### 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 I had missed that question on space explorations SE entirely. Thanks. Wondering if I should close this one as a dup.

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.

don't you like RATAN-600?

I mean, what else is Pierce Brosnan going to throw Sean Bean off of?

@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 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...

@Joe ...and was way farther than this and did not use the spacecraft's transponder like this.

@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 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 :-)

one year ago

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.

@RobJeffries Oh, absolutely; I had been ignoring cost.

@Rob Jeffries: When adjusted for inflation, and equipped with equivalent cost/capability electronics? (And equivalent bureaucracy :-()

There is already since 2011 a larger single dish radio transmitter in China: FAST (Five-hundred-meter Aperture Spherical Telescope)

@pba As far as I can tell from that article, FAST is not a transmitter

@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.)

Isn't radar astronomy being done at the Haystack observatory as well?