The Variable Thermal Environment

The local thermal environment interacts with a telescope, or its enclosure, and determines to a large extent the thermal behavior of a telescope. The thermal environment is therefore investigated with the intention to derive global parameters for the design and operation of a telescope and its enclosure. The global parameters are site dependent and dependent on the season, the month, and the day of the year. Each telescope site needs its own investigation for a considerable length of time to obtain reliable statistical data. A telescope may reach a (quasi-)equilibrium state in the thermal environment, or may follow its variations with a certain time delay and a reduced amplitude. The interaction of structural components with the environment can be reduced by paint, insulation, and ventilation, or even be excluded to a large extent by placing the telescope in an enclosure. Important for the design and operation of a telescope are the time constants of the thermal environment and the thermal time constants of the telescope components.