For the first, the Astrographic Catalogue, the entire sky was to be photographed to 11 mag to provide a reference catalogue of star positions that would fill the magnitude gap between those previously observed by transit and meridian circle instrument observations down to 8 mag & ndash; this would provide the positions of a reasonably dense network of star positions which could in turn be used as a reference system for the fainter survey component ( the Carte du Ciel ).
