(1) Departamento de Astronomia, Observatorio do Valongo/UFRJ, Brazil (2) Observatorio Nacional/CNPq, Brazil
We present results of CCD observations for 85 quasars with declinations -80° < \delta < +20°, made at the 1.60m Telescope of the Laboratorio Nacional de Astrofisica (LNA) - Brazil. The positions are referred to either the Carlsberg Series (CAMC), the ACRS and PPM catalogues. We discuss a method which allows to obtain positions based only on the CCD observations.
Auxiliary frames formed by the Guide Star Catalogue (GSC) and fainter reference stars are measured on digitized images of the Digitized Sky Survey (DSS) of the Hubble Space Telescope Science Institute. No observations with great field astrographs are needed. The original GSC star positions are corrected to the local system of the astrometric catalogue using common coordinates in the tangential plane. Small DSS fields containing enough GSC stars are used to reduce the positions of the fainter reference stars also present in the CCD fields. Then, the CCD fields are reduced and the positions of the quasars are finally determined.
The uncertainties in the many steps of the reduction process are accessed and a comparison of the results with VLBI positions is made. For the later, we used arclength differences to select fiducial sources for the determination of orientation between optical and radio frames. For 60 quasars then selected, the optical positions agree within 0".15 (in the rms sense) with VLBI's, with less than 0".02 average offsets. The orientation angles a1, a2 and a3 (and their formal errors) were -0''.035 (0''.021), -0''.024 (0''.022) and +0''.025 (0''.022), respectivelly. The results support the GSC correction method developed and the use of the DSS in combination with CCDs, for high precision optical astrometry of quasars. The procedures are very usefull for simple precise astrometry of natural satellites of planets, asteroids and faint objects in general, as well as for spectroscopy with fiber optics.