Atmospheric dispersion

The atmospheric dispersion corrector, located near the first pupil image, uses a pair of normal-field Risley prisms, which are rotated. The glasses, Schott PK51A and Corning B29-52, correct a wide spectral bandpass, from 380 nm up to 950 nm. The residual chromatism in the 600-850nm band is on average 1/4 of the Airy peak diameter, corresponding to $0.03\arcsec$ at 750nm. With the 1.5m telescope and the 6mm pupil, the prisms correct up to $60\degr$ from zenith. We adjusted visually the prism settings although pre-calculated settings would be preferable, especially for faint stars.