Atmospheric dispersion correction
PrincipleAtmospheric dispersion has to be corrected to use narrow slits and to get round images in the UV and Blue. Correctors for the atmospheric dispersion are described in a paper presented by Avila et al. (1996)
ImplementationAtmospheric dispersion correctors (ADC) were built for the FORS instruments of the ESO VLT. The ADC were moved together with the FORS instruments to the respective unit telescopes. For reasons explained in the Landskrona paper a new concept called Longitudinal ADC (LADC) which consists of two silica prisms of 590mm diameter and 22.5mm centre thickness were choosen. They are shown in the following pictures:
LADC prism during acceptance test
One mounted LADC prism
PerformanceThe following figures show the atmospheric dispersion normalized to a wavelength of 650nm (red curve) and the residual atmospheric dispersion which is not corrected by the LADC (blue curve). Both measurements were taken on the sky with the slits aligned in parallel to the parallactic angle.