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19 th International Symposium on Space Terahertz Technology P9-4 Characterization of ALMA Calibration Targets A. Murk 1 , A. Duric 1 , F. Patt 2 1 University of Bern, Switzerland 2 European Southern Observatory, Germany The Atacama Large Millimeter Array (ALMA) has the challenging goal to achieve an absolute calibration accuracy better than 5% at frequencies between 30 - 950 GHz. This poses stringent requirements on its two black-body calibration targets, which will be operated at ambient and elevated temperatures. The targets need to have a high emissivity, which is equivalent to a low value of integrated scattering into all possible directions. Even more critical is the coherent backscatter of the target, since it will lead to frequency dependent standing waves between the target and the receiver. In addition the thermal properties of the target need to be well understood to ensure that its surface brightness temperature corresponds to the reading of the thermometers in its body under changing environmental conditions in the receiver cabin. We present detailed active backscatter measurements between 30- 700GHz of different calibration target designs, which were obtained with a vector network analyzer. They were done at varying angles of incidence and polarizations in a quasi-optical setup with similar beam parameters as in the actual ALMA receivers. In addition, the integrated scattering was determined by passive radiometric measurements using a 90GHz and a 323GHz SIS receiver. In a third test setup the thermal gradients over the aperture of the elevated temperature target has been investigated with an IR camera. The initial ALMA prototype target under test consists of an array of sharp Aluminum pyramids covered with a thin casted layer of absorbing material. An alternative conical target design made out of an injection molded plastic absorber has been tested in parallel, as well as several off-the-shelf microwave absorbers. We will compare the test results of the different targets and discuss their advantages and disadvantages for the ALMA calibration system. 148
19 th International Symposium on Space Terahertz Technology P9-5 Near field beam and cross-polarization pattern measurements of ALMA band 8 cartridges Masato Naruse 1,2 , Mamoru Kamikura 1,2 , Yutaro Sekimoto 1,2 , Tetsuya Ito 2 , Masahiro Sugimoto 2 , Yoshizo Iizuka 2 , Naohisa Satou 2 , Kazuyoshi Kumagai 2 1 Department of Astronomy, School of Science, the University of Tokyo 2 Advanced Technology Center and ALMA-J project office National Astronomical Observatory of Japan, National Institutes of Sciences To develop sensitive astronomical receivers, high precision measurements of beam pattern, cross-polarization pattern and polarization alignment are required. We have developed a vector beam measurement system covering 385-500 GHz which measures the amplitude and phase at near field with wide dynamic range of 50dB. We measured corrugated horns, OMTs [1] (Orthomode Transducer), optics blocks at room temperature, also ALMA band 8 cartridges [2] cooled in a cartridge-test-cryostat. The amplitude and phase at near field are transformed to far field, and compared to physical optics calculations with Grasp 9 and CORRUG as shown in Figure1. The co-polar beams were symmetrical, consistent with the simulations at 385, 415, 442, 470, 500GHz, and the calculated beam efficiency at the sub-reflector was greater than 92%. Side lobe was less than -30dB. The peak of cross polarization relative to co polarization was less than -20dB. In addition, to evaluate accuracy of measurements, we studied both effects of standing wave and stability of the amplitude and phase. The error of the far field was found to be less than 0.2dB between 0 and -20dB range. Relative Power [dB] 0 -10 -20 -30 -40 -50 Symmetric Plane Simulation h-pol. v-pol. cross-pol. -60 -7.5 -5 -2.5 0 2.5 5 7.5 Angle [deg.] Figure 1: Beam pattern and cross polarization pattern of the optics block compared to simulation with Grasp9 at 500GHz. Reference [1] Kamikura et al. This conference [2] Sekimoto et al. This conference 149
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