Heat Stop

The Heat Rejecter (HR) lies at the prime focus of the telescope and operates as first field stop of the optical system. The aim of the HR is to reject the solar radiation falling outside the selected Field Of View, avoiding the development of thermal plumes, i.e. avoiding internal seeing.

The science requirements state that observations up to a distance of 200 arcsec from the solar limb should be possible. Various concepts were analysed to meet the requirement. The flat heat rejecter alternative was chosen as the most suitable for the telescope configuration. The flat heat rejecter consists essentially in a flat reflective surface inclined at 45º to the optical axis and will be described in the following.

The solar radiation, in the optical layout of the telescope, generates a total thermal load on the heat rejecter of about 13.4 kW assuming a ~95% reflectivity for the primary mirror (M1). The M1 has a diameter of 4.1 m with a central hole of 1.358 m and a focal length of about 6.2 m. Accordingly, the sun image size at the focal plane is about 59.1 mm and the heat load ranges between 2016 W and 672 W, for an HR surface reflectivity of 85% and 95%, respectively.

The cooling system required to remove the HR thermal load has been designed to avoid the formation of air turbulence in the optical path due to the temperature increment on the HR heated surface. Due to the large heat load applied on the HR heated surface, the proposed cooling system combines a liquid cooling system and an air suction system.

In the design process, the following guidelines were taken into account: a) coolant flow velocity below 5 m/s in all circuit parts in order to avoid vibrations and reduce surface washout; and b) coolant bulk temperature equal to ambient temperature in order to avoid internal seeing and prevent moisture condensation.

A new design is under study into the framework of SOLARNET project.

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