<p>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 <a href="https://en.m.wikipedia.org/wiki/Five-hundred-meter_Aperture_Spherical_Telescope" rel="nofollow noreferrer">FAST radio telescope</a>, rather than an array with similar collecting area, is evidence of that.</p>
<p>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.</p>

CC BY-SA 4.0
2020-12-08T10:47:08.060Z
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
CC BY-SA 4.0
2020-12-08T11:04:26.423Z
@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
CC BY-SA 4.0
2020-12-08T11:22:53.427Z
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
CC BY-SA 4.0
2020-12-08T11:30:37.630Z
@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
CC BY-SA 4.0
2020-12-08T14:31:15.793Z
@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
CC BY-SA 4.0
2020-12-08T14:38:56.470Z
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
CC BY-SA 4.0
2020-12-08T16:18:17.970Z
@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
CC BY-SA 4.0
2020-12-08T16:30:17.187Z
@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
CC BY-SA 4.0
2020-12-08T16:32:30.613Z
@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
CC BY-SA 4.0
2020-12-07T16:44:58.867Z
2023-02-24T07:37:16.200Z
2023-02-24T07:37:16.200Z
ProfRob
<p>Arecibo wasn't just a radio telescope, it was a <a href="https://en.wikipedia.org/wiki/Radar_astronomy" rel="noreferrer">radar</a> 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 <a href="https://en.wikipedia.org/wiki/Airy_disk" rel="noreferrer">Airy disk</a> rather than a complicated pattern formed by tens or hundreds of such disks.</p>

CC BY-SA 4.0
2020-12-08T10:47:49.397Z
Why does the simpler beam pattern make it superior, given modern signal processing techniques are available?
Steve Linton
CC BY-SA 4.0
2020-12-08T11:07:46.920Z
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
CC BY-SA 4.0
2020-12-08T11:12:08.607Z
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
CC BY-SA 4.0
2020-12-08T21:04:51.387Z
@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
CC BY-SA 4.0
2020-12-09T18:23:37.323Z
@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
CC BY-SA 4.0
2020-12-08T02:33:25.920Z
2020-12-08T02:33:25.920Z
Mark
<p>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.</p>

CC BY-SA 4.0
2020-12-10T21:49:23.627Z
2020-12-10T21:49:23.627Z
user6030
<p>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.</p>
<p>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, <a href="https://science.nrao.edu/facilities/vla/docs/manuals/oss/performance/resolution" rel="noreferrer">the Very Large Array's minimum baseline is 35 meters</a>. 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.</p>
<p>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.</p>
<p>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.</p>

CC BY-SA 4.0
2020-12-08T16:13:10.820Z
Arecibo dish area is 0.29 km$^2$. I bet you it cost less than 29% of the SKA.
ProfRob
CC BY-SA 4.0
2020-12-08T16:17:56.627Z
@RobJeffries Oh, absolutely; I had been ignoring cost.
HDE 226868
CC BY-SA 4.0
2020-12-09T03:29:02.027Z
@Rob Jeffries: When adjusted for inflation, and equipped with equivalent cost/capability electronics?  (And equivalent bureaucracy :-()
jamesqf
CC BY-SA 4.0
2020-12-09T12:43:36.903Z
There is already since 2011 a larger single dish radio transmitter in China: [FAST (Five-hundred-meter Aperture Spherical Telescope)](https://en.wikipedia.org/wiki/Five-hundred-meter_Aperture_Spherical_Telescope)
pba
CC BY-SA 4.0
2020-12-10T11:54:54.220Z
@pba As far as I can tell from that article, FAST is not a transmitter
Simon
CC BY-SA 4.0
2020-12-10T17:20:16.483Z
@pba Indeed, FAST does not have transmitters, so cannot do radar astronomy. (There is now only one such instrument in operation, Goldstone Solar System Radar.)
Keeley Hoek
CC BY-SA 4.0
2020-12-10T22:24:00.320Z
Isn't radar astronomy being done at the Haystack observatory as well?
Brian Borchers
CC BY-SA 4.0
2020-12-08T15:59:49.710Z
2020-12-08T21:18:51.247Z
2020-12-08T21:18:51.247Z
HDE 226868
<p>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.</p>
<p>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?</p>

CC BY-SA 4.0
2020-12-07T16:22:41.257Z
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](https://space.stackexchange.com/a/48673/12102) to *Arecibo: Advantages of Giant Dish?*
uhoh
CC BY-SA 4.0
2020-12-07T16:37:26.647Z
@uhoh I had missed that question on space explorations SE entirely. Thanks. Wondering if I should close this one as a dup.
Steve Linton
CC BY-SA 4.0
2020-12-07T16:55:04.027Z
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
CC BY-SA 4.0
2020-12-08T14:57:14.233Z
don't you like RATAN-600?
Ivan Borsuk
CC BY-SA 4.0
2020-12-08T22:12:48.760Z
I mean, what else is Pierce Brosnan going to throw Sean Bean off of?
John Doe
CC BY-SA 4.0
2020-12-09T17:44:00.543Z
@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
CC BY-SA 4.0
2020-12-09T20:51:10.917Z
@Joe oh thank you very much for that! I mention a couple of papers about that in [this question](https://space.stackexchange.com/q/16644/12102) 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?](https://space.stackexchange.com/q/17059/12102), since even though it wasn't completely "dead" it was somewhat disabled and the radar detection was completely passive...
uhoh
CC BY-SA 4.0
2020-12-09T20:53:07.403Z
@Joe ...and was way farther [than this](https://space.stackexchange.com/q/20582/12102) and did not use the spacecraft's transponder [like this](https://space.stackexchange.com/q/37753/12102).
uhoh
CC BY-SA 4.0
2020-12-10T00:17:56.410Z
@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
CC BY-SA 4.0
2020-12-10T00:56:41.310Z
@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
<observational-astronomy><radio-astronomy><radio-telescope><radar>
Is there any role today that would justify building a large single dish radio telescope to replace Arecibo?
Steve Linton
# Is there any role today that would justify building a large single dish radio telescope to replace Arecibo?

(Asked by: username_0 on 2020-12-07)

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?

**Tags:** observational-astronomy, radio-astronomy, radio-telescope, radar
> **Comments:**
> **username_1**: 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](https://space.stackexchange.com/a/48673/12102) to *Arecibo: Advantages of Giant Dish?*  
> **username_0**: @uhoh I had missed that question on space explorations SE entirely. Thanks. Wondering if I should close this one as a dup.  
> **username_1**: 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.  
> **username_2**: don't you like RATAN-600?  
> **username_3**: I mean, what else is Pierce Brosnan going to throw Sean Bean off of?  
> **username_4**: @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  
> **username_1**: @Joe oh thank you very much for that! I mention a couple of papers about that in [this question](https://space.stackexchange.com/q/16644/12102) 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?](https://space.stackexchange.com/q/17059/12102), since even though it wasn't completely "dead" it was somewhat disabled and the radar detection was completely passive...  
> **username_1**: @Joe ...and was way farther [than this](https://space.stackexchange.com/q/20582/12102) and did not use the spacecraft's transponder [like this](https://space.stackexchange.com/q/37753/12102).  
> **username_4**: @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.  
> **username_1**: @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 :-)  

---

## 4 answers

### Answer by username_12 on 2020-12-08. Score: 18 | Accepted answer

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](https://science.nrao.edu/facilities/vla/docs/manuals/oss/performance/resolution). 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.

> **Comments:**
> **username_5**: Arecibo dish area is 0.29 km$^2$. I bet you it cost less than 29% of the SKA.  
> **username_12**: @RobJeffries Oh, absolutely; I had been ignoring cost.  
> **username_13**: @Rob Jeffries: When adjusted for inflation, and equipped with equivalent cost/capability electronics?  (And equivalent bureaucracy :-()  
> **username_14**: There is already since 2011 a larger single dish radio transmitter in China: [FAST (Five-hundred-meter Aperture Spherical Telescope)](https://en.wikipedia.org/wiki/Five-hundred-meter_Aperture_Spherical_Telescope)  
> **username_15**: @pba As far as I can tell from that article, FAST is not a transmitter  
> **username_16**: @pba Indeed, FAST does not have transmitters, so cannot do radar astronomy. (There is now only one such instrument in operation, Goldstone Solar System Radar.)  
> **username_17**: Isn't radar astronomy being done at the Haystack observatory as well?  

---

### Answer by username_9 on 2020-12-08. Score: 36

Arecibo wasn't just a radio telescope, it was a [radar](https://en.wikipedia.org/wiki/Radar_astronomy) 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](https://en.wikipedia.org/wiki/Airy_disk) rather than a complicated pattern formed by tens or hundreds of such disks.

> **Comments:**
> **username_0**: Why does the simpler beam pattern make it superior, given modern signal processing techniques are available?  
> **username_5**: 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.  
> **username_5**: 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.  
> **username_9**: @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.  
> **username_10**: @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.  

---

### Answer by username_5 on 2020-12-07. Score: 25

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](https://en.m.wikipedia.org/wiki/Five-hundred-meter_Aperture_Spherical_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.

> **Comments:**
> **username_0**: 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).  
> **username_5**: @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.  
> **username_6**: 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?  
> **username_5**: @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.  
> **username_7**: @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.  
> **username_8**: 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%.  
> **username_5**: @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?  
> **username_8**: @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.  
> **username_5**: @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.  

---

### Answer by username_11 on 2020-12-10. Score: 2

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.


---

