A gravitational telescope deformation model for geodetic VLBI

We have measured the geometric deformations of the Onsala 20m VLBI telescope utilizing a combination of laser scanner, laser tracker, and electronic distance meters. The data put geometric constraints on the electromagnetic raypath variations inside the telescope. The results show that the propagated distance of the electromagnetic signal inside the telescope differs from the telescope's focal length variation, and that the deformations alias as a vertical or tropospheric component. We find that for geodetic purposes, structural deformations of the telescope are more important than optic properties, and that for geodetic modelling the variations in raypath centroid rather than focal length should be used. All variations that have been identified as significant in previous studies can be quantified. We derived coefficients to model the gravitational deformation effect on the path length and provide uncertainty intervals for this model. The path length variation due to gravitational deformation of the Onsala 20m telescope is in the range of 7-11mm, comparing elevation 0 degrees and 90 degrees, and can be modelled with an uncertainty of 0.3 mm.