CHIME Progress Report

Submitted by Mark Halpern and Mateus Fandino
(Cassiopeia – Autumn/Automne 2015)

Construction of the structure for the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope was completed in August 2015 and the receiver electronics and custom correlator will be built and installed on site over the coming winter. At 8,000 m2, CHIME is now the largest telescope in continental North America with 2% more collecting area than the Green Bank telescope.

CHIME is a new radio interferometer located at the Dominion Radio Astrophysical Observatory (DRAO). It will map the 21 cm emission of neutral Hydrogen from redshifts 0.8 to 2.5 culminating in the largest-volume survey of the distribution of matter in the Universe ever made. This redshift range encompasses the critical period when Dark Energy becomes comparable to the energy density of matter and drives the late accelerated expansion of the Universe. The survey will allow the team to measure the expansion rate of the Universe through this critical epoch, thus helping to constrain the Dark Energy equation of state.

CHIME Cosmology

To map the history of the expansion rate of the Universe, the experiment will measure the relic of Baryon Acoustic Oscillations (BAO), spherical shells of matter over-density in which galaxies and gas are more likely to be found today. The radius of these shells was established by conditions in the early Universe (up to ~400,000 years after the Big Bang), and remains constant in co-moving coordinates afterwards. What this means is that, for the past 13 billion years, this characteristic distance scale evolved solely due to the expansion of the Universe, and hence provides a standard ruler to measure the expansion rate.

The BAO scale has been measured before using galaxy surveys to map the distribution of matter. This is a long and difficult process that requires resolving each individual galaxy and has a limited redshift range. CHIME will map the distribution of matter using the 21 cm emission of intergalactic hydrogen at a resolution much lower than that of individual galaxies, but high enough to measure the BAO scale (~150 Mpc today). This technique, known as Hydrogen Intensity (HI) Mapping, is much faster and will allow for a much larger survey volume than has ever been observed. It avoids reliance on the non-linear physics of galaxy formation since galaxies are not resolved and counted, and is well suited to measuring structure on large physical scales.

Ancillary Science

In addition to measuring the expansion rate of the Universe, the design and operation of CHIME make it well suited to pursue a variety of very important ancillary science objectives. The instrument will produce an independent map of the intensity and polarization of the Milky Way visible from the northern hemisphere every half MHz across the CHIME band. CHIME can time the pulse arrival times for a very large number of known pulsars, which pass through the beam every day, and a program is under way to perform these measurements with the CHIME Pathfinder. The full CHIME telescope will be a powerful instrument to detect Fast Radio Bursts and a separate back-end processor to perform the high-cadence de-dispersion this requires has been funded by the CFI. CHIME will act as a scientific and technical pathfinder for the SKA, pioneering the measurement of very low-surface brightness phenomena and developing key correlator hardware.

The CHIME Instrument

CHIME is a radio interferometer composed of four parabolic cylinders, 100 m long and 20 m wide each, oriented in the North-South direction. The focal line will be populated by dual polarization feeds whose main beams resemble thin cigar-shaped stripes ~100° long by ~2° wide. It has no moving parts and scans half the sky every day as the Earth rotates. Frequency and spatial North-South resolution are achieved by Fourier-transforming and correlating the signals from all feeds. The instrument operates over the frequency band from 400 to 800 MHz, corresponding to a redshift of 2.5 to 0.8 for 21 cm radiation. The signals are brought to a custom FX correlator, which performs a Fourier transform in time/frequency with 500 kHz resolution and then at each frequency performs a spatial correlation.

Figure 1 - The telescope structure for CHIME. The telescope consists of four parabolic cylinders 20 m wide and 100 m long with a focal length of 5 m. 256 dual-polarization antennas will be placed along the focal line of each cylinder and the signals are brought to a custom 2048-input correlator. The instrument has no moving parts.

Figure 1 – The telescope structure for CHIME. The telescope consists of four parabolic cylinders 20 m wide and 100 m long with a focal length of 5 m. 256 dual-polarization antennas will be placed along the focal line of each cylinder and the signals are brought to a custom 2048-input correlator. The instrument has no moving parts.

Figure 2 - The CHIME telescope structure under construction in July 2015. The fourth cylinder (on the left) was being assembled. The full structure is now completed and the receiver electronics and custom correlator will be built and installed on site over the coming winter.

Figure 2 – The CHIME telescope structure under construction in July 2015. The fourth cylinder (on the left) was being assembled. The full structure is now completed and the receiver electronics and custom correlator will be built and installed on site over the coming winter.

The CHIME Pathfinder

The CHIME Pathfinder is a smaller-scale prototype of full CHIME that has informed its design, will shape the analysis strategy and will produce sensitive maps of neutral hydrogen and of the Galaxy. The Pathfinder is composed of two cylinders 37 m long by 20 m wide whose focal lines are populated with 64 dual-polarization feeds each, totalling 256 analog signal channels.

The analog signal chain is composed of low-noise amplifiers and anti-aliasing filters made affordable by the use of components developed for the cell phone industry. After digitization, the 256 channels are Fourier-transformed and correlated by a purpose-built hybrid FPGA/GPU FX correlator based on consumer-grade GPUs and custom optimized data handling and processing software. With 32,896 baselines and 400 MHz bandwidth, the Pathfinder correlator is amongst the largest in the World and is already fully operational. It has a larger signal throughput than all North American telephone conversations combined.

Status

Construction of the structure of the full instrument was completed in August and its many analog and digital components are being finalized. Meanwhile, the Pathfinder is now fully assembled and its first stable science run will happen during the winter, initiating a period of signal integration necessary to extract the 21 cm signal.

CHIME is a collaboration between the University of British Columbia, the University of Toronto, McGill University and the Dominion Radio Astrophysical Observatory.

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