Night Sky Brightness
at Cerro Pachon

Alistair Walker, Cerro Tololo Inter-American Observatory

INTRODUCTION

The growth of the population centers in the vicinity of Cerro Tololo and Cerro Pachon (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 at the observatories in the decades to come. Many of the issues are illustrated (text in Spanish) at http://www.ctio.noao.edu/light_pollution.

Here we present models of the light pollution calculated for Cerro Pachon.

A Sky Brightness Primer

The natural night-sky brightness is made up from several components. From Benn & Ellison, La Palma report 114, and for the V band at zenith:

The calculation of the amount of light pollution at a given site has been treated by Garstang (PASP 101, 306, 1989a) and the evolution with time discussed by Garstang (ARAA, 27,19, 1989b). Here we show (Figure 1) an example the contributions from direct light, scattering by aerosols, and Rayleigh scattering by molecules vary as a function of zenith distance, calculated by Garstang (1989) for Mt. Graham.

Figure 1

It is often not appreciated that the brightness of the night sky varies around the solar cycle, with a B or V-band peak-peak of over half a magnitude, This is well shown (Figure 2) by Krisciunus (PASP, 109, 1181, 1997) for Mauna Kea.

Figure 2

Predicted man-made contributions for several observatories that have strong light pollution are shown in Figure 3, from 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.

Figure 3

Night Sky Brightness at Cerro Pachon

MODEL CALCULATIONS

Dr. Roy Garstang has calculated the light pollution at Cerro Pachon using the following inputs:

• latitude, longitude and elevation of Cerro Pachon
• latitude, longitude and elevations for La Serena-Coquimbo, Vicuna, Andacollo and Ovalle. The cities are treated as point sources.
• Population estimates for each center (1990, 2000, 2010, 2020, 2030, 2040).
• Light output per person of 150L (1990) and 300L (others).

Note that the model does NOT include:

• The effects of intervening mountains. Almost all the direct light (see Figure 1) is scattered at angles just above the horizontal, so a substantial amount of the direct light contamination is removed by intervening mountains. In our case Ovalle is not directly visible, and large fractions of Vicuna and La Serena-Coquimbo are occulted.

• On approximately 50% of nights La Serena and Coquimbo are totally covered by low cloud which reduces 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 light are all Sodium and there are virtually no Mercury lamps, the V band is a good test case. We also have some calculations for 2010 (ie near next solar minimum) looking at zenith distance 45 degrees to and away from La Serena-Coquimbo.

Night Sky Brightness at Cerro Pachon

Cerro Pachon Model 1:

Our 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 (note we have added in the population of Monte Patria) 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 (note that we have some new official predictions to 2005 that we will use to update these figures soon, but they make little difference to the results. They show Vicuna "flat", Andacollo increasing slowly, and the population of LaS-Coq. a little higher in 2000 than our figures).

Population growth:

Light per person model:

The light output per person in a "typical uncontrolled US city" is used as base, 1000L 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 2 malls for 250000 people, and the active steps being taken (successfully) to install environmentally friendly fixtures. However we realize our estimates are ad-hoc, and will re-do the calculations when better estimates are available.

Results:

Figure 4

Night Sky Brightness at Cerro Pachon

Cerro Pachon Model 2:

Our model 2, with results scaled from model 1, uses the same light output per person figure as model 1, but a much more agressively increasing population model. We do not think this is very likely, but it is the model we will use for planning purposes.

Results:

Figure 5

Cerro Pachon Model 3:

This has the same population growth as in model 2, but assumes NO light control whatsoever, and 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.

Results:

Figure 6

Night Sky Brightness at Cerro Pachon

RESULTS

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

  Year	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!

• Even the worst case model 3 shows that by 2040 the sky brightness at solar minimum is still only about half that at an un-polluted solar maximum

• The model 1 2010 figures for 45 deg 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.

We plotted the three models (Model 1, Model 2, and Model 3) together with an un-polluted model, where the solar cycle induced background variations are obtained from smoothing the V-magnitude curve in Figure 2.

Finally (Figure 7) we plot the three model results and compare them to "dark-sky" observatories elsewhere. Remember, the solar-cycle variation of 0.6-mag peak-peak has been removed! And for a comparison, the Kitt Peak 1980-2000 predictions are thought to be approximately a factor 2 worse than reality, due to lighting ordinances being introduced, similar to those that are being adopted for the cities near us.

Figure 7

Measurements at Cerro Tololo

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.

We have, near sunspot minimum 1987-1988, from measurements by A. Walker

U = 22.0 mag/arcsec2
B = 22.7
V = 21.8
R = 20.9
I = 19.9

And compare these to measurements by M. Phillips in 1997

B = 22.8
V = 22.2

and at ZD=45 deg in the direction of La Serena+Coquimbo

B = 22.6
V = 21.8

Given that the likely error on each measurement is approximately +/-0.15 mag (see discussion in Krisciunas 1997) we conclude that there is NO increase in light pollution detectable in broad-band colors. We will continue to monitor through the next solar cycle.