# Is there any role today that would justify building a large single dish radio telescope to replace Arecibo?

It seems that most of the modern radio astronomy instruments and observation 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 Arecibo by anything the same size?

• 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 7 '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 7 '20 at 16:37
• I mean, what else is Pierce Brosnan going to throw Sean Bean off of? – John Doe Dec 8 '20 at 22:12
• @uhoh : it's my understanding that it was Arecibo's radar ability that allowed them to find and recover SOHO : en.wikipedia.org/wiki/… – Joe Dec 9 '20 at 17:44
• @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 0:17

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.

• Arecibo dish area is 0.29 km$^2$. I bet you it cost less than 29% of the SKA. – ProfRob Dec 8 '20 at 16:13
• @RobJeffries Oh, absolutely; I had been ignoring cost. – HDE 226868 Dec 8 '20 at 16:17
• There is already since 2011 a larger single dish radio transmitter in China: FAST (Five-hundred-meter Aperture Spherical Telescope) – pba Dec 9 '20 at 12:43
• @pba As far as I can tell from that article, FAST is not a transmitter – Simon Dec 10 '20 at 11:54
• @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 Dec 10 '20 at 17:20

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.

• Why does the simpler beam pattern make it superior, given modern signal processing techniques are available? – Steve Linton Dec 8 '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 8 '20 at 11:07
• 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 8 '20 at 11:12
• @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 8 '20 at 21:04
• @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 9 '20 at 18:23

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. Otherwise that is what people would have done in the past. Arrays are built because you can synthesize a larger diameter of aperture. In addition, the noise properties of 100 receivers, each with 1/100 of the collecting area of a big dish do not give the same sensitivity, because you will essentially be getting additional noise for each receiver that is added.

• 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 8 '20 at 11:22
• @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 8 '20 at 11:30
• @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 8 '20 at 14:31
• 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 8 '20 at 14:38
• @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 8 '20 at 16:32

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.