Laser scanner and terrestrial surveying applied to gravitational deformation monitoring of large VLBI telescopes’ primary reflector

Laser scanning surveys were performed on the primary mirror of the VLBI (Very Long Baseline Interferometry) telescopes situated at Medicina and Noto observatories, with the specific purpose of investigating (i) gravity deformation patterns of the radio telescopes’ primary reflector and (ii) the magnitude and relative variations of focal length as the antennas are steered in elevation. Both instruments have AZ-EL mounts and have 32 m parabolic mirrors which were surveyed in steps of 15 degrees spanning the 90-15 degrees elevation range. The scanning sessions were performed from two standpoints using a GS200 Trimble-Mensi; the sampling interval was set to 2 cm at a distance of 15 m. The complete surface of the main reflector at every elevation position was obtained by merging the two separate point clouds acquired from the two standpoints; each elevation is represented by at least 1.3 millions points. The merged clouds were compared for determining relative deformation patterns and magnitude. As the elevation decreases from 90 deg to 15 deg, the edges of the primary mirror of both telescopes fold in by a couple of cm. A least squares adjustment was applied to point clouds corresponding to different elevations aimed at estimating the parameters of the rotational paraboloids that better fit the experimental data: this led to estimate the focal length variations induced by the structure deformative behaviour. The focal lengths of the best fit surfaces were compared; their largest variation is found to be 2.5 cm at Medicina, between the 90 deg and the 15 deg positions. The clouds were also used to attempt a direct computation of the incoming radio signal’s path length variation due to primary reflectors’ deformations. Finally, two Leica total stations, a TDA5005 and a TC2003, were used to perform a survey of the local ground control network and of some selected targets placed on the edge of the dish. The comparison of the distances determined with the two terrestrial surveying methods (laser scanner vs. triangulation and trilateration) highlights a statistically significant scale factor of about 1.0005 ± 0.0002, being the laser estimates smaller than those obtained with total stations. This study proves that laser scanners can be efficiently used to determine gravitational influences on large VLBI telescopes’ primary reflectors: deformation patterns are clearly and reliably depicted, focal length and incoming radio signal path variations are precisely quantified.