The Canadian Hydrogen Intensity Mapping Experiment (CHIME) has discovered the second repeated fast radio burst recently. However, its structure is simple apparently. I wonder why we did not build a telescope before. What is the challenge? I just know that its data volume is large. It is difficult to update the telescope?

There are 4 cylinders and 1024 antennas. What determines the distribution of the 1024 antennas? I mean why is it not just one longer cylinder?


There are many radiotelescopes that consist of large arrays of antennas. We usually think of an array individual dishes and VLA and ALMA are two of the most commonly recognized.

But if you want to keep an eye on a huge patch of sky in order to try to catch a very rare event, you want a wide field of view more than ultrahigh spatial resolution. And if what you are looking for is super-hot science, you want to build your device quickly which means a modest budget.

So wide field, lower resolution means a smaller baseline, and CHIME is of order 100 meters on a side. but building 1000 small say 3 meter dishes or 1000's of log-periodic dipole arrays and trying to steer them all would be a mess.

So instead CHIME built four, simple, parabolic cylinders and told all those antennas to be nice to each other and share!

Each antenna picks up signals from a huge area of the cylinder. If you include all directions it's like 25x25 meter section. The antennas are spaced only about a wavelength or so apart, but that's plenty far to do interferometry with the signals in a computer.

The ArXiv preprint The CHIME Fast Radio Burst Project: System Overview goes into great detail about the design and optimization of the antenna, considering what it is supposed to do and the necessity to make it straightforward to build an have a reasonable budget.

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Figure 1. Photograph of the CHIME telescope on 15 September, 2016, looking North-West. The shipping containers housing the X-Engine and CHIME/FRB backend can be seen adjacent to the right-most cylinder. The receiver huts containing the F-Engine are beneath the reflectors and cannot be seen here. The DRAO Synthesis Telescope (Kothes et al. 2010) can be seen in the background. See Table 1 for detailed properties of CHIME.

The Field of View in the North-South direction (the axis of the parabolic cylinders) is about 110 degrees, but in the East-West direction it's of course very narrow, only 2.5 to 1.3 degrees.

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I've borrowed the following text, images and links from my question How does Canada's radiotelescope CHIME manage polarization-dependent apodization of the fast f/0.25 optics?

Here's a picture of some of the antennas, looking down into the trough:

below x2: from The cloverleaf antenna: A compact wide-bandwidth dual-polarization feed for CHIME

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Fig. 1. At left, the shape of each petal consists of two perpendicular straight lines, two 45 degree circular arcs with radius R and one half an ellipse. W is the major axis of the ellipse and L is the length from the intersection of the straight sides to the outer edge of the ellipse. The shape is illustrated here for the adopted values of gap, R, L, and W. Each of the four tabs shown at the centre is connected to one side of a vertical microstrip transmission line and in each case the full width of the adjacent petal is connected to the other lead. At right, CST simulated currents for one linear polarization at 600MHz are shown. Note the small asymmetry in the curent distribution near the centre becasue of the tab geometry.

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Fig. 3. A linear array of eight cloverleaf antennas installed at the focal line of the CHIME Pathfinder at the Dominion Radio Astrophysical Observatory in Penticton, BC, Canada. The picture is taken through the wire mesh reflective surface (mesh spacing 19 mm) illustrating a photonsview of the antennas and ground plane. Notice that each feed has an image-feed in the ground plane, 1/2 λ away at the passband centre frequency. Notice also the four slots cut to remove dielectric material from the gaps between the petals.

below: From https://chime-experiment.ca/instrument

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.

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  • $\begingroup$ 2 quick questions first. you mean the hardware is not very difficult and people just designed a new configuration? If we build every cylinder longer and make them not so compact, we would localize FRBs easily? BTW, why is it 4? $\endgroup$ – questionhang Jan 14 '19 at 14:02
  • $\begingroup$ @questionhang that's a lot of questions and they are very good questions! If you made the cylinders longer and you kept the same antenna spacing, you'd need a much bigger, more expensive computer to do the interferometry, it would cost more and take longer to fund and build. If you kept the same number of antennas but made the cylinders longer, the spacing would become many wavelengths and then solving for the location would become a challenge, there would be ambiguities due to multiple orders (like a large-pitch, high-order diffraction grating). $\endgroup$ – uhoh Jan 14 '19 at 14:07
  • $\begingroup$ @questionhang About 4, I think that isn't special, it probably just comes out of the optimization somehow. I'm going to add a new section now, hang on... $\endgroup$ – uhoh Jan 14 '19 at 14:08
  • $\begingroup$ so the main problem is computation power? As long as we have better computers..... But it seems people are pessimistic about the localization in the near future. At least it is not easy. $\endgroup$ – questionhang Jan 14 '19 at 14:16
  • $\begingroup$ updating CHIME, better localization for every FRB, and multi-wavelength follow-ups for repeated FRBs, problem solved! $\endgroup$ – questionhang Jan 14 '19 at 14:21

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