Abstract
The actual zenith distance of observation in the Danjon astrolabe is a function of the prism angle and of the atmospheric refraction variation during the observations. The zenith distance instabilities preclude the obtention of reliable group corrections and, consequently, of the individual corrections to star positions. In this paper we present group corrections in zenith distance for astrolabe data at Valinhos (θ = -23° 00',λ = +3h07m), freed from systematic effects of instrumental origin by means of two different methods: 1) measurements of the instantaneous prism angle, with the help of a specially made auto-collimating eyepiece, and 2) estimation of the prism thermal gradient by the "astrolabe minus air" temperature difference, which is available in nearly all astrolabe stations.
