<p>There are many radiotelescopes that consist of large arrays of antennas. We usually think of an array individual dishes and <a href="https://en.wikipedia.org/wiki/Very_Large_Array" rel="nofollow noreferrer">VLA</a> and <a href="https://en.wikipedia.org/wiki/Atacama_Large_Millimeter_Array" rel="nofollow noreferrer">ALMA</a> are two of the most commonly recognized.</p>

<p>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.</p>

<p>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.</p>

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

<p>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.</p>

<hr>

<p>The ArXiv preprint <a href="https://arxiv.org/abs/1803.11235" rel="nofollow noreferrer">The CHIME Fast Radio Burst Project: System Overview</a> 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.</p>

<p><a href="https://i.sstatic.net/kfSYm.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/kfSYm.jpg" alt="enter image description here"></a></p>

<blockquote>
  <p>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.</p>
</blockquote>

<p>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.</p>

<p><a href="https://i.sstatic.net/Z1lZH.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Z1lZH.png" alt="enter image description here"></a></p>

<hr>

<p>I've borrowed the following text, images and links from my question <a href="https://astronomy.stackexchange.com/q/28615/7982">How does Canada's radiotelescope CHIME manage polarization-dependent apodization of the fast f/0.25 optics?</a></p>

<p>Here's a picture of some of the antennas, looking down into the trough:</p>

<p><strong>below x2:</strong> from <a href="https://arxiv.org/abs/1708.08521" rel="nofollow noreferrer">The cloverleaf antenna: A compact wide-bandwidth dual-polarization feed for CHIME</a></p>

<p><a href="https://i.sstatic.net/u6J0Q.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/u6J0Q.png" alt="enter image description here"></a></p>

<p>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.</p>

<p><a href="https://i.sstatic.net/1Sb4J.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/1Sb4J.png" alt="enter image description here"></a></p>

<p>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.</p>

<p><strong>below:</strong> From <a href="https://chime-experiment.ca/instrument" rel="nofollow noreferrer">https://chime-experiment.ca/instrument</a></p>

<blockquote>
  <p>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. <strong>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.</strong> 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.</p>
</blockquote>

<p><a href="https://i.sstatic.net/NSO9C.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/NSO9C.jpg" alt="enter image description here"></a></p>

CC BY-SA 4.0
2019-01-14T14:02:43.553Z
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?
questionhang
CC BY-SA 4.0
2019-01-14T14:07:33.457Z
@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).
uhoh
CC BY-SA 4.0
2019-01-14T14:08:11.603Z
@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...
uhoh
CC BY-SA 4.0
2019-01-14T14:16:25.093Z
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.
questionhang
CC BY-SA 4.0
2019-01-14T14:21:31.840Z
updating CHIME, better localization for every FRB, and multi-wavelength follow-ups for repeated FRBs, problem solved!
questionhang
CC BY-SA 4.0
2019-01-14T14:22:16.823Z
@questionhang I don't think it's so easy to say "the main problem". It's an optimization problem and you have to look at how every aspect changes as you change the size.
uhoh
CC BY-SA 4.0
2019-01-14T14:29:01.250Z
Yes, you are right. But what is the real obstacle? The input and output is not a linear relation.
questionhang
CC BY-SA 4.0
2019-01-14T14:40:00.687Z
@questionhang to "why didn't they build it before", these projects take a long time, from initial design, proposal, final design, funding etc... maybe this took ten years.
uhoh
CC BY-SA 4.0
2019-01-14T13:52:47.553Z
2019-01-14T14:13:45.043Z
2019-01-14T14:13:45.043Z
uhoh
<p>The  Canadian Hydrogen Intensity Mapping Experiment (<a href="https://en.wikipedia.org/wiki/Canadian_Hydrogen_Intensity_Mapping_Experiment" rel="nofollow noreferrer">CHIME</a>) has <a href="https://arstechnica.com/science/2019/01/new-canadian-telescope-spots-a-second-repeating-fast-radio-burst/" rel="nofollow noreferrer">discovered</a> 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?</p>
<p>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?</p>

CC BY-SA 4.0
2019-01-14T13:36:28.763Z
You can see the individual antennas in the question [How does Canada's radiotelescope CHIME manage polarization-dependent apodization of the fast f/0.25 optics?](https://astronomy.stackexchange.com/q/28615/7982)
uhoh
<observational-astronomy><radio-astronomy><data-analysis><interferometry>
What are the challenges for the building and data analysis of the CHIME telescope?
questionhang
# What are the challenges for the building and data analysis of the CHIME telescope?

(Asked by: username_0 on 2019-01-14)

The Canadian Hydrogen Intensity Mapping Experiment ([CHIME](https://en.wikipedia.org/wiki/Canadian_Hydrogen_Intensity_Mapping_Experiment)) has [discovered](https://arstechnica.com/science/2019/01/new-canadian-telescope-spots-a-second-repeating-fast-radio-burst/) 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?

**Tags:** observational-astronomy, radio-astronomy, data-analysis, interferometry

> **Comments:**
> **username_1**: You can see the individual antennas in the question [How does Canada's radiotelescope CHIME manage polarization-dependent apodization of the fast f/0.25 optics?](https://astronomy.stackexchange.com/q/28615/7982)  

---

## 1 answers

### Answer by username_1 on 2019-01-14. Score: 1 | Accepted answer

There are many radiotelescopes that consist of large arrays of antennas. We usually think of an array individual dishes and [VLA](https://en.wikipedia.org/wiki/Very_Large_Array) and [ALMA](https://en.wikipedia.org/wiki/Atacama_Large_Millimeter_Array) 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](https://arxiv.org/abs/1803.11235) 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.

[![enter image description here](https://i.sstatic.net/kfSYm.jpg)](https://i.sstatic.net/kfSYm.jpg)

> 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.

[![enter image description here](https://i.sstatic.net/Z1lZH.png)](https://i.sstatic.net/Z1lZH.png)

---

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?](https://astronomy.stackexchange.com/q/28615/7982)

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](https://arxiv.org/abs/1708.08521)

[![enter image description here](https://i.sstatic.net/u6J0Q.png)](https://i.sstatic.net/u6J0Q.png)

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.

[![enter image description here](https://i.sstatic.net/1Sb4J.png)](https://i.sstatic.net/1Sb4J.png)

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.

[![enter image description here](https://i.sstatic.net/NSO9C.jpg)](https://i.sstatic.net/NSO9C.jpg)

> **Comments:**
> **username_0**: 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?  
> **username_1**: @username_0 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).  
> **username_1**: @username_0 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...  
> **username_0**: 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.  
> **username_0**: updating CHIME, better localization for every FRB, and multi-wavelength follow-ups for repeated FRBs, problem solved!  
> **username_1**: @username_0 I don't think it's so easy to say "the main problem". It's an optimization problem and you have to look at how every aspect changes as you change the size.  
> **username_0**: Yes, you are right. But what is the real obstacle? The input and output is not a linear relation.  
> **username_1**: @username_0 to "why didn't they build it before", these projects take a long time, from initial design, proposal, final design, funding etc... maybe this took ten years.  

---

