The MICADO atmospheric dispersion corrector

© Joost van den Born

Atmospheric dispersion, the wavelength dependent differential refraction of light passing through the atmosphere, will cause severe degradation of image quality and contrast on the upcoming Extremely Large Telescope (ELT). To counteract this adverse effect, MICADO -- the near-infrared camera and spectrograph for the ELT -- will employ an atmospheric dispersion corrector (ADC), which aims to reduce the dispersion to below 2.5 milli arcseconds. Aside from the unprecedented correction requirement, it will also be the first ADC that has to operate in cryogenic conditions (77 K, vacuum), which comes with additional challenges. The ambitious scientific goals of MICADO 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 colloquium I will present my PhD work on this topic. I will first introduce the scientific context and need for this component, what will ultimately limit its performance and how the quality of the optics will impact the astrometric ambitions of the instrument. As part of the design process, the NOVA team at ASTRON built a prototype to test the performance and assess the expected lifetime of the design. Although the initial performance was excellent, the simulated lifetime was not. I will discuss some of the observed issues and their causes, which required us to perform a detailed characterisation of the stepper motor that will be used to position the ADC optics. To close, I present a novel method on how we might do the performance verification once the instrument arrives at the commissioning phase.