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Quasi-Optical Verification of the Band 9 ALMA Front-End

The front-end optical design for band 9 (600 to 720GHz) of the Atacama Large Millimeter Array (ALMA) is now completed. A frequency independent design approach is used to couple radiation to the two orthogonal polarized mixed detectors from the large 12m ALMA Cassegrain telescope. As it is a heterodyne receiver, two local oscillator beam paths are integrated into the front-end optical system. Due to the large number of interferometer elements (64 antenna units) to be built, installed and maintained in the remote site of the Atacama Desert, reliability of the optical system should be ensured. A modular and compact optical design is also important. In addition a cheaper fabrication process is considered, at these more tolerant higher frequencies, by milling the mirror surfaces near the surface roughness limit. In this paper we verify the optical design and estimate system efficiency by means of experimental measurement and software simulation comparisons. Precision planar scans of near field beam patterns (amplitude and phase) have been measured. Experimental beam measurements were taken at the output of the mirror coupling system (telescope focal plane location) for both polarization paths and for both local oscillator beam guides. At the same measurement locations, software simulations of a highly accurate geometrical model of the mirror coupling system were predicted using the commercial package GRASP8 c ©. These comparisons at some fundamental locations along the beam paths, allow the assessment of the quasi-optical beam coupling system design. The local oscillator power budget analysis is carried out from results obtained using GRASP8 c ©. In the conclusion we summarize the current status and describe future analysis plans.