Radiative Coupling towards Sky and Ground (External View Factor)

Abstract

A significant radiative coupling can occur between the outer surfaces of a telescope, and enclosure, and the sky and the ground. This radiative coupling is illustrated in Fig. 8.1 for the IRAM 30-m telescope. In the left picture, the telescope is at horizon position so that the upper part of the reflector surface sees the warmer ground while the lower part sees the cooler sky. There exists an up.down temperature difference of the reflector panels (panel surfaces) of 4.5° C. In the right picture, the telescope points towards 45° elevation and the reflector surface sees more or less uniformly the cool sky so that the panels have a lower and more uniform temperature compared to the situation shown for horizon position. The measured temperatures are summarized in Fig. 8.2. In this interpretation, it is assumed that the contribution of reflected radiation from the sky and the ground is small (TiO₂ painted panels with little specular reflection). A consequence of the asymmetric radiative cooling is shown in Fig. 8.3 for the IRAM 15-m telescope, which pointed during night towards horizon. The lower part of the reflector, facing primarily the cool sky, is iced up at the panel gaps, and the upper part, facing primarily the warmer ground (even when covered with snow), is free of ice.