Phase-Corrected Near-Field Measurements of the TELIS Telescope at 637 GHz

Results are presented for the telescope optics verification of the TELIS instrument which is developed and constructed at the Deutsches Zentrum fU¿r Luft- und Raumfahrt (DLR), Germany. TELIS is a balloon-borne limb-sounding radiometer which measures pressure-broadened emission lines of various species of the earth's atmosphere. Due to the strong brightness gradient in the limb-viewing geometry the antenna beam of the instrument needs to be well-defined in order to ensure proper operation. Near-field measurements made at 637 GHz of the TELIS telescope were used to derive the far-field pattern. Error compensation included correction of the phase error of the near-field data due to cable flexing. The phase error due to cable flexing was measured using a technique based on heterodyne reception of the signal reflected at the end of the cable. The results are compared to an optics model based on physical optics. Discrepancies between measurements and the model are resolved by misaligning the mirrors in the optics model while minimizing the deviations from theory and experiment.