C-BASS

Abstract:

Context. The use of large-format focal plane imaging arrays employing multiple feed horns is becoming increasingly important for the next generation of single dish sub-mm telescopes and cosmology experiments. Such receivers are being commissioned on both general purpose, common user telescopes and telescopes specifically designed for mapping intensity and polarisation anisotropies in the cosmic microwave background (CMB). Telescopes are currently being constructed to map the CMB polarisation that employ hundreds of feeds and the cost of manufacturing these feeds has become a significant fraction of the total cost of the telescope. Aims. We have developed and manufactured low-cost easy-to-machine smooth-walled horns that have a performance comparable to the more traditional corrugated feed horns that are often used in focal plane arrays. Our horns are much easier to fabricate than corrugated horns enabling the rapid construction of arrays with a large number of horns at a very low cost. Methods. Our smooth walled horns use multiple changes in flare angle to excite higher order waveguide modes. They are designed using a genetic algorithm to optimise the positions and magnitudes of these flare angle discontinuities. We have developed a fully parallelised software suite for the optimisation of these horns. We have manufactured prototype horns by traditional electroforming and also by a new direct drilling technique and we have measured their beam patterns using a far-field antenna test range at 230 GHz. Results. We present simulated and measured far-field beam patterns for one of our horn designs. They exhibit low sidelobe levels, good beam circularity and low cross-polarisation levels over a fractional bandwidth of 20%. These results offer experimental confirmation of our design technique, allowing us to proceed confidently in the optimisation of horns with a wider operational bandwidth. The results also show that the new manufacturing technique using drilling is successful, enabling the fabrication of large format arrays by repeatedly drilling into a single aluminium plate. This will enable the construction of focal plane arrays at a very low cost per horn. Conclusions. We have developed a new type of high performance feed horn that is fast and easy to fabricate. Having demonstrated the efficacy of our horn designs experimentally, we are building and testing a prototype focal plane array of 37 hexagonally close packed horns. This prototype array will be an important step towards building a complete CMB mapping receiver using these feed horns. © 2011 ESO.

Abstract:

The design and preliminary results from testing the performance of a 230GHz unilateral finline SIS mixer, fabricated on a silicon substrate are presented. The mixer will be employed in the single baseline heterodyne interferometer-GUBBINS (220-GHz Ultra-BroadBand INterferometer for S-Z), which aims to measure the null frequency in the Sunyaev-Zel'dovich spectrum at ~227GHz. The mixer is operated in the frequency range of 180GHz~280GHz with an IF bandwidth of 1-13 GHz. An important feature of this mixer is its ultra-wide IF bandwidth, so as to achieve very high brightness sensitivity in the observation of the galaxy clusters in the faint cosmic microwave background radiation. This SIS mixer described is deposited on an 80um silicon substrate with the dielectric constant of 11.9. The incoming RF signal from the feed horn is coupled via the waveguide mode to a unilateral finline, which is tapered to a 2.5um slotline. The signal is coupled from the slotline to the microstrip with the aid of two radial stubs. The employment of a silicon substrates gives a slotline impedance of 36Ω, which is ideal for coupling to the microstrip, where the SIS junction is fabricated. Also, the employment of silicon allows the generation of the trenches around the device so that individual devices can be separated from the batch without dicing. A 2-stage notch was fabricated at the front end of the substrate to match the loaded waveguide to the free space. The capacitance of the SIS junction was tuned out using a wide band circuit consisting of two stubs and a 3-stage Chebyshev transformer. This is followed by a 3-stage RF choke to prevent RF signal from leaking into the IF output port. The mixer chip was fabricated at KOSMA, Cologne using Nb-AlOx-Nb tunnel junction defined with E-beam lithography. The device used in this experiment had a normal resistance of 20Ω, a current density of 14KA/cm2, an area of 1μm2 and an intrinsic capacitance of 75fF. Measurement of the mixer performance was done using a local oscillator which gives sufficient power to pump the mixer in the frequency range 200GHz~260GHz using 12μm beam splitter. Measurement of the mixer sensitivity was done using the hot/cold Y-factor method. Uncorrected noise temperature of 75K was obtained at 208GHz, with 30 K contributed by mismatch to the IF system. Future characterization and improvements of the IF system and the measurement with better-tuned devices will also be reported.

Abstract:

An anomalous radio continuum component at cm-wavelengths has been observed in various sources, including dark clouds. This continuum component represents a new property of the interstellar medium. In this work, we focus on one particular dark cloud, the bright reflection nebula M78. The main goal of this work is to investigate the cm-wave continuum emission in a prominent molecular cloud, nearby and with complementary observational data. We acquired Cosmic Background Imager (CBI) visibility data of M78 at 31 GHz with an angular resolution of ∼ 5.8arcmin, and CBI2 data at an angular resolution of ∼ 4.2arcmin. A morphological analysis was undertaken to search for possible correlations with templates that trace different emission mechanisms. Using data from Wilkinson Microwave Anisotropy Probe and the Rhodes/HartRAO 2326-MHz survey, we constructed the spectral energy distribution (SED) of M78 in a 45-arcmin circular aperture. We used results from the literature to constrain the physical conditions and the stellar content. The 5-31 GHz spectral index in flux density (α= 1.89 ± 0.15) is significantly different from optically thin free-free values. We also find closer morphological agreement with IR dust tracers than with free-free sources. Dust-correlated cm-wave emission that is not due to free-free is significant at small scales (CBI resolutions). However, a free-free background dominates at cm-wavelengths on large scales (∼ 1°). We correct for this uniform background by differencing against a set of reference fields. The differenced SED of M78 shows excess emission at 10-70 GHz over free-free and a modified blackbody, at 3.4σ. The excess is matched by the spinning dust model from Draine & Lazarian. © 2010 Universidad de Chile. Journal compilation © 2010 RAS.