1 Introduction

The photographic plates of the Astrographic Catalogue can serve two different purposes, namely: (1) They constitute the oldest epoch which covers the entire sky to a minimum limiting magnitude of 11.0 photographic magnitude, yielding accurate positions for about four million stars, and (2) They constitute a large set of plates of uniform characteristics which cover large areas of the sky with a generous overlap between plates and thus are ideal for testing plate reduction methods designed to determine and eliminate field distortion and other systematic effects in coordinates measured on the plates.

Theoretical considerations may lead to a mathematical formulation of the field distortion expected for a given telescope, but deformations or misalignments of the optical surfaces can lead to additional terms which are difficult or impossible to predict. The HIPPARCOS Catalogue greatly facilitates the test of any reduction method since from it accurate positions can be derived even for an epoch as remote as that of the Astrographic Catalogue. At the same time it provides information on the colors of the stars.

The Naval Observatory at San Fernando, presently the Royal Naval Institute and Observatory at San Fernando, Spain, was in charge of the observations of the zone of the sky situated between two degrees and ten degrees southern declination. The plates cover a field of $2 \times 2$degrees approximately. Plates were taken along every full degree of declination, separating them in right ascension by a little less than two degrees. Thus a small overlap is obtained in right ascension between neighboring plates of the same zone, while a half-plate overlap is obtained between neighboring zones. The zones of even degrees in declination are displaced in right ascension by one degree with respect to the uneven zones. In this manner every star appears on at least two plates. A few stars may appear on as much as five plates.

The plates of the San Fernando zone have already been reduced by S. Urban (1996), who made the original measured coordinates available to us. His reduction is based on a second order polynomial in the coordinates X' and Y', the latter being the original measured coordinates X and Y corrected by an a priori known distortion correction (Zacharias et al. 1992). The ACRS catalogue (Corbin et al. 1991) was used as the reference system.

In this paper we make use of an improved method originally proposed by Abad (1993) to which magnitude and color dependent terms have been added. Here the HIPPARCOS Catalogue is used as the reference system.

The method makes full use of the plate overlaps, but we should point out here that under certain circumstances this may not lead to the best solution.

One of the reasons to chose the San Fernando zone for the test of the method is precisely that one of us (A.Z.) is a member of the San Fernando Observatory.