The Danjon Astrolabe is a highly specialised instrument developed in the 1950s for use as an alternative to conventional transit instruments, which had many sources of potential error. In July 1959, a Danjon astrolabe (OPL No.9) was brought into service at the RGO Herstmonceux under the direction of C. A. Murray, primarily in order to extend the study of its effectiveness in determining corrections to the catalogue positions of bright stars. Before commencing operations, the method of recording an observation was changed from recording the time at which a star crossed the instrumental almucantar to recording the altitude of the star at precise instants close to the predicted time of transit. After four years of observations, the instrument was transferred to Royal Observatory, Cape of Good Hope, South Africa, where it was employed for four years from April 1965 on a programme of observations of selected stars from standard catalogues. It was subsequently returned to Herstmonceux, where a perfunctory attempt was made to adapt it as a reflecting astrolabe before it was decommissioned. From 1961 until termination in 1979, the programme at RGO and the Cape was directed by Dr David V. Thomas. The final analysis of the Cape results, culminating in corrections to the catalogue positions of over 1000 southern stars, was not completed until 1979.

The papers in RGO 87 comprise reports, notes, tabulated data of observations and analysis, computer program information, calculations, charts, and correspondence relating to the astrolabe. Titles in inverted commas are the original file titles.

The papers in this series cover the modifications made to the instrument and their effectiveness, the observing programme, the methods of reduction and analysis, and the determination of corrections to the adopted positions of several hundred stars observed at Herstmonceux. In common with the experience of other observatories, the angle of the prism (notionally 60 degrees), which determines the altitude of observation, was found to vary significantly during the course of a night’s observations. Empirical corrections were determined and applied at Herstmonceux, which should significantly increase the accuracy of the results. Air conditioning was installed in the astrolabe pavilion at Cape Town, and analysis of the results showed that systematic errors due to nightly variation of prism angle had been largely eliminated.