This paper describes the design, implementation, and verification of a test-bed for determining the noise temperature of radio antennas operating between 400-800 MHz. The requirements for this test-bed were driven by the HIRAX experiment, which uses antennas with embedded amplification, making system noise characterization difficult in the laboratory. The test-bed consists of two large cylindrical cavities, each containing radio-frequency (RF) absorber held at different temperatures (300K and 77 K), allowing a measurement of system noise temperature through the well-known ‘Y-factor’ method. The apparatus has been constructed at Yale, and over the course of the past year has undergone detailed verification measurements. To date, three preliminary noise temperature measurement sets have been conducted using the system, putting us on track to make the first noise temperature measurements of the HIRAX feed and perform the first analysis of feed repeatability.
The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is a planned interferometric radio telescope array that will ultimately consist of 1024 close packed 6 m dishes that will be deployed at the SKA South Africa site. HIRAX will survey the majority of the southern sky to measure baryon acoustic oscillations (BAO) using the 21 cm hyperfine transition of neutral hydrogen. It will operate between 400-800 MHz with 391 kHz resolution, corresponding to a redshift range of 0:8 < z < 2:5 and a minimum Δz/z of ~0.003 (frequency resolution 500 < R < 1000). One of the primary science goals of HIRAX is to constrain the dark energy equation of state by measuring the BAO scale as a function of redshift over a cosmologically significant range. Achieving this goal places stringent requirements on the mechanical and optical design of the HIRAX instrument which are described in this paper. This includes the simulations used to optimize the mechanical and electromagnetic characteristics of the instrument, including the dish focal ratio, receiver support mechanism, and instrument cabling. As a result of these simulations, the dish focal ratio has been reduced to 0.23 to reduce inter-dish crosstalk, the feed support mechanism has been redesigned as a wide (35 cm diam.) central column, and the feed design has been modified to allow the cabling for the receiver to pass directly along the symmetry axis of the feed and dish in order to eliminate beam asymmetries and reduce sidelobe amplitudes. The beams from these full-instrument simulations are also used in an astrophysical m-mode analysis pipeline which is used to evaluate cosmological constraints and determine potential systematic contamination due to physical non-redundancies of the array elements. This end-to-end simulation pipeline was used to inform the dish manufacturing and assembly specifications which will guide the production and construction of the first-stage HIRAX 256-element array.
High precision calibration is essential for the new generation of radio interferometers looking for Baryon Acoustic Oscillation signatures in neutral hydrogen emissions which is a signal buried in foregrounds. Yet, differences in instrument design and non-redundancies in the subsystems consisting such instruments can pose great challenges to proper calibration. For instance, Canadian Hydrogen Intensity Mapping Experiment (CHIME) offers a large instantaneous field of view with fast mapping speed with a sensitivity equivalent to a single dish in terms of total collecting area. However, the calibration of its 1000+ individual receivers poses major obstacle to harness the full capability of the new technology. In future instruments, it is planned to use redundancy at the level required for science experiments (typically below 1%). Although, this idea is against the traditional knowledge of having the radio receivers accurate to 1/20th of the observed wavelength, we plan to meet this goal by solving the redundancy issue in the front-end subsystems using precise metrology and alignment methods. To achieve this goal, we work on a 2-element interferometer called Deep Dish Development Array (D3A), with a targeted precision of 1/1000th of a wavelength at 300 MHz, located at Dominion Radio Astrophysical Observatory, located in Penticton, BC, Canada. The D3A will serve as test bed for dish prototyping, composite dish repeatability, antennae back ends which can be scaled to larger arrays without sacrificing the precision. The two dishes are made out of fiber glass composite and measured in the shop condition and in the field after fabrication. The relative pointing accuracy was obtained as 0.02°. The elevation axes of the dishes were placed in East-West direction within 0.04° of error. The surface RMS error was obtained as 0.389 mm which meets the 1/1000th of observation wavelength. RMS error due to temperature variation and assembly error on the dishes were obtained at 1 mm. This article presents the metrology principles applied to obtain the results and challenges. The tests performed in D3A will be implemented in Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) and the Canadian Hydrogen Observatory and Radio Transient Detector (CHORD).