Thermal Model Calculations

Abstract

Model calculations are made to obtain for certain load cases information on the thermal behavior of a telescope, or a telescope and its enclosure or of a specific structural component. The temperatures predicted from model calculations are used for design and/or operational purposes. Design questions addressed in thermal model calculations may concern the type of insulation required to obtain temperature stability, the type and amount of ventilation required to obtain temperature uniformity of a BUS under asymmetric solar illumination, the heat required to counterbalance radiative cooling of a BUS during the night, etc. The calculations provide numbers, like 4 cm thick insulation, 10,000m³ h^-1 ventilation, 10kW heating etc., which are then realized in the construction. Thermal model calculations and associated finite element calculations may investigate for operational purposes the pointing stability of an alidade support, the focus stability of a BUS under the influence of the thermal environment, the development of transient reflector surface deformations due to temperature asymmetry in a yoke structure, etc. The thermal load case in the finite element calculation may be of a static nature in which the telescope and enclosure experiences one particular temperature change, or of a dynamic nature in which the telescope and enclosure experience a time variable temperature change. This temperature change may be of an artificial nature, for instance a sudden temperature drop to determine the thermal time constant of a structure, or it may represent the response of the telescope and enclosure to the change of the thermal environment in which they operate or are expected to operate. The quality of an answer of a thermal problem depends, evidently, on the quality of the thermal model, which is constructed from the basic relations of heat conservation and heat transfer. The confidence of an answer can be judged from thermal calculations of a similar structure of which temperature measurements are available for comparison.