Design and Verification of ALMA Band 9 Receiver Optics

The Atacama Large Millimeter Array (ALMA) is an interferometer consisting of 64 antennae of 12 m diameter. It will be placed in Chile at a high altitude plateau (5000 m) with exceptionally good atmospheric conditions for astronomical observations at sub-mm wavelengths. The ALMA frequency coverage (30 GHz - 950 GHz) is divided into ten bands corresponding to the atmospheric transparency windows. The receiver for each band is mounted as a separate module in the ALMA front-end cryostat, which provides 4K, 12K, and 90K temperature levels. We would like to report on the design of the ALMA band 9 (602-720 GHz) receiver module. A detailed optics layout for coupling between the telescope secondary and the SIS mixer feed horns will be presented. The local oscillator insertion optics will be described allowing for mounting a local oscillator module inside the receiver module at the 90K level. To verify our intended production technique (CNC machining without any need for adjustment), a two-minor prototype, representative of the signal path, has been produced. The output beam of this prototype was measured in phase-and-amplitude sensitive set-up. A superlattice device, used as a subharmonically pumped mixer, was mounted instead of the SIS device into the mixer housing. This allowed the evaluation of the optics (including the influence of machining tolerances) to be performed at room temperature. The receiver beam was measured in the near field at several signal frequencies using a Gunn-multiplier chain to generate the probe signal. A homodyne detection technique was used to retrieve both phase and amplitude, achieving a 60 dB signal-to-noise ratio. The measured patterns are then compared with theoretical predictions. Far-field beam patterns are computed from the measured near-field data allowing to predict the illumination of the telescope secondary mirror. Finally, the data was also used to verify if our CNC production method is suitable for production of mirrors with high enough quality for sub-mm wavelengths