Measuring and analyzing thermal deformations of the primary reflector of the Tianma radio telescope

摘要

The primary reflector of the Tianma Radio Telescope (TMRT) distorts due to the varying thermal conditions, which dramatically reduces the aperture efficiency of Q-band observations. To evaluate and overcome the thermal effects, a thermal deformations measurement system has been established based on the extended Out-of-Focus holography (e-OOF). The thermal deformations can be measured in approximately 20 min with an illumination-weighted surface root mean square (RMS) accuracy of approximately 50 mu m. We have measured the thermal deformations when the backup and front structure were heated by the sun respectively, and used the active surface system to correct the thermal deformations immediately to confirm the measurements. The thermal deformations when the backup structure is heated are larger than those when the front structure is heated. The values of half power beam width (HPBW) are related to the illumination-weighted surface RMS, and can be used to check the thermal deformations. When the backup structure is heated, the aperture efficiencies can remain above 90% of the maximum efficiency at 40 GHz for approximately two hours after one adjustment. While the front structure is heated, the aperture efficiencies can remain above 90% of the maximum efficiency at 40 GHz, and above 95% after one adjustment in approximately three hours.