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Given the loss of the Arecibo telescope: Is anybody aware of any efforts or studies towards a floating, ocean-going radio telescope of with 500m diameter and more?

The idea is floating (pun intended) in my head since I heard about Japanese design studies about foldable aircraft carriers (actually it is just the runway which is expandable and thin).

In the age rockets self-landing on a small sea-born platform, I assume that we have the wave movements under control, at least in principle.

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    $\begingroup$ What does "swimming" mean? Does it mean underwater/submerged? Could you edit your post so it explains so? $\endgroup$ – fasterthanlight Dec 7 '20 at 14:56
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    $\begingroup$ tragic loss Not really tragic as happily no one was injured or killed and it was going to be demolished anyway. Cannot imagine any advantage to offshore facilities over land based ones anyway and several disadvantages - e.g. maintenance costs. $\endgroup$ – StephenG Dec 7 '20 at 15:12
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    $\begingroup$ I've made an edit to help clarify, please have a look and feel free to edit further. Thanks! $\endgroup$ – uhoh Dec 7 '20 at 16:11
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I would be extremely concerned about the ability of such a telescope to make adequately precise measurements, given the motion of the water. The leading radio telescopes have their mirrors and receivers very exactly aligned. For example, Arecibo's Gregorian dome could be aligned with any location on the order of millimeters, while the Green Bank Telescope's mirrors are aligned to the tenth of a millimeter. I would certainly be concerned about replicating this performance in the ocean. It's fine for an aircraft carrier's surface to wobble by a couple orders of magnitude more than that; it's less so for a world-class radio telescope. I would expect to see mirror deformation and pointing errors due to the motion of the water.

There's a basket of additional logistical issues - constructing the telescope on water, transporting personnel to and from the structure, combating temperature variations, protecting electrical components from short-circuiting, staving off large waves, etc. All of these drive up construction and operating costs, and astronomers aren't exactly rolling in cash.

I can admittedly see two possible benefits from the plan. First, radio frequency interference (RFI) from artificial sources would presumably be nonexistent throughout much of the open issue (although Connor Garcia makes the excellent point that the increased amount of water vapor could be problematic!). RFI is the bane of the radio astronomer's existence, which is why radio quiet zones are so important. As a counter point, of course, there are available formal and natural radio quiet zones which could be taken advantage of. Second, you could change the telescope's latitude, which would give it a wider range of sources. On the other hand, it's not as if any significant parts of the radio sky can't be seen by radio telescopes, so it's unclear as to the benefits of this.

Overall, I'd say that the precision issues outweigh the potential benefits. If the National Science Foundation were to fund future replacements for Arecibo, there are plenty of better options, some of which are already being planned. If an aquatic radio telescope were ever to be built, I'd guess that it would be entirely privately funded by a billionaire with money to spare who would be willing to take significant risks. I wouldn't complain about something like that, but at the same time . . . I think a conventional radio telescope would be a much better choice.

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    $\begingroup$ I think that one of the reasons Arecibo and FAST are where they are is the hollow shape of the terrain. These aren't usually built on flat ground nor flat water, and high towers attached to bedrock are needed to suspend quite heavy receiver what-nots at the primary focus and scan them. There's no bedrock to anchor to in the ocean, this would have to be an unbelievably massive steel structure, or it would not be steerable. Also support of the people working there is much easier when there are roads... $\endgroup$ – uhoh Dec 7 '20 at 16:16
  • $\begingroup$ Boeing built a sea-based very large radar. it needs pretty precise positioning as well. And if was accomplished for this radar. $\endgroup$ – Natsfan Dec 7 '20 at 18:43
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    $\begingroup$ RF interference from atmospheric humidity is a problem, depending on your collection RF. That's why they put ALMA in the high desert. Corrosion is also a huge problem. Unless you put a giant radome over the dish, it's going to be periodically soaked with salt water. Not good for mechanical and RF components. On the other hand, it would be pretty cool to see a fleet of super-container ships fitted with big dishes that could swing away to load/unload containers. We could call it the VLSBI - Very Large Shipping Baseline Interferometer! $\endgroup$ – Connor Garcia Dec 7 '20 at 19:50
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    $\begingroup$ @B--rian Regardless of design, you'd still need plenty of folks to be on-site for regular maintenance and operation. $\endgroup$ – HDE 226868 Dec 8 '20 at 14:45
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    $\begingroup$ NASA has proposed a telescope attached to a sub-orbital balloon: nasa.gov/content/… How do they solve the precision problem in that case? $\endgroup$ – Shawn V. Wilson Dec 8 '20 at 16:13

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