A simulation of the thermal behaviour of the UK-NL millimeter wave telescope

Abstract

To keep the thermally induced average telescope deformations within 15 μm, a thermal design goal has been set which limits the temperature difference between all locations in the panels, backing structure and subframe to 1°C. Temperature gradients are caused by non-homogeneous exposure and by difference in response to a changing exposure for elements with different time constants. Thermal model simulations are used to identify the critical elements in the thermal design of telescope and enclosure. It is shown that an enclosure with a highly reflective paint with high infrared emissivity on the outside surface and an inside zinc coating with low infrared emissivity works as a highly effective radiation shield. A sprayed aluminium coating on the telescope makes free convection the dominant heat transfer mechanism. An enclosure with louvres works as a chimney forcing the inside air temperature to follow the ambient temperature to within a few degrees. The resulting temperature stratification in the area where the telescope reflector is located does not exceed 1.5°C. Structural analysis shows that temperature gradients in the subframe cause reflector surface deformations with a regular pattern, like gravitational deformations, which means that an error budget summation based on independent rms values is not necessarily valid. Knowing the structural analysis coefficients for differential thermal expansion one can decide which time constants in backing structure and subframe have to be matched to achieve optimal performance.