Chile Sky Background at ESO/la Silla in the Visible and Near IR Leonardo Vanzi -Olivier R. Hainaut European Southern Observatory La Silla - Chile

Chile 2Abstract We present measurements of the sky background collected at the ESO - La Silla Observatory. Our data include both the optical and near-infrared astronomical bands. We discuss the major contributions to the sky background and show that La Silla observatory is one of the best sites available for astronomical observations. The effect of light pollution from nearby towns is still relatively negligible. We point to the importance of preserving this situation."

Chile 3 The Site: ESO – La Silla 2400m altitude 85km from La Serena 75km from Vallenar

Chile 4 Near IR The natural near-infrared (NIR) sky background is dominated by the emission lines produced by the OH radical. The OH is generated in reactions between H and O_3 in the high atmosphere at an altitude of about 87 Km. The NIR atmospheric windows centered at 1.25, 1.65 and 2.2  m, corresponding to the astronomical broad band filters J, H and K, are particularly rich of OH emission lines. Their effect on the astronomical observations has been reviewed by Oliva & Origlia (1992) and Rousselot et al. (2000). At wavelengths greater than 2  m the effect of the thermal radiation starts to be important, this is mostly due to the telescope and its surroundings rather than to the sky. We have measured the sky brightness at the zenith in the J, H and Ks filter at the ESO La Silla Observatory during bright (full moon) and dark (new moon) time. All measures have been obtained at the ESO 3.5m NTT with the NIR camera- spectrometer SOFI. The values obtained are listed in Table 1. These measures compare well with the average values obtained at other astronomical sites worldwide recognized amongst the best, see Table 2, and make of La Silla a site with excellent background conditions.

Chile 5 SkyJHK Dark Bright SiteJHK Mauna Kea Las Campanas Paranal Table 1. NIR Sky brightness at La Silla (mag/arcsec2) in bright and dark time. Table 2. Average NIR Sky brightness at top quality astronomical sites (mag/arcsec2).

Chile 6 Pollution in the NIR To quantify the possible polluting effect of artificial illumination we have measured the NIR background at different altitudes (ALT) and azimuth (AZ). All measures were obtained during dark time to avoid the effect of the moon. The values relative to the zenith background are plotted in Fig. 1. Blue, green and red lines represent the J, H and Ks filters respectively. The solid lines are measures at ALT=20, the dashed lines at ALT=45. The brightest artificial spots on the La Silla horizon are produced by the cities of La Serena, about 85 Km to the south-west (AZ=30) and Vallenar about 75 Km to the north (AZ=150). The approximate positions of the two towns are marked in Fig. 1. There is no correlation between these positions and the variation of the NIR sky background. The pick at AZ about 330 is related to the Galactic Center that was rising at that position during the observations. Apart from this the most obvious trend is a maximum toward the north (AZ=180) and a minimum toward the S-W, direction of the coast.

Chile 7 Figure 1: Sky background as a function of Azimuth [relative to Zenith brighness] Blue, green and red lines represent J, H and Ks bands. Measurements have been taken at ALT=20 (solid lines) and ALT=45 (dashed lines). VL=Vallenar, LS=La Serena. Alt = 20 o Azimuth Alt = 45 o Surf.brightness (relative to Zenith)

Chile 8 Near IR Spectra In Figure 2 we compare the NIR sky spectrum obtained at a ALT=15, AZ=30 (red line) toward La Serena and at ALT=15, AZ=220. Two conclusions can be drawn from the spectra: 1.No artificial emission line is detectable in the direction of La Serena but only a very weak increase in the thermal background. 2.The increase of the background toward the North is mostly due to the OH emission lines. This is consistent with the fact that the maximum is higher in the H band where the OH lines are stronger.

Chile 9 Figure 2: NIR sky spectrum observed from La Silla at ALT=15 toward La Serena (red line) and toward AZ=220 (blue line), the darkest region of the sky.

Chile 10Visible In the visible, the natural sky background is dominated by the light diffusion, with some emission lines, esp. toward the red end of the spectrum ( R and I bands). sigma-clipping method to reject the astronomical objects. Results for zenithal surface brightnesses are listed in Table 3. It shows that La Silla has an excellent sky in terms of darkness. The sky brightness was measured on all the moonless archive images of the SuSI-2 instrument with exposure time longer than 200s. The background was estimated using an iterative

Chile 11 Table 3. Visible sky brightness at Zenith SiteUBVRI La Silla Paranal Tololo

Chile 12 Pollution in the Visible Visually, the La Silla sky in direction of La Serena and Vallenar displays a faint, but clearly visible glow. In order to quantify this pollution, we used the SuSI-2 imager to measure the sky background toward various azimuths. In order to quantify that effect, and to measure the variations as a function of the Altitude, we performed a series of measurements with Alt in the o range in various directions: toward La Serena, toward Vallenar, and toward the Andes. The resulting magnitudes are plotted in Figure 3. Except in the U-band, the effect of La Serena is only marginally detectable for Altitudes lower than 20deg. In U, it will be necessary to confirm these measurements spectroscopically.

Chile 13 Figure 3. Visible sky brightnesses from La Silla on a moonless night, as a function of Alt, for 3 azimuths. The region from 55 to 75deg was affected by the Milky Way.

Chile 14 Full Moon For completeness, we also present the variation of the visible sky brightness at Full Moon. Figure 4 displays it as a function of the altitude, while the Moon was very low on the horizon. Figure 4. Sky brightness at full Moon.

Chile 15Conclusions The sky brightness at la Silla was measured in the visible and Near IR. The values measured are similar to those obtained at other observatories. The pollution from la Serena and Vallenar, which is clearly visible by naked eye, does not affect yet the observable sky (Alt>20deg) at a significant level. At very low altitudes, the effect of la Serena is already detected. It is therefore important for the La Silla sky to maintain the illumination of La Serena at its current level or better.