1,721,086 research outputs found
Design and Verification of ALMA Band 9 Receiver Optics
No full textThe 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Design and Verification of ALMA Band 9 Receiver Optics
No full textThe 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
Satellite antenna measurement at 322 GHz using a computer-generated hologram as the focusing element
No full textComputer-generated holograms can be applied in space terahertz technology; they are particularly suitable as focusing elements in compact test ranges, for example, in testing of high-gain antennas and measuring radar cross section (RCS) of scaled models. Radio holograms have also been demonstrated in other millimeter- and submillimeter-wave beam-shaping applications, such as the creation of non-diffracting mm-wave Bessel beams, mm-wave vortices, and custom-designed beam patterns. During summer 2003 we built a temporary measuring facility at the High Voltage Institute of the Helsinki University of Technology to test the ADMIRALS antenna built by EADS Astrium. The measurement results obtained at 322 GHz are so promising that we are currently proposing a hologram CATR test for the Planck telescope at 500 GHz
Satellite antenna measurement at 322 GHz using a computer-generated hologram as the focusing element
No full textComputer-generated holograms can be applied in space terahertz technology; they are particularly suitable as focusing elements in compact test ranges, for example, in testing of high-gain antennas and measuring radar cross section (RCS) of scaled models. Radio holograms have also been demonstrated in other millimeter- and submillimeter-wave beam-shaping applications, such as the creation of non-diffracting mm-wave Bessel beams, mm-wave vortices, and custom-designed beam patterns. During summer 2003 we built a temporary measuring facility at the High Voltage Institute of the Helsinki University of Technology to test the ADMIRALS antenna built by EADS Astrium. The measurement results obtained at 322 GHz are so promising that we are currently proposing a hologram CATR test for the Planck telescope at 500 GHz
Design and Verification of ALMA Band 9 Receiver Optics
No full textThe 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
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