On-sky measurements of atmospheric dispersion: I. Method validation

TL;DR

This paper introduces a new on-sky measurement method for atmospheric dispersion in optical astronomy, validating it with UVES spectroscopic data to improve atmospheric models and ADC design accuracy.

Contribution

A novel on-sky measurement technique for atmospheric dispersion using cross-dispersion spectrographs, validated with UVES data, achieving better than 50 mas accuracy.

Findings

Measured instrumental dispersion of 47 mas in blue, 15 mas and 23 mas in red arms.

Achieved ~17 mas accuracy in the 315-665 nm range.

Identified a 4% deviation in UVES pixel scale.

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. Atmospheric dispersion correction residuals above the level of ~ 100 milli-arcseconds (mas) will affect astronomical observations, in particular radial velocity and flux losses. The design of an ADC is based on atmospheric models. To the best of our knowledge, those models have never been tested on-sky. In this paper, we present a new method to measure the atmospheric dispersion on-sky in the optical range. We require an accuracy better than 50 mas that is equal to the difference between atmospheric models. The method is based on the use of cross-dispersion spectrographs to determine the position of the centroid of the spatial profile at each wavelength of each spectral order. The method is validated using cross-dispersed spectroscopic data acquired with the slit spectrograph UVES. We measure an instrumental dispersion of 47 mas in the blue arm, 15 mas, and 23 mas in the two ranges of the red arm. We also measure a 4 % deviation in the pixel scale from the value cited in UVES manual. The accuracy of the method is ~ 17 mas in the range of 315-665 nm. At this level, we can compare and characterize different atmospheric dispersion models for better future ADC designs.