I recently watched an interesting talk[1] about the NANOGrav project which looks for extremely low-frequency gravitational waves. The sources for these would be super-massive binary black holes, unlike the compact binaries ("regular" black holes and neutrons) that LIGO can observe.<p>They use radio telescopes to look for disturbances in the timing of millisecond pulsars due to gravitational waves, and they get about half of their data from Arecibo.<p>The project has been going for almost 15 years and they had just begun to see interesting stuff in their data. As the the project relies on continuous measurements of the pulsars, in order to detect variations in their timing, the presenter mentioned that more than six months of downtime would have a significant impact on the quality of their their overall data, and as mentioned on their page[2] many of the pulsars can only be measured by the Arecibo telescope due to its sensitivity.<p>[1]: <a href="http://pirsa.org/20100068/" rel="nofollow">http://pirsa.org/20100068/</a><p>[2]: <a href="http://nanograv.org/announcement/2020/08/20/Arecibo.html" rel="nofollow">http://nanograv.org/announcement/2020/08/20/Arecibo.html</a>