Mitigating wavefront error caused by sky subcooling of the ELT structure

Telescopes suffer from radiative subcooling against the night sky (“low-wind effect”), depending on wind speed, sky exposure and the emissivity of the surface finish. Critical structural elements in the ELT are the spider vanes, the vertical trusses in the telescope tube and the M4 Tower. We present an improved sky temperature model and 3D computational fluid dynamics simulations of the airflow inside the ELT dome with local temperatures and derive the wavefront error map in the pupil from it, using ray tracing. We discuss the optimal shape that M4 can achieve to correct the errors and report the wavefront residuals.