages were computed for dry and rainy seasons for both, photometric and spectroscopic night qualities.Twelve sites with five year averages of CSF for spectroscopic nights larger than 30% during the dry seasons were found to be suitable for astronomical observations. The best site with (220±42) spectroscopic clear nights per year is located in the Andes of Venezuela (70 • 28'48"W, 9 • 5'60"N) at an altitude of 3480 meters.Lower quality regions were found in Sierra Nevada de Santamarta and Serranía del Perijá with (126±34) and (111±27) clear nights per year, respectively. Sites over the Andes are identified in Norte de Santander with (107±23) and in the north-east part of Boyacá with a mean of (94±13) clear nights per year. Two sites at low latitude located in Ecuador with more than 100 clear nights per year and with similar seasonal CSF percentages were also identified. Five year evolution suggest a possible correlation between the lowest percentages observed during the rainy seasons of 2010 and 2011 with positive values of the Southern Oscillation Index. Subject headings: Astronomical Instrumentation. Recently, Marín et al. (2015) computed the CSF at high altitude sites by using a new clearance algorithm based on global meteorological models. CSF values were then used for precipitable water vapour estimations at two astronomical sites in the north of Chile. Even though most of the methods are based on the comparison of radiances, fluxes or temperatures at each pixel of the satellite image with threshold values previously defined there is no standard methodology for CSF computations. Cavazzani & Zitelli (2013) used -5monthly thresholds defined as the maximum radiance at clear nights along the month.Thresholds may also be defined by using an atmospheric model (9). In this study, we introduce a novel methodology involving the use of monthly thresholds of temperature at different altitudes from the surface up to the upper troposphere. Our thresholds definitions were based on the altitude of the terrain, long-term records of weather stations and radiosonde data archives.The thresholds definitions are based on monthly mean records of both, air temperature at the site and vertical temperature profile between 8 and 10 km of altitude which make more precise the predictions for clear...

Low Dimensional Embedding of Climate Data for Radio Astronomical Site Testing in the Colombian Andes

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New pixel-level low-cost Polar Star tracking technique and device for the instantaneous evaluation of atmospheric turbulence (seeing)

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