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It seems that most of the modern radio astronomy instruments and observations that make the news are interferometers or phased array systems of one kind of another.

Is there any application left for which a single huge dish is better (or cheaper) than the same collecting surface in the form of some kind of array of smaller dishes, whether relatively compact, like ALMA, or widely distributed like the Event Horizon telescope or the square kilometer array? In other words is there any point at all in considering replacing the recently destroyed Arecibo dish with anything of similar size?

ProfRob
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Steve Linton
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    Arecibo was also a radar transmitter for radar astronomy, and a single dish with a single point feed has both practical and optical beam-forming advantages over a sparse array of smaller transmitting dishes. As a receiver, having the noise of one front end is better than having 100 receivers each contributing noise, though I'm guessing it will vary only as $\sqrt{N}$ For more see this answer to Arecibo: Advantages of Giant Dish? – uhoh Dec 07 '20 at 16:22
  • @uhoh I had missed that question on space explorations SE entirely. Thanks. Wondering if I should close this one as a dup. – Steve Linton Dec 07 '20 at 16:37
  • It happens once in a while, no need to close I think now that there's a link and since there's already a nice answer here. – uhoh Dec 07 '20 at 16:55
  • don't you like RATAN-600? – Ivan Borsuk Dec 08 '20 at 14:57
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    I mean, what else is Pierce Brosnan going to throw Sean Bean off of? – John Doe Dec 08 '20 at 22:12
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    @uhoh : it's my understanding that it was Arecibo's radar ability that allowed them to find and recover SOHO : https://en.wikipedia.org/wiki/Solar_and_Heliospheric_Observatory#Near_loss_of_SOHO – Joe Dec 09 '20 at 17:44
  • @Joe oh thank you very much for that! I mention a couple of papers about that in this question four years ago but at that time I didn't really appreciate the communications aspects and completely missed that they'd used radar confirmation. I think you have a new answer to What is the current record for the farthest detection of a “dead” spacecraft?, since even though it wasn't completely "dead" it was somewhat disabled and the radar detection was completely passive... – uhoh Dec 09 '20 at 20:51
  • @Joe ...and was way farther than this and did not use the spacecraft's transponder like this. – uhoh Dec 09 '20 at 20:53
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    @uhoh : I answered your question ... But SOHO was actually closer than Rosetta. (although I don't know if I'd count Rosetta as "dead" like SOHO). The real "winner" was STEREO-Behind. But "winner" in a horrible sense here. – Joe Dec 10 '20 at 00:17
  • @Joe yes indeed you did an a wonderful answer at that! I will dig in this weekend to read more about those, it's all news to me :-) – uhoh Dec 10 '20 at 00:56

4 Answers4

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Arecibo wasn't just a radio telescope, it was a radar telescope, bouncing megawatt-level radio signals off various bodies in the Solar System. A single-dish transmitter is far superior to a phased-array or other composite system, because the beam pattern is a simple Airy disk rather than a complicated pattern formed by tens or hundreds of such disks.

Mark
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    Why does the simpler beam pattern make it superior, given modern signal processing techniques are available? – Steve Linton Dec 08 '20 at 10:47
  • Why is an Airy disk simpler than other diffraction pattern? Personally I've always found Bessel functions harder to work with than simple sinusoids. An array, which could have a much larger effective aperture, could have a much narrower beam. I don't think this is why it's better to use a dish as a radar. – ProfRob Dec 08 '20 at 11:07
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    Quoting from the wiki on the Deep space radar network: "By 2025, the 70 meter antennas at all three locations will be decommissioned and replaced with 34-meter BWG antennas that will be arrayed." I think the big dish concept is purely down to sensitivity. – ProfRob Dec 08 '20 at 11:12
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    @RobJeffries, the DSN's primary mission is talking to spacecraft. An array of antennas works just fine for that, since you're not trying to extract every possible scrap of information from the signal. – Mark Dec 08 '20 at 21:04
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    @Mark: If one is talking to a spacecraft, the strength of the signal received from the spacecraft will generally be independent of the strength of the signal sent to it. If one is trying to image a planet, doubling the amount of signal the planet receives will double the amount of signal it reflect. – supercat Dec 09 '20 at 18:23
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Having a large dish gives you a large collecting area and hence better sensitivity. Building a multitude of receivers with the same collecting area, each having its own feed and electronics, is more expensive, not less. So yes, there is still a role for big dishes - at least on the scale of a few hundred metres. The fact that the Chinese have already built the 500-m diameter FAST radio telescope, rather than an array with similar collecting area, is evidence of that.

Arrays are built chiefly because you can synthesize a larger diameter of aperture and get better angular resolution. In addition to cost, the noise properties of 100 receivers, each with 1/100 of the collecting area of a big dish would not give the same sensitivity, because you will essentially be getting additional noise for each receiver that is added.

ProfRob
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  • Presumably above a certain size steerable dishes become more expensive than multiple smaller dishes, and/or simply infeasible, for mechanical engineering reasons (I admit Arecibo wasn't steerable). – Steve Linton Dec 08 '20 at 10:47
  • @SteveLinton Yes, that is another plus for the array concept - you can point them where you like and set them up in different configurations to concentrate on different angular scales. – ProfRob Dec 08 '20 at 11:04
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    Is it not also simply a matter of area? Phased arrays give amazingly large linear extends in the form of long baselines, which is all that's needed for high resolution but the area only scales linearly with the number of units. In a single dish meanwhile, the area scales quadratically with radius, and the round-trip sensitivity scales quadratically with area, so basically a single dish's radar quality is $\propto \ell^4$ whereas with an array you only get $\propto \ell^2$. Does that make sense? – leftaroundabout Dec 08 '20 at 11:22
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    @leftaroundabout That is what the first sentence says. To build an array with the same collecting area it would obviously have to cover much more (geographical) area. If you make them the same collecting area there is no difference in sensitivity beyond the additional noise you get from combining a lot of receivers. – ProfRob Dec 08 '20 at 11:30
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    @SteveLinton Arecibo was steerable to a limited degree. Obviously the dish itself is fixed, but they could steer it by moving the azimuth arm and sub-reflector around on the receiver platform. – Darrel Hoffman Dec 08 '20 at 14:31
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    My understanding of arrays was that they start making sense when the cost of scaling up a single sensor starts to grow superlinearly. To scale up Arecibo by 10% would require scaling up its support structure by far more than 10%. – MSalters Dec 08 '20 at 14:38
  • @MSalters I do not think we are at that limit (not for fixed dishes anyway). The Chinese FAST 500m dish is surely evidence of that? – ProfRob Dec 08 '20 at 16:18
  • @RobJeffries: It looks like the support towers at the edges of FAST are clearly taller than those in the middle. And the number of towers would already grow linearly with the area of the dish. Similar thing for the cables that hold the sensors up near the focal point - they need to be both longer and thicker, or made of stronger (more expensive) materials. You can move the focal point up so the curvature of the disc can be lower, but that's a trade-off.Guess you'd need to do the math to find out the tipping point. – MSalters Dec 08 '20 at 16:30
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    @MSalters I don't need to, the Chinese have done it. If they could build an array telescope with the same sensitivity, cheaper, then they would surely have done that. Especially since it comes with many other advantages in terms of resolving power and steerability. – ProfRob Dec 08 '20 at 16:32
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Single-dish telescopes have advantages over interferometers in a few areas; existing answers have touched on some of them. Collecting area is extremely important, as Rob Jeffries mentioned, and you need extremely large arrays to compensate for this. Granted, such arrays are certainly possible (ignoring the fairly sizable cost cost), as demonstrated by the coming Square Kilometer Array in 2027, which will have a collecting area of ~1 km. On the other hand, the SKA is in many ways the exception, not the rule, collecting area included.

Another advantage of single-dish setups arises when investigating large-scale structures. An interferometer's upper limit to its spatial frequency depends on the shortest baseline between any two dishes. Even in its most compact configuration, the Very Large Array's minimum baseline is 35 meters. If you want to quickly map large areas of the sky - important for large, extended sources - you want a single-dish telescope. In cases when you require both large-scale and small-scale spatial sensitivity, a combination of an interferometer and a single-dish may be warranted.

Let's say you want to swap out receivers, or upgrade your instrument for a specialized purpose. In that scenario, it's much more convenient to just have one dish to deal with. It would be much, much simpler to install a new receiver in one dish instead of dozens. Similarly, an interferometer won't be able to easily shift between configurations, which gives an advantage when it comes to scheduling - particularly in an era when radio telescopes are often massively oversubscribed.

As a final note, from a spatial perspective, observations of point sources - pulsars, FRBs, radio transients, etc. - can be done just as easily with single-dish telescopes - which, again, usually have larger collecting areas.

HDE 226868
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I've read of plans to build a large single disk radio telescope somewhere on the Moon, to give access to wavelengths blocked by Earth's atmosphere. It would be best to choose a place that gets no radio signals from the Sun or from the Earth, which would probably be in a crater near either the Moon's north pole or it's south pole. Such a location could not see the entire sky, so a second one may be needed on the opposite side of the Moon.

user6030
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