Program and Abstract Book - SRON

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19 th International Symposium on Space Terahertz Technology A novel heterodyne interferometer for millimetre and submillimetre astronomy 10-7 Paul K. Grimes, Christian M. Holler, Michael E. Jones, Oliver G. King, Jamie Leech, Angela C. Taylor, Ghassan Yassin Astrophysics, Oxford University, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK. pxg@astro.ox.ac.uk We describe a novel heterodyne interferometer currently under construction at Oxford, funded by the Royal Society's Paul Instrument Fund to investigate the Sunyaev-Zeldovich (S-Z) effect at 230 GHz. An important feature of this instrument is that it employs several pioneering technologies of importance to mm and sub-mm wave heterodyne interferometry. The instrument is a 190-260 GHz single-baseline interferometer employing SIS mixers, very wide bandwidth low-noise IF amplifiers, and a wide-band analogue correlator. The unique feature of this instrument is its exceptionally high brightness sensitivity on large angular scales combined with moderate frequency resolution. This novel, portable instrument may be considered as a first demonstration for a future large format international S-Z project. We expect the instrument to be commissioned in 2010 and astronomical observations will be carried out at the Chajnantor Observatory, Chile, close to the core ALMA site. The instrument is a single baseline tracking interferometer with 0.4m diameter offset parabolic primary mirrors and a 0.5m baseline. The antennas feed two SIS mixers contained in a single closed cycle cryostat and driven by photonic local oscillator using a single laser source. The interferometer will be phase switched by introducing phase shifts in the LO signals fed to each mixer. The SIS mixers are single-chip balanced mixers using back-to-back finline geometry. The mixers have been designed for extremely wide IF band, up to 2-20 GHz, and feed high performance cryogenic HEMT amplifiers developed by Sander Weinreb. Some major RF modifications need to be done to the standard finline mixer design to allow for very wide IF band operation, including the use of microstrip and capacitor RF bandpass filters between the finline feed and the SIS junction, and the use of low parasitic microstrip RF chokes, tuning and IF matching circuits. We will demonstrate the extremely high brightness sensitivity of this technology, by measuring the zerocrossing frequency of the Sunyaev-Zeldovich (S-Z) effect in a few of the brightest galaxy clusters. The S-Z effect is the distortion of the spectrum of the CMB due to its passage through a cluster of galaxies; the effect is negative at low frequencies and positive at high frequencies, with the null close to 220 GHz. The exact null frequency varies depending on the temperature of the cluster gas and is approximated by (217+0.45(T/keV) GHz where T is the cluster gas temperature. Our instrument will initially observe two 10-GHz bands spaced 8 GHz either side of 217 GHz, each split into eight sub-bands. We should be able to measure the null frequency for a few bright clusters with a few nights of observing each. This unique measurement will be a convincing demonstration of the capabilities of the new technology. As well as observing clusters, to verify the performance of the instrument we will carry out extensive observations of bright calibrator sources, and of signals from the atmosphere. The wide spacing and frequency resolution of the bands, plus the fact that we will measure both the total power from each antenna and the correlated power between them, will be ideal for characterising the contribution to observed fluctuations from wet and dry components of the atmosphere. A future ground-based instrument based on these same principles, with more baselines and hence greater sensitivity, could obtain cluster temperatures faster than the best current X-ray satellite observatories, at a tiny fraction of the cost. Other applications of the same techniques could include highly sensitive pseudo-correlation polarimetry (for example for a future CMB satellite observatory), or spectroscopy of atmospheric emission lines in limb-sounding applications. All of the technology developed for this instrument has applications for instrumentation throughout the mm and sub-mm wave region and can easily be extended to the THz spectrum. 88
19 th International Symposium on Space Terahertz Technology 10-8 A 600 GHz Imaging Radar for Contraband Detection Goutam Chattopadhyay, Ken B. Cooper, Robert Dengler, Tomas E. Bryllert, Erich Schlecht, Anders Skalare, Imran Mehdi, and Peter H. Siegel Jet Propulsion Laboratory, California Institute of Technology 4800 Oak Grove Drive, Pasadena, CA 91109, USA. ABSTRACT We have developed and demonstrated 3D imaging for contraband detection using a submillimeter-wave frequency modulated continuous wave (FMCW) radar with a fast microwave chirp and phase coherent detection. The technique provides an important advantage over more traditional CW RF imaging because of the ability to time-gate the return signals. This can be used to discern specific objects by greatly reducing clutter from unwanted targets or specular reflections. The prototype system uses a 590 GHz RF signal with a 28.8 GHz chirp producing a 1.2 MHz/usec sweep yielding a range resolution of approximately 1 cm or less. Lateral resolution on the scene is set by a 40cm diameter reflector producing approximately 0.5cm at 4m distance. The RF transmit power (generated by a W-band power amplifier and an in-house frequency multiplier chain) is less than 0.5 mW but when coupled with a heterodyne downconverter (in-house developed fundamental balanced Schottky diode mixer, 4000K DSB noise temperature) yields a measured dynamic range of more than 70dB. A more compact and higher resolution system is currently under construction and will employ a commercial 30 GHz microwave chirp for sub-cm range resolution and a higher power transmitter, dramatically increasing resolution and dynamic range. This paper will describe the design and implementation of the radar with some results we have obtained on test targets at 4 m and 25 m. The research described herein was carried out at the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA, under contract with National Aeronautics and Space Administration. Fig: A 600 GHz radar image of a concealed gun under a shirt at 4 m stand-off distance. The picture on the left shows the optical image, the figure in the middle shows radar data range-gated at the front surface, and the image on the right shows the radar data range-gated at the back surface. 89
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Program and Abstract Book - SRON

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