By/par Cherry Ng, Andre Renard, and Seth Siegel
(Cassiopeia – Autumn/l’automne 2017)
CHIME, the $16M new Canadian radio telescope, saw its “First Light” on September 7th and was celebrated at a ceremony in Penticton, BC involving Federal Minister of Science, Kirsty Duncan.
The telescope is designed to simultaneously tackle major astrophysics and cosmology topics, including studying the nature of dark energy by making unprecedented maps of the distant universe, studying pulsars, and determining the origin of the mysterious phenomenon of Fast Radio Bursts.
Now that all the major components are in place, the first data from the instrument is starting to be collected. “After years of work it’s fantastic to finally see the graphs showing real sky data coming through the system on all channels.” says Nolan Denman, a graduate student at the University of Toronto, who produced the first light plots after an overnight session collecting data during the transit of Cygnus A (a nearby galaxy that is bright at radio wavelengths and is a useful source for calibrating the instrument). This new instrument will serve as a powerful tool to explore a number of interesting cosmological and astrophysical topics.
CHIME first light. Cross-correlation of the signal measured by two CHIME antennas on different cylinders during the transit of Cygnus A. Top panel shows the magnitude, real, and imaginary component of the cross-correlation. Bottom panel shows the phase. Radio waves from Cyg A reach the two antennas at slightly different times. As the source moves across the sky, the delay between antennas changes. This results in the fringe pattern observed in the real and imaginary component, with the envelope tracing out the antenna beam pattern. Note that this is just one pair of antennas (or baseline) at a single frequency; in total CHIME measures the cross-correlation for over 2 million baselines at 1024 frequencies.
Science
CHIME will probe the fundamental nature of dark energy, the mysterious agent invoked to explain the accelerated expansion of the universe. To accomplish this, it will produce a three-dimensional map of the 21-cm emission from neutral hydrogen that covers the entire northern sky and spans redshifts 0.8 to 2.5. This will enable a measurement of Baryon Acoustic Oscillations (BAO) in the large scale distribution of neutral hydrogen — a relic that originates from sound waves propagating in the baryon-photon plasma of the early universe. The size of the BAO feature will be used as a standard ruler to measure the expansion history of the universe during the epoch when dark energy generated the transition from decelerated to accelerated expansion.
Two further key science projects are currently under commissioning and will soon be conducted simultaneously alongside the cosmology experiment. These include a blind survey for Fast Radio Bursts (FRBs), energetic single pulses of radio emission arriving in random directions from unknown sources well beyond our galaxy. FRB appears to be a new class of radio transient with unknown astrophysical origin and have drawn a lot of attention among the astrophysics community. “There are currently more theories in the literature than the number of known FRB sources” said graduate student Utkarsh Giri at the Perimeter Institute. So far progress in resolving the mystery has been limited by the low survey efficiency of traditional single dish telescopes. With its huge field of view and broad frequency coverage, CHIME is a nearly ideal instrument for finding and studying many of these bursts. Like what McGill postdoc Emmanuel Fonseca said, “It has taken almost 10 years to observe 25 FRBs with different telescopes; CHIME is expected to detect 25 FRBs within one week of operation.” Pinning down the FRB event rate will be crucial for determining the origin of FRBs and all eyes are on CHIME to revolutionize the field.
The other commensal project that CHIME will carry out is pulsar timing. CHIME will monitor the pulses from all known pulsars in the Northern hemisphere visible from Penticton, every day. Among other things, this information will aid in the search for gravitational waves – travelling ripples in space-time – passing through our galaxy.
Instrumentation
CHIME is a transit telescope that surveys the northern half of the sky every day as the earth rotates. It is composed of four cylindrical reflecting surfaces that resemble snow-board half-pipes and have a total collecting area equivalent to five hockey rinks (8,000 square meters). It records the information from all the radio waves falling across its surface with over a thousand antennas. “These cloverleaf-shaped antennas are compact and have an excellent broadband coverage. They are made out of conventional low loss circuit boards and can be mass produced economically.”, said Meiling Deng, a graduate student at UBC who has led the design of these antennas.
CHIME at night. The telescope consists of four parabolic cylinders that are 20 m wide and 100 m long with a focal length of 5 m. The telescope has no moving parts, instead relying on the earth’s rotation to move the sky across its field of view. The focal line of each cylinder is populated with 256 dual-polarization antennas that feed into a custom 2048-input radio correlator.
The CHIME correlator is a sophisticated digital network and signal processing instrument that converts the massive amount of information that is contained in the radio waves incident on the cylinders into an image of the overhead sky. Measured in number of analog inputs (N=2048) squared times bandwidth (400 MHz), the CHIME correlator is the largest radio correlator in the world — and it was built for a comparatively low price. The correlator employs 128 field programmable gate arrays (FPGAs) to digitize the analog radio signals collected by the antennas and channelize their full bandwidth into 1024 narrow frequency bins. The FPGAs are interconnected through custom, full-mesh backplanes that enable a massive reorganization of 6.6 Terabit/second of data into the format required to compute the N2 correlation matrix of the signals measured by the antennas. The data is then transmitted over more than a thousand fiber optic cables to a supercomputer.
Using the data from the FPGAs, the CHIME supercomputer correlates the inputs into “visibility” matrices used to created detailed sky maps, and performs real-time beamforming which is used for the FRB and pulsar applications. This requires a huge amount of computing power, which was made possible thanks to the existence of low cost Graphics Processing Units (GPUs) from AMD, which were developed primarily for computer games, but are increasingly leveraged by scientists to perform complex calculations. In total CHIME has 1024 high end GPUs, spread out over 256 servers. Together they are able to perform over 7 quadrillion (a million billion) operations per second.
Undergraduate and graduate students played a key role in the assembly, testing, and on-site installation of the instrument. “My favourite part of working on CHIME has been interacting with all the wonderful people involved in this project. The team’s enthusiasm and devotion is contagious” said Emilie Storer, an undergraduate student at McGill who participated in the testing of FPGA motherboards.
People at work. (Top left) Postdoc Emmanuel Fonseca and summer intern Tristan Simmons raising feeds onto the focal line; (top right) Postdoc Cherry Ng connecting some of the 2048 50m-long coaxial cables; (bottom left) Graduate student Juan Mena Parra installing FPGA motherboards; (bottom right) Graduate student Nolan Denman assembling GPUs in the X-engine.
Future of CHIME
CHIME is now in its commissioning phase, in preparation for science operations. This new telescope will bring Canada to the forefront of an emerging important and technically challenging domain of radio astronomy. More information on CHIME can be found here.