The MICADO atmospheric dispersion corrector

Abstract

Earth’s atmosphere disperses light into a spectrum of colours, not unlike a glass prism. The next generation of extremely large telescopes, with mirror diameters above 25 meters, will make it possibly to study the skies at unprecedented angular resolution. To obtain such sharp images, it is crucial to minimise the atmospheric dispersion. For that reason, MICADO, a near-infrared camera being built for the 39 meter Extremely Large Telescope (ELT), will employ an atmospheric dispersion corrector (ADC). This ADC is the subject of this dissertation.The basic principle of an ADC is simple; a set of prisms is positioned such that the dispersion of incoming light is reduced and results in a sharp image. However, due to the high resolution provided by the ELT, the MICADO ADC will have to operate more accurately and more precisely than has so far been necessary on previous instruments. The ambitious scientific goals of the instrument make it necessary to study in detail the impact that the ADC will have on future astronomical observations as well as on the instrument itself.In this dissertation, different models of atmospheric dispersion are compared, the impact of the quality of the optics is studied with respect to the scientific goals and a new calibration method for the ADC is developed. In addition, the MICADO ADC will have to operate under cryogenic conditions (-196 °C, vacuum), which comes with considerable challenges. Two chapters describe a series of experiments that go into some of these challenges and their explanations.