Atmospheric dispersion correction for the FORS focal reducers at the ESO VLT

The need for atmospheric dispersion correction on large telescopes is well known. Therefore it was decided to implement atmospheric dispersion correctors for FORS, the focal reducer/spectrographs of the ESO Very Large Telescope. The boundary conditions at the VLT Cassegrain foci excluded however all previously known ADC concepts and therefore we were forced to design a new one, the Longitudinal Atmospheric Dispersion Corrector (LADC) consisting of two thin prisms with variable distance. This design has several advantages compared to the ''classical concepts'': among others it avoids tilting the pupil axis and uses only one material (silica) which has a very high transmission over the operating wavelength range of PORS (330 - 1000 nm).