The Night Sky at the Calar Alto Observatory

Abstract

We present a characterization of the main properties of the night-sky at the Calar Alto observatory for the time period between 2004 and 2007. We use optical spectrophotometric data, photometric calibrated images taken in moonless observing periods, together with the observing conditions regularly monitored at the observatory, such as atmospheric extinction and seeing. We derive, for the first time, the typical moonless night-sky optical spectrum for the observatory. The spectrum shows a strong contamination by different pollution lines, in particular from Mercury lines, which contribution to the sky-brightness in the different bands is of the order of ∼0.09 mag, ∼0.16 mag and ∼0.10 mag in B, V and R respectively. Regarding the strength of the Sodium pollution line in comparison with the airglow emission, the observatory does not fulfill the IAU recomendations of a dark site. The zenith-corrected values of the moonless night-sky surface brightness are 22. 39, 22.86, 22.01, 21.36 and 19.25 mag arcsec −2 in U , B, V , R and I, which indicates that Calar Alto is a particularly dark site for optical observations up to the I-band. The fraction of astronomical useful nights at the observatory is ∼70%, with a ∼30% of photometric nights. The typical extinction at the observatory is κ V ∼0.15 mag in the Winter season, with little dispersion. In summer the extinction has a wider range of values, although it does not reach the extreme peaks observed at other sites. The analysis of the Winter and summer extinction curves indicates that the Rayleigh scattering is almost constant along the year. The rise of the extinction in the summer season is due to an enhance of the Aerosol extinction, most probably associated with an increase of dust in the atmosphere. The median seeing for the last two years (2005-6) was ∼0.90 ′′ , being smaller in the Summer (∼0.87 ′′ ) than in the Winter (∼0.96 ′′ ). We conclude in general that after 26 years of operations Calar Alto is still a good astronomical site. Its main properties are similar in many aspects to those of other major observatories where 10m-like telescopes are under operation or construction, being a natural candidate for future large aperture optical telescopes.

Night-sky brightness monitoring in Hong Kong

“…On the other hand, this figure does present a characteristic level of night-sky brightness in Hong Kong as the survey covered a wide variety of locations and a wide range of time frames under diverse natural and artificial conditions. The current level is significantly brighter than the natural NSB level of 21.6 mag arcsec −2 (a 5.5-mag arcsec −2 difference, or 160 times brighter in flux) recommended by the International Astronomical Union for conducting astronomical observations (Smith 1979), not to say the darker skies of 21.7-22.0 mag arcsec −2 measured at research observatory locations (Pedani 2009;Patat 2008;Sanchez et al 2007;Krisciunas et al 2007;Pilachowski et al 1989). On the other hand, the darkest sites in Hong Kong at 20.8 mag arcsec −2 remain to be reasonably good sites for public enjoyment on the starry night and for astronomy work despite the heavy urbanization in many parts of the city and it is essential that efforts should be made for these locations to remain so as discussed in "Introduction".…”

“…On the other hand, professional astronomers use astronomical telescopes equipped with CCD cameras to capture night-sky images. Using the standard technique of astronomical photometry, the brightness levels of the star-free region(s) on CCD images can be used to determine accurately the NSB from an observatory site (Taylor et al 2004;Krisciunas et al 2007;Sanchez et al 2007;Patat 2008;Stalin et al 2008;Pedani 2009). Another approach to estimate the level of light pollution is by remote sensing techniques using night-time satellite images covering a wide area (Fig.…”

The low-metallicity QSO HE 2158 − 0107: a massive galaxy growing by accretion of nearly pristine gas from its environment?

“…Twilight observations were obtained to construct a fibre flat-field to correct for the difference in fibre-to-fibre transmission. Photometric standard star observations allowed an absolute photometric calibration of the data including the correction for the atmospheric extinction at the Calar Alto site (Sánchez et al 2007). A high S/N sky spectrum was extracted from a blank sky area within the PMAS field of view, which we subsequently subtracted from the entire datacube.…”

Contributions of artificial lighting sources on light pollution in Hong Kong measured through a night sky brightness monitoring network

“…Apart from the effects of the Sun and the Moon, the chief contributor to the NSB for a highly populated metropolitan city is the artificial lighting source. The level of the NSB had mostly been studied at professional astronomical observatories using the traditional astronomical technique of photometry -the amount of light detected in star-free regions on the CCD images was extracted to estimate the NSB at different wavelength bands (Pedani 2009;Stalin et al 2008;Patat 2008;Sanchez et al 2007;Krisciunas et al 2007;Taylor et al 2004;Liu et al 2003). This kind of observation helped astronomers to reveal potential light pollution threats to the observatory and aid in their search for a new potential dark observing location.…”