CHIME is a novel radio telescope that has no moving parts. Originally conceived to map the most abundant element in the universe – hydrogen – over a good fraction of the observable universe, this unusual telescope is optimized to have a high “mapping speed”, which requires a large instantaneous field of view (~200 square degrees) and broad frequency coverage (400-800 MHz). The digitized signals collected by CHIME will be processed to form a 3-dimensional map of hydrogen density, which will be used to measure the expansion history of the universe. At the same time, these signals can be combed for fast, transient radio emission, making CHIME a unique telescope for discovering new “Fast Radio Bursts” and for monitoring many pulsars on a daily basis.
Telescope
CHIME consists of four adjacent 20m x 100m cylindrical reflectors oriented north-south. The focal axis of each cylinder is lined with 256 dual-polarization antennas, each of which receives radiation from a large swath of sky that nearly stretches from the northern horizon to the southern horizon. This gives CHIME its enormous field of view. The CHIME antennas are custom-designed to have good sensitivity from 400 to 800 MHz, in both linear polarizations. This gives CHIME its large frequency coverage. Signals from the antennas are amplified in two stages, using low-noise amplifiers developed by the cell-phone industry. This novel application of consumer technology makes CHIME affordable. The 2048 signals from these inputs (256 antennas x 2 polarizations x 4 cylinders) are fed to the F-Engine for the initial stage of digital processing.
F-Engine
The FPGA-based F-Engine is housed in two specially shielded 20-foot shipping containers located between the cylinders. The system digitizes each analog signal input 800 million times per second and converts each microsecond of data (2048 samples) into a 1024-element frequency spectrum between 400 and 800 MHz, with a frequency resolution of 0.39 MHz. It then organizes the data by frequency bin and sends it over optical fibre to the X-Engine for spatial correlation. The input data rate processed by the F-Engine is 13 terabits/sec!
X-Engine
The GPU-based X-Engine is housed in two specially-shielded 40-foot shipping containers located just east of the cylinders. Each container contains 128 compute nodes housed in 15 refrigerator-sized racks. Each node is responsible for processing 4 of the 1024 frequency bins by collecting its share of digitized signals from the F-Engine and forming the product of each telescope input with all other inputs, every millisecond. These “correlation matrices” are averaged over a few seconds of time and written to disk for later processing into sky maps. The total system requires 250 kilowatts of power to operate.
Fast Radio Burst Detector
To search for FRBs, CHIME will continuously scan 1024 separate points or “beams” on the sky 24/7. Each beam is sampled at 16,000 different frequencies and at a rate of 1000 times per second, corresponding to 130 billion bits of data per second to be sifted through in real time. The data are packaged in the X-engine and shipped via a high-speed network to the FRB backend search engine, which is housed in its own 40-foot shipping container under the CHIME telescope. The FRB search backend will consist of 128 compute nodes with over 2500 CPU cores and 32,000 GB of RAM. Each compute node will search eight individual beams for FRBs. Candidate FRBs are then passed to a second stage of processing which combines information from all 1024 beams to determine the location, distance and characteristics of the burst. Once an FRB event has been detected, an automatic alert will be sent, within seconds of the arrival of the burst, to the CHIME team and to the wider astrophysical community allowing for rapid follow up of the burst.
Pulsar Monitoring Instrument
CHIME has the sensitivity to monitor practically all known pulsars in the Northern sky. The pulsar monitoring instrument will receive ten sky-tracking beams produced by CHIME’s X-engine. CHIME will sample each of the ten sky-tracking beams at a resolution of 6.4 billion bits per second. The signals are sent from the X-engine to the pulsar timing monitor via a high-speed network. The timing monitor processes the signals in real time on ten dedicated compute nodes. Each node consists of a high-powered Intel processor that assembles the incoming data before passing it to an NVIDIA TitanX GPU for timing analysis. The results from the timing analysis are stored on a local multi-disk server before being shipped to Compute Canada. The data output from the pulsar timing monitor is approximately 500,000 gigabytes per year. The high-cadence of these observations will allow us to investigate the properties of neutron stars and ionized gas in the interstellar medium as well as verify the predictions of general relativity and search for gravitational waves.