The expeditionary character of the San Luis Observatory calls for a more detailed statement regarding plant, equipment and operations than would be required in the case of a permanent observatory. A full description of essential features is therefore offered, that those who handle the catalogue may judge of its merits and deficiencies.
SCOPE OF THE CATALOGUE
The San Luis Catalogue, made possible through a cooperative arrangement between the Carnegie Institution of Washington and the Dudley Observatory at Albany, New York, embodying the concluded positions of stars observed at San Luis, Argentina, constitutes one of the steps in a general program formulated by Lewis Boss, with the object of producing the positions and motions of all stars brighter than magnitude 7.0, together with those fainter stars which, at the time of the conception of the undertaking, gave promise of yielding determinate motions. Since the derivation of the motion of a star involves the comparison of its positions over a considerable lapse of time, and since in many instances no present-day observations existed, it became necessary to reobserve these stars. Thus the San Luis Catalogue fills many gaps in observational material. It likewise helps to amend the disparity between observations taken in the northern and southern hemispheres.
In addition to the above mentioned motives for undertaking the observations of southern stars, it was realized that a program including observations taken in both the northern and southern hemispheres might be arranged to reconcile some of the bothersome discrepancies encountered in the formation of a smooth system of fundamental star places extending from pole to pole. Past experience in forming fundamental systems has indicated the difficulty in fixing the equator, due to widely varying systematic errors, found even in the case of modern catalogues of high precision. Combinations of catalogues observed nearly simultaneously from northern and southern stations have afforded the best means for linking the star places from pole to pole. These results, however, include outstanding errors due to personality and to the peculiar functioning of the various telescopes employed. Therefore, in formulating the general program it was arranged to observe a selected list of stars with the same instrument, both from the northern and southern hemispheres; moreover, the same observers were in large part engaged upon both series of observations, those taken at Albany, New York, and those at San Luis, Argentina. How far the anticipations were realized may be judged from the presentation of the results in the following pages.