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