The pointing of the IRAM 30-m telescope

The original pointing model of the IRAM 30-m millimeter wavelength radio telescope is based on nine parameters which are updated from special pointing measurements made approximately every two weeks. The pointing models derived from these measurements provide a pointing accuracy of similar to 3.5'' (rms) which, however, degrades slowly with time. Application by the observer of instantaneous pointing corrections recovers the inherently precise operation of the telescope. The data analyzed here show that the number of pointing parameters used for the 30-m telescope can be reduced to six because the astronomical positions of pointing sources are today accurately known, and because the telescope does not show hysteresis or long-term irreversible changes of its mechanical behaviour so that the gravitational bending can be described by a unique and time-independent, function, as should be the case. There are indications that the inclination of the azimuth axis is influenced by seasonal thermal effects, which merit further monitoring for eventual quasi real time correction. The other pointing parameters show random variations in time which makes it difficult to trace any particular source which sets their ultimate accuracy, although we suspect residual thermal effects to be the main source of perturbation.