ABSTRACT. The contribution of the Carlsberg Meridian Catalogue La PalmaNumber 4 to the determination of the optical reference frame is discussed. This catalogue of almost 51000 stars provides one of the most accurate optical reference frames at the present epoch, having a density of 1 star per square degree, and an average accuracy of 0"l2 in position and 0"003 per year in proper motion for stars with V<9. The catalogue also contains positions of reference stars with V>11 in the fields of benchmark extragalactic radio sources which can be used in linking the optical reference frame defined by the FK5 to the extragalactic frame.

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The primary purpose of the FK5 is to provide an absolute optical reference frame against which to measure the positions and motions of other objects. One of the main tasks of meridian circles is to check and improve that reference frame by repeated observation of the fundamental stars. The meridian circle at Bordeaux, France and the Carlsberg Automatic Meridian Circle (CAMC) at La Palma are carrying out programmes of observation of FK5 stars partly with this aim in view.The positions from which the systematic differences are derived are the means of at least 10, but normally about 30 (Bordeaux) and 50 (CAMC), independent observations of each FK5 star made in the period 1984–87. The systematic differences are displayed in pairs of 3-dimensional plots, Bordeau–FK5 and CAMC–FK5. The similarities of the corresponding pairs of plots is striking, particularly in declination. This implies that systematic errors exist in the FK5 which reach about in places at the epoch 1986.0.

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