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NOAO Newsletter - Cerro Tololo Inter-American Observatory - September 1999 - Number 59


Pachon-Tololo Skies: Dark, and Keeping Them That Way

Recent measurements and computer models for Cerro Pachon indicate that it is one of the darkest sites in the world. However, the growth of the population centers in the vicinity of Cerro Pachon and Cerro Tololo (La Serena-Coquimbo, Vicuna, Andacollo, Ovalle), and the rapid growth of the coastal region as a center of tourism, has caused concern that poor lighting-control practices could threaten the dark skies in the decades to come. Even very pessimistic projections, with aggressive population growth and no lighting-control measures, indicate that Pachon will continue to be a very dark site until at least 2020. This worst-case scenario predicts an augmentation of 0.1 mag above the natural solar-minimum background of 21.9 V mag per square arcsec at the zenith in 2020. This should be compared with the 0.6 mag variation of night sky brightness induced by the solar cycle (Figure 1 and Model 3 below).

image Caption: Sky brightness model prediction for Cerro Pachon with (upper curve) and without (lower curve) light pollution from artificial sources. The dominant effect is the variation in natural sources induced by the solar cycle. This model projects rapid population growth and modest light control measures.

Moderate controls would extend our present very dark sky even further into the future. CTIO has begun a campaign of public awareness and specific pro-active and corrective measures in the surrounding municipalities to secure this future now . This has included working closely with Vicuna in the development of attractive, low-cost, low-impact street lighting, and with large international mining operations in Andacollo for the selection and positioning of outdoor industrial safety lighting. On the national level, lighting control regulations intended to protect Chile's world class scientific resources for observational astronomy have been crafted by CONAMA (the Chilean EPA), with technical assistance from CTIO and other observatories, and are making their way into law.

Here we present some models and measurements of light contamination for Pachon and Tololo, followed by a "success story" illustrating a collective response to a recent case of poor lighting practice.

Models and Measurements for Pachon and Tololo

The natural night-sky brightness is made up of several components. From Benn and Ellison, La Palma Technical Note 115, and for the V band at zenith:

     Zodiacal light (mean value):          34 nL *
     Airglow (varies with solar cycle): 11-50 nL
     Backround stars (V>20 +
         interstellar scattering):         13 nL
     Galaxies:                            0.1 nL
     Aurorae (for b < 40):                 0 nL
     Total:                             58-97 nL

*nano Lamberts

It is often not appreciated that the brightness of the night sky varies with the solar cycle, with a B or V-band peak-peak of over half a magnitude; this is shown for Mauna Kea by Krisciunus (PASP, 109, 1181, 1997).

The dispersed brightness from artificial sources -- light pollution -- includes contributions from direct light, aerosol scattering, and Rayleigh scattering by molecules. These vary as a function of zenith distance. The calculation of the amount of light pollution at a given site has been treated by Roy Garstang (PASP, 101, 306, 1989a) and the evolution with time discussed by Garstang (ARAA, 27,19, 1989b). Predicted man- made contributions for several observatories that have strong light pollution are shown in Figure 3 of Garstang (1989b). Note that the solar cycle modulation of 0.6 mag amplitude has been removed. The pollution growth models are based on population estimates and assume that no attempts are made to reduce the light output per person by installation of environmentally friendly light fixtures, etc.

Garstang has kindly calculated the evolution of light contamination at Cerro Pachon by his methods, using the location and elevation of Pachon and neighboring communities (La Serena-Coquimbo, Vicuna, Andacollo, Ovalle-Monte Patria); population measures or estimates for each center by decade, from 1990 to 2030; and light output per person of 150 L (1990) or 300 L (all other dates).

Note that the model does not include:

The effects of intervening mountains. Almost all the direct light is scattered at angles just above the horizontal, so intervening mountains remove a substantial amount of the direct light contamination. In our case, Ovalle is not directly visible, and large fractions of Vicuna and La Serena-Coquimbo are occulted.

Cloud cover. On approximately 50% of nights La Serena and Coquimbo are totally covered by low clouds, which reduce light contamination from these cities to near zero. Until 2010, La Serena-Coquimbo contribute around half of the light contamination, and more in later years.

The model calculates V magnitude light output at zenith. Since the street lights are all high pressure sodium, and there are virtually no mercury lamps, the V band is a good test case. We also have some calculations for 2010 (i.e., near next solar minimum) looking at zenith distance 45 to and away from La Serena-Coquimbo.

Garstang's calculation, which we call Model 1, assumes a moderate population growth. Andacollo is a small mining center, with little growth potential both for population and mining; Ovalle is an agricultural service center that also has little growth potential. La Serena and Coquimbo have turned into major tourist centers in the past 10 years, but there is no industry, and our model shows the population growth slowing down over that experienced in the last 15 years. Official figures show Vicuna as having near-stagnant population. We have chosen to increase it rather drastically to allow for an increase of tourism-related activity in the Elqui Valley. Here is the population model used for Model 1:

      City/Year   1990   2000   2010   2020   2030   2040

      LaS-Coq     220K   240K   260K   280K   300K   320K
      Andacolla    12K    12K    12K    12K    12K    12K
      Vicuna       20K    22K    28K    30K    32K    34K
      Ovalle      105K   110K   115K   120K   125K   130K

The light output per person in a "typical uncontrolled US city" is 1000 L per person. Our estimates relevant to the cities here were based on installed street lighting figures for Vicuna and La Serena, population density compared to a US city (about a factor 5 smaller), the lack of any industry apart from tourism, the existence of only two malls for 250,000 people, and the active steps being successfully taken to install environmentally friendly fixtures. However we realize our estimates are ad-hoc, and will re-do the calculations when better estimates are available.

We have also produced a Model 2 by scaling results from Model 1. We use the same light output per person but a much more aggressively increasing population model. We do not think this is very likely, but it is the model we will use for planning purposes:

      City/Year   1990   2000   2010   2020   2030   2040

      LaS-Coq     220K   240K   307K   393K   503K   644K
      Andacolla    12K    12K    14K    16K    18K    21K
      Vicuna       20K    22K    28K    35K    40K    50K
      Ovalle      105K   110K   120K   140K   165K   200K

Finally, we calculate a Model 3, with Model 2's population growth but no light control whatsoever. Additionally, the light output per person is assumed to be the "US uncontrolled city model." We consider that this model is very pessimistic and very much a worst-case scenario.

The outstanding results of this study are:

Cerro Pachon is an extremely dark site! The V-magnitude per arcsec2 above the natural background (assuming solar minimum sky) is predicted to be (in mag units):

             Model 1     Model 2     Model 3

      1990     0.010       0.010       0.010
      2000     0.022       0.022       0.033
      2010     0.025       0.042       0.075
      2020     0.027       0.062       0.130
      2030     0.028       0.082       0.220

Compare these to the 0.6 mag increase in sky background from solar minimum to maximum shown in the figure! Even the worst case model 3 shows that by 2030 the sky brightness at solar minimum is still only about half that at an unpolluted solar maximum.

The model 1 2010 figures for 45 zenith distance towards La Serena-Coquimbo and directly away are +0.041 mag and +0.031 mag respectively (cf +0.025 mag at zenith). Note that the sky naturally brightens as zenith distance increases.

The accompanying Figure shows our planning model, Model 2, together with a natural-brightness-only model including the effects of the solar cycle. Facilities presently under construction on Cerro Pachon face a long, dark future!

Cerro Tololo is closer to La Serena-Coquimbo and Vicuna than Cerro Pachon and is roughly estimated to suffer 65% more light contamination than Cerro Pachon. But since Pachon is so dark, how much is this quantitatively for Tololo, and how has it evolved?

We have, near sunspot minimum 1987-1988, from measurements by A. Walker: U = 22.0 mag/"2, , B = 22.7, V = 21.8, R = 20.9, and I = 19.9.

Compare these to measurements by M. Phillips in 1997: B = 22.8 and V = 22.2; and at ZD = 45 in the direction of La Serena + Coquimbo: B = 22.6 and V = 21.8.

Given that the likely error on each measurement is ~+- 0.15 mag (see discussion in Krisciunas 1997) we conclude that there is no increase in light pollution detectable in broad-band colors at zenith distances less than 45 between these observations made at the last two sunspot minimum. We will continue to monitor the sky brightness through the present solar cycle.

For more details and graphs, see http://www.ctio.noao.edu/site/pachon_sky/.

Alistair Walker (awalker@noao.edu)
Chris Smith (csmith@noao.edu)


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