The astronomer community couldn't save it the same reason they can't build a new one - these are national projects, funded from national funds. The scale of academic science funding is simply on the wrong scale for major projects like Arecibo, or CERN, or the NIF. No grant of a few thousand dollars is going to crowdsource a radiotelescope. In any event, the NSF would be the one giving the astronomers the funds in the first place, so if it was important, they would simply have repaired or replaced the telescope, if this was the intent.

I heard estimates that fixing it could have cost up to $150M.Yet just in the last few years, Astronomers were able to find funding for:

MeerKAT: $330M

ELT: > $1B

GMT: $1B

Thirty Meter: $2.4B

Vera Rubin: $473M

James Webb: $10B++

WFirst: $4B

FAST (China): $180M

SKA: $1B

NOEMA: $70M

The last three are radio telescopes, which show both the range of funding available for new radio telescopes, and also serve as evidence we've gone beyond Arecibo.

I am no astro-rocket-surgeon, but I've been led to believe the problem with Arecibo is that it's giant dish was only an advantage for sending signals, while astronomy's overwhelming needs are to receive fainter signals which networks of dishes like SKA can do a much more amazing job at.

Note that FAST was built by China alone, SKA is an international collaboration but without US involvement and NOEMA does have a partnership agreement for data access with the university of Michigan, but is operated and payed for by IRAM, a collaboration between France, Germany and Spain. Meerkat (at the top of the list) is also a radio telescope and while it is not build solely by South Africa and Germany, I have not heard of any US involvement that goes beyond US citizens hiring on at Hartrao or Cape Town. And in the last few years NSF has not just stopped funding Arecibo Observatory, but also Greenbank Observatory. (But they were more successful in finding their on funding and staying operational.)

And while the planetary radar at Arecibo was one of a kind, and something that simply no other facility has right now, it is not the only food thing about AO. The huge collecting area and the low noise receivers were actually rather good at finding faint signals. Networks of dishes shine when it comes to resolving power, but they have their own drawbacks such as poor coverage of the u-v plane or the inability to image extended faint objects.

What’s a good budget to get it done?