Atmospheric Dispersion Corrector: from design phase to on-sky commissioning

Astronomical observations with ground-based telescopes are affected by differential atmospheric dispersion, a consequence of the wavelength-dependent index of refraction of the atmosphere. In high resolution astronomical instruments, an Atmospheric Dispersion Corrector (ADC) is mandatory to avoid wavelength dependent losses. Even though an ADC seems a simple component, but from the design phase to on-sky commissioning, several problems can occur. The design of an ADC is based on atmospheric models that, to the best of our knowledge, were never tested on-sky. Different models shows a variation of 50 milli-arcseconds (mas), a value close to the required residuals from current ADCs. During the commissioning, detecting a variation of 50 mas in a PSF of 1 arcseconds, is not an easy task. We will present a method to measure on-sky the atmospheric dispersion based on measuring the PSF centroid of each wavelength using cross-dispersed spectra. We are able to characterize different atmospheric models with an accuracy of 18 mas. As for the on-sky commissioning, we present a simple concept based on the ellipse fit of intensity contour plots of the PSF. This method will allow us to better align the ADC in terms of prisms angles and total dispersion direction using on-sky measurements. In this talk we show the study we did to improve the phases of an ADC from design to on-sky commissioning