Bachar Wehbe
IA / FCUP

Abstract
Observations with ground based telescopes are affected by differential atmospheric dispersion due to the wavelength-dependent index of refraction of the atmosphere. The usage of an Atmospheric Dispersion Corrector (ADC) is fundamental to compensate this effect. Insufficient, or wrong correction of the atmospheric dispersion, will produce a spectrally elongated shape instead of a circular white one for the target observed.
The commissioning tests on ADCs with on-sky observation are not an easy task. In fact, the residual dispersion is expected to be of a few tens of milli-arcsec, where the object is almost 1 arcsec. We are developing a procedure to better test and characterize the alignment and behavior of an ADC on-sky. This procedure is based on the concept of ellipse fitting where we will be able to measure the ellipticity and the atmospheric dispersion direction.
This procedure was tested on ESPRESSO using the guiding camera images, where we detected a misalignment of 3 degrees of the ADC of UT3. By analyzing these images, we were able to correct this misalignment where the ellipticity dropped from 0.089 to 0.004 before and after the correction, respectively.
In this cookie seminar, I will describe the work done on the ADC of UT3 and present the results obtained.

2019 March 06, 15:00

IA/U.Porto
Centro de Astrofísica da Universidade do Porto (Classroom)
Rua das Estrelas, 4150-762 Porto