Local Seeing Control
The proposed baseline to control the local seeing around the telescope structure is to locate the telescope in open air conditions with a completely foldable enclosure. Under these conditions the telescope environment benefits from optimal natural air flushing, as well as during low wind periods, which prevent hot air plumes degrading the local seeing.
With the telescope placed in open air, it is necessary to provide liquid-cooled sunshields to the telescope structure to protect it from the solar irradiation.
The telescope mirrors shall be cooled to avoid mirror seeing, although they take advantage of natural air flushing provided in open-air conditions to reduce the mirror seeing. Mirror cooling by air impingement jets at the back of the mirror is proposed for M1 and M2 due to the compatibility with the mirror actuators, while liquid cooling is proposed for the remaining mirrors in the optical path. Additionally, a dedicated air flushing system is proposed on the M1 surface in order to guarantee the mirror flushing in very low wind periods. The flushing of the M1 surface is crucial since M1 supports the maximum heat load and the size of the mirror is large, being subjected to air temperature gradients that affect the local seeing.
The heat stop placed at the prime focus receives the maximum heat density. The open air configuration allows implementing a reflecting heat rejecter, reflecting away most of the heat load. If the telescope were to operate with a conventional enclosure, it would be necessary to implement a heat trap absorbing all received heat load at the primary focus, thereby increasing dramatically the heat that would need to be removed by the cooling system. An air suction system is additionally proposed around the heat rejecter surface to absorb any warmer air from the surface to avoid crossing the optical beam.
Liquid cooling by plate coils is also proposed for the telescope platform. Preliminary thermal analysis of the telescope platform shows that its temperature increases by up to 7 ºC with respect to the ambient temperature in low wind conditions if thermal control is not provided (see Figure), even if it is painted in white with high visible reflectivity and infrared emissivity paint.
Working in open air conditions, it is observed that the solar radiation reflected on the telescope platform warms up the lower part of the telescope structure, producing a similar effect as direct solar radiation; hence, it will also be necessary to provide sunshades for the lower part of the structure to protect it from the reflected radiation. Painting the telescope platform in white with high visible reflectivity and infrared emissivity paint to reduce the thermal load to be removed by its liquid cooling system produces a greenhouse effect on the lower part of the telescope structure, since the platform reflects the solar radiation on the telescope structure but prevents infrared exchange between the structure and the cold sky. To avoid this effect, it is proposed to paint the telescope azimuth platform in silver, with low infrared emissivity, which will allow the infrared exchange between the lower part of the structure and the sky, although it will increase the thermal load to be removed by the thermal control of the azimuth platform.