Where the shadows lie – The dark skies of Chile

 

Ferdinando Patat – European Southern Observatory

 

For ground-based astronomy, the quality of the observing site is fundamental and this necessarily forces the astronomers to place their telescopes in the most desolated and waste Earth’s places. In fact, frontline optical and infrared observations require very good transparency conditions, very low humidity, a high number of clear nights, steady and non-turbulent winds and, of course, a dark sky. This has become even truer during the last fifty years, during which the growth of cities and industries, especially in Europe and North America, has added a new kind of environmental contamination, which is usually referred to as light pollution.

 

Good bye, blue sky

 

The problem has become indeed serious, so that practically no professional astronomical observations are carried out from continental Europe and all large European telescopes have been placed either in the Canary Islands or in some remote places, usually in the northern and southern American continents.

To stop the light pollution and in order to diffuse the awareness of the problem and the possible solutions to it, in 1988 the International Dark-Sky Association was founded[1]. These noble tasks, though, are unfortunately very difficult to achieve and it will probably take a long time before new lighting systems are put in place and the bright halos surrounding our towns disappear from the night sky. It is very sad to notice that the Milky Way is already a forgotten spectacle for many of us. This is just another sign that we are loosing contact with Nature and its beauty.

Fig.1 – Satellite map from the World Atlas of Artificial Sky Brightness. Credit: P. Cinzano, F. Falchi (Padua University – Italy), C.D. Elvidge (NOAA National Geophysical Data Center, Boulder, CO). Copyright of the Royal Astronomical Society. Reproduced from the Monthly Notices of the RAS by permission of Blackwell Science).

The effect of light pollution on astronomical observations is in fact devastating. Street lamps do not illuminate just the ground. Some radiation is reflected towards the sky and, in the most deprecated cases, even directly sent upwards by lamps with a bad design. When this light reaches the troposphere, it is reflected back to the Earth and it overwhelms the natural night sky brightness, making the detection of faint and remote astronomical sources very difficult, if not impossible.

The artificial illumination increases the sky luminosities chiefly through emission lines of Mercury and Sodium vapor lamps, which affect in particular the visual band, but have also intense emissions in the blue and violet parts of the spectrum. Other lines are produced by special elements (like rare earths) which are introduced in some lamps (like the compact fluorescent lamps or CFL), in order to generate a spectrum which resembles that of the Sun, to which the human eye is tuned and which allows a perfect color recognition. Finally, there is a small contribution of incandescence lights, which produce a continuum spectrum.

The effects of artificial light contamination are clearly visible in the upper panel of Fig.2, where the night time spectrum obtained with the 1.8m reflector of the Asiago Observatory (Italy) is shown. The original scientific spectrum, which was aimed at studying a faint Supernova, was obtained with the telescope pointing almost at zenith, in order to minimize the impact of atmospheric extinction. Notwithstanding this precaution, the sky spectrum, which has been extracted on an object-free part of the field of view, shows very clear signs of artificial emission lines of Mercury and Sodium, which are identified with red marks. In particular, the broad Sodium feature at about 590nm is a clear imprint of high pressure lamps, which are the most disturbing sources of light pollution.

The 1.8m Copernicus telescope is placed on the top of Mount Ekar, at about 1300 meters on the sea level. In spite of its relatively good location, the brightest feature in its night sky spectrum is the Mercury line at 436nm (the only red line in Fig.2), which is much brighter than the natural Oxygen line at 558nm, the most intense feature emitted by the night sky.

Fig.2 – Comparison between a night sky spectrum obtained in a light polluted site (Asiago Astrophysical Observatory – Italy, upper panel) and a dark site (ESO-Paranal – Chile, lower panel). Spectral line identifications for the main features are traced in red for artificial sources and in blue for the natural ones. The emissions generated by street lighting are clearly visible, mainly in the form of strong lines of Mercury and Sodium, which fall not only in the visible range (500-600nm), but also in the in blue and violet parts of the spectrum.

The presence of these lines causes the sky to become artificially brighter, and it turns into a disaster for broad band imaging. In a relatively protected site like Mount Ekar, in the B and V filters the enhancement is more than a magnitude. But when one goes close to a relatively big town, this degradation can reach two or more magnitudes, causing many astronomical objects to be lost, not only for the visual observers, but also for the evolved amateurs equipped with modern CCD cameras. A possible solution to the problem, at least for photographic and digital imaging, is the use of sky-suppression filters, which are designed to have a very low transmission in the spectral regions centered on the main Mercury and Sodium emission lines.

Of course, also professionals are affected by light pollution, which makes deep imaging a very difficult task, since the augmented sky background introduces an additional noise in the images, increasing the detection threshold. The contamination is a bit less of a problem for spectroscopy, since in that case one can still observe “in-between” the emission lines. Unfortunately, the region where the high pressure Sodium broad feature is sitting is also very interesting for several astrophysical fields. If it is true that one can subtract the sky spectrum from its science spectrum, this is not the case for the added noise, which cannot be removed and degrades the data quality in an unrecoverable way, especially for low and medium resolution spectral dispersions.

The artificial emission lines are so bright in large towns that one can use them to achieve an accurate wavelength calibration, without the need of obtaining dedicated exposures on special arc lamps. A bitter advantage, though.

Fig.3 – Tracings of night sky spectra at a polluted (red) and at a dark site (blue). Main line identifications for artificial features are marked. The colored curves on the top of the figure are the transmission functions of UBVR standard astronomical passbands. The light pollution appears to be maximum in the V passband, which is very close to the human eye sensitivity region. The B band is also severely contaminated, making astrophotographer’s life quite difficult.

Moving to the waste lands

 

As we have seen, the problem is getting serious. Not only the amateur astronomer who wants to observe with her telescope from the balcony is disturbed by the light pollution. This is the case for the professionals too. All the more. Good astronomical sites are hard to find, because a long series of requirements, having to do with natural geographical and meteorological characteristics need to be fulfilled. As a matter of fact, nowadays professional astronomers perform their observations mainly in remote places, either in dry and high altitude deserts or on the top of high and isolated volcanoes. Nevertheless, even going in the most abandoned places of the planet does not solve the problem once and forever. In fact, once an observatory has been built, one has to make sure that the growing human activities are not degrading the excellent conditions for which the site has been chosen. For this reason, the Commission n. 50 of the international Astronomical Union for the protection of professional observatories has created a working group to control the light pollution both at existing and potential sites. The latter, in fact, could guest in the next 20 years, the extremely large telescopes, reaching up to a hundred meters in diameter, and for them one needs to plan everything well in advance, including the prevention of contamination by human activities. For this reason, all major observatories around the world have sky brightness surveys, which serve both to detect any signs of light pollution and to study long term trends, seasonal variations and so on. This is true also for the largest optical and near-IR observing facility in the world, the European Southern Observatory[2], which represents the most important resource for all European researchers. This international organization, originally founded in 1962, is supported by eleven countries and operates two large observatories located in the Chilean Atacama Desert, La Silla and Cerro Paranal, the latter hosting since 1998 the four 8.2m telescopes.

Fig.4 – Cerro Paranal (Chile) as seen from the base camp (credit European Southern Observatory).

In those sites the sky is absolutely dark. In moonless nights the southern Milky Way is of an amazing beauty, the Magellanic Clouds shine like atmospheric clouds and Omega Centauri is clearly visible to the naked eye. No matter how many times you have seen it, it will always surprise you, being one of the most fascinating Nature’s aspects one can experience.

The price astronomers have to pay for having such excellent conditions is relatively high. Being far from any city, everything needs to be brought there and this makes all buildings and infrastructures pretty expensive. Also from the living conditions point of view things are not easy. Humidity is extremely low (typical values range from 10% to 20%), there is a constant wind blowing from a dominant direction (25-30 km per hour), the elevation is quite high (more than 2500m at both observatories) and, besides observing, there is not much one can do. But the reward which is given back in terms of site quality is enormous. Seeing conditions are very good (0.7 arc seconds on average), the number of clear nights is larger than 300 per year, the transparency is very good and, of course, the sky is dark. As dark as it can naturally get.

Fig.5 – The summit of Paranal Observatory with the four 8.2m telescopes, which have been named using the Mapuche language: Antu (the Sun), Kueyen (the Moon), Melipal (the Southern Cross) and Yepun (Venus). (Credit European Southern Observatory).

The night sky at Paranal

 

The four 8.2m ESO telescopes are located on the top of Cerro Paranal in the Atacama Desert in the northern part of Chile, one of the driest areas on Earth. Cerro Paranal (2635 m) is at about 108 km south of Antofagasta (225,000 inhabitants), 280 km south-west from Calama (121,000 inhabitants) and 12 km inland from the Pacific Coast.  This ensures that the astronomical observations to be carried out there are not disturbed by adverse human activities like dust and light from cities and roads. Nevertheless, for the reasons we have outlined above, a systematic monitoring of the sky conditions is mandatory in order to preserve the high site quality and to take appropriate action, if the conditions are proven to deteriorate. Besides this, it also sets the stage for the study of natural sky brightness oscillations, both on short and long time scales, such as micro-auroral activity, seasonal and sunspot cycle effects. For all these reasons ESO has started a systematic monitoring campaign, which automatically extracts the sky background information from the images that are obtained for scientific purposes. The results of this survey confirmed that Paranal, similarly to La Silla and other Chilean sites, is one of the darkest places on the planet and that there are no signs of artificial contamination.

Nevertheless, even at dark sites like Paranal, the night sky is not completely black.  In fact,  when one is observing from the ground, there are several sources that contribute to its brightness, some of which are of extra-terrestrial nature (e.g. unresolved stars/galaxies, diffuse galactic background, zodiacal light) and others are due to atmospheric phenomena (airglow and auroral activity in the upper Earth's atmosphere). While the extra-terrestrial components vary only with the position on the sky and are therefore predictable, the terrestrial ones are known to depend on a large number of parameters (season, geographical position, solar cycle and so on) which interact in a largely unpredictable way. In fact, airglow contributes with a significant fraction to the optical global night sky emission (up to 50%) and hence its variations have a strong effect on the overall brightness, which changes from passband to passband.

Fig.6 – Night sky spectrum obtained at Paranal with the Focal Reducer low dispersion Spectrograph (FORS1) mounted at the 8.2m telescope Kueyen. The dashed curves indicate the BVRI passband response functions.

In the B filter, the spectrum is rather featureless and it is characterized by the so called airglow pseudo-continuum, which arises in layers at a height of about 90-100 km (mesopause) and extends all the way from 400nm to 700nm. All visible emission features, which become particularly marked below 400nm and largely dominate the U passband, are due to Herzberg and Chamberlain bands of molecular Oxygen (O₂).

The V passband is chiefly dominated by Oxygen 558nm and to a lesser extent by Sodium D doublet 590nm and OI 630,636 nm doublet. The relative contribution to the total flux of these three lines is 17%, 3% and 2%, respectively. Besides the aforementioned pseudo-continuum, several Oxydryl (OH) Meinel vibration-rotation bands are also present in this spectral window; in particular, there is one which is clearly visible on the red wing of NaI D lines and  another two on the blue wing of OI 630nm. All these features are known to be strongly variable and show independent behavior. In fact, OI 558nm, which is generally the brightest emission line in the optical sky spectrum, arises in layers at an altitude of 90 km, while OI 630,636 nm is produced at 250-300 km. The OH bands are emitted by a layer at about 85 km, while the Na ID is generated at about 92 km, in the so called Sodium-layer which is used by laser guide star adaptive optic systems. In particular, OI 630,636 shows a marked and complex dependency on geomagnetic latitude which turns into different typical line intensities at different observatories and it is known to undergo abrupt intensity changes on the time scale of hours.

In the R passband, besides the contribution of NaI D and OI 630,636nm, which account for 3% and 10% of the total flux in the spectrum, strong OH Meinel bands begin to appear, while the pseudo-continuum remains constant. Finally, the I passband is dominated by OH Meinel bands; the broad feature visible at 860-870nm, and marginally contributing to the I flux, is due to molecular Oxygen.

Due to these phenomena, the sky is darker in the blue (22.6 magnitudes per square arcsecond in B) and becomes progressively brighter as one goes to the red (19.7 magnitudes per square arcsecond in I). Then, when one enters the infra-red domain, the natural sky background becomes more and more the dominant source of radiation, making ground-based observations very difficult.

As many surveys have demonstrated, the dark time sky brightness shows strong variations within the same night on the time scales of tens of minutes to hours. This variation is commonly attributed to airglow fluctuations. Moreover, as first pointed out by Lord Rayleigh, the intensity of the OI 558nm line depends on the solar activity. Similar results were found also for other emission lines (NaI D and OH). Moreover, B and V sky brightness is well correlated with the 10.7 cm solar radio flux so that, during a full sunspot cycle, it changes by about 60%, with the highest fluxes reached during the maximum of solar activity.  This means that the difference in sky brightness between solar minima and maxima can reach about half a magnitude.

Fig.7 – Paranal’s southern horizon as seen from the telescopes platform. The Southern Cross is just rising; its brightest star, Alpha Crucis, is at an elevation of 6 degrees, while Beta Crucis is at only 2 degrees. The sky is so clear and the humidity so low that other stars are visible almost until they disappear below the horizon (Credit L. Vanzi, European Southern Observatory).

Besides the atmospheric airglow, there are other natural sources. Among these, the dominant one is the Sun light diffused by the dust distributed on the ecliptic plane, known as the zodiacal light. This can contribute up to 50% of the total brightness if one is observing at low ecliptic latitudes and, in general, it depends on the position of the sky where one is pointing. In dark sites, like Paranal, this is clearly visible also to the naked eye, as a diffused and wide cone, just after and before the evening and morning twilights respectively.

 

The night sky must have impressed the human beings since the beginning of their evolution, at the very origin of Astronomy. Its beauty continues to astonish everybody who turns his eyes to the heavens, no matter whether he or she is an astronomer or simply a lover of Nature. Sadly, less and less people can nowadays enjoy this wonderful experience, which is unfortunately becoming a privilege. One more reason to love and protect it.

Fig.8 – The radio galaxy Centaurus A. The image was obtained combining three B, V and R frames, about 5 minutes of exposure time each, taken with FORS2 mounted at the 8.2m telescope Kueyen. The image quality is 0.6 arcseconds (credit European Southern Observatory).