Optical atmospheric extinction over Cerro Paranal

Abstract

Aims. The present study was conducted to determine the optical extinction curve for Cerro Paranal under typical clear-sky observing conditions, with the purpose of providing the community with a function to be used to correct the observed spectra, with an accuracy of 0.01 mag airmass −1 . Additionally, this work was meant to analyze the variability of the various components, to derive the main atmospheric parameters, and to set a term of reference for future studies, especially in view of the construction of the Extremely Large Telescope on the nearby Cerro Armazones. Methods. The extinction curve of Paranal was obtained through low-resolution spectroscopy of 8 spectrophotometric standard stars observed with FORS1 mounted at the 8.2 m Very Large Telescope, covering a spectral range 3300-8000 Å. A total of 600 spectra were collected on more than 40 nights distributed over six months, from October 2008 to March 2009. The average extinction curve was derived using a global fit algorithm, which allowed us to simultaneously combine all the available data. The main atmospheric parameters were retrieved using the LBLRTM radiative transfer code, which was also utilised to study the impact of variability of the main molecular bands of O 2 , O 3 , and H 2 O, and to estimate their column densities. Results. In general, the extinction curve of Paranal appears to conform to those derived for other astronomical sites in the Atacama desert, like La Silla and Cerro Tololo. However, a systematic deficit with respect to the extinction curve derived for Cerro Tololo before the El Chichón eruption is detected below 4000 Å. We attribute this downturn to a non standard aerosol composition, probably revealing the presence of volcanic pollutants above the Atacama desert. An analysis of all spectroscopic extinction curves obtained since 1974 shows that the aerosol composition has been evolving during the last 35 years. The persistence of traces of non meteorologic haze suggests the effect of volcanic eruptions, like those of El Chichón and Pinatubo, lasts several decades. The usage of the standard CTIO and La Silla extinction curves implemented in IRAF and MIDAS produce systematic over/under-estimates of the absolute flux.

Atmospheric extinction properties above Mauna Kea from the Nearby SuperNova Factory spectro-photometric data set

“…Fig. 21), we found that both quantities were highly correlated, reinforcing the findings of Patat et al (2011) and validating our water telluric correction approach. Furthermore, using an orthogonal distance regression we find a saturation exponent of ρ = 0.62 ± 0.01 from the best power law describing the distribution (cf.…”

“…Nevertheless, we do expect to have more stable values since the measurements are made at night far above the inversion layer, and the mean optical depth value is compatible with aerosols from maritime origins. Patat et al (2011) showed that the water band equivalent width at 7200 Å is well correlated with the Preciptable Water Vapor (PWV) at the Paranal site in Chile. In order to check the consistency of our approach, we compare our water intensity measurement to the PWV amount at Mauna Kea.…”

“…For many studies accurate flux calibration is critical for deriving the maximum amount of information from observations with these telescopes, and correction for the optical atmospheric and instrumental transmissions is one of the main limitations of astronomical flux measurements from the ground (see Burke et al 2010;Patat et al 2011, for the most recent reviews). Therefore, as part of our Nearby SuperNova Factory project A&A 549, A8 (2013) limited wavelength ranges (3100-3900 Å and 3650-5850 Å, respectively).…”

Auroral meridian scanning photometer calibration using Jupiter

“…During this study, we identified a systematic difference between our flux calculations for Jupiter and the corresponding magnitude value provided by widely available astronomy software (Downey, 2015) using the formula (Thuillier et al, 2003), albedo of Jupiter (Karkoschka, 1998), and atmospheric extinction at Cerro Paranal (Patat et al, 2011). Column 5 is the product of solar irradiance at 1 AU and Jupiter albedo (defined as B(λ) in Eq.17) with units of watts per meter squared per nanometer.…”

“…For this study, we use values from Patat et al (2011) to provide a lower bound on extinction effects. The upper bound is estimated using an empirical model based on Vargas et al (2002) and Zhang et al (2013).…”