Correcting Gravitational Deformation at the Tianma Radio Telescope

The primary reflector of the Tianma Radio Telescope (TMRT) distorts due to gravity, which dramatically reduces the aperture efficiency of high-frequency observations. A technique known as out-of-focus holography (OOF) has been developed to measure gravitational deformation. However, the TMRT has a shaped dual-reflector optical system, so the OOF technique cannot be used directly. An extended OOF (e-OOF) technique that can be used for a shaped telescope is proposed. A new calculation method is developed to calculate the extra phase and illumination. A new measurement strategy is proposed that uses only one feed, reduces the length of the scan pattern, and allows the telescope to scan smoothly at low speed. At the TMRT, the time required for each measurement is under 20 min, the achieved accuracy is approximately 50 mu m, and the repeatability is sufficient. We have acquired a model for the gravitational deformation of the TMRT. After applying the model, there is a 150%-400% improvement in the aperture efficiency at low and high elevations. The model flattens the gain curve between 15 degrees-80 degrees elevations with an aperture efficiency of approximately 52%. The final weighted root-mean-square error is approximately 270 mu m. The e-OOF technique reduces the constraints on the telescopes.