1 Introduction

A program for solar observations with a CCD astrolabe, aiming to monitoring variations of the apparent diameter, has been pursued since 1997, January, at the Observatório Nacional (ON), Rio de Janeiro ($\varphi = -22^\circ 54'$). The program is part of a collaboration with the Observatoire de la Côte d'Azur (CERGA), and the analysis of its first results was published elsewhere (Jilinski et al. 1998). The measurements follow the fundamental principle of astrolabe observations, relying on the coincidence of the direct and mirrored images as way to fix a given zenith distance. They are largely independent from reference definitions and from instrumental biases, provided the instrument stability is met between the transits of the upper and lower limbs of the Sun. The required stability is at daytime seeing level for the pointing and at one milli-second for time recording. Details of the method and description of the instrument were also produced by the french group (Irbah et al. 1994; Sinceac et al. 1998). Here the results of the campaign from 1997, January 10th to 1998, June 30th are presented.

Observations at the ON were made with a variable angle prism, between 27$^\circ$ and 58$^\circ$ of zenith distance. The operation without rotating shutter was preferred (Chollet 1996). The images are acquired by a COHU $\char93$ 4710 CCD camera. The camera is IR sensitive (4000-10000 Å). The effective wavelength is 6400 Å. Observations are taken daily, East and West of the solar meridian transit. The use of the variable angle prism, combined with the latitude of astrolabe and usually uncloudy weather enabled to get the measurements all year round.