Its made of panels of thin perforated aluminium. See these photos https://www.researchgate.net/figure/A-close-view-of-the-perforated-panels-that-make-up-the-pri

Its made of panels of thin perforated aluminium. See these photos https://www.researchgate.net/figure/A-close-view-of-the-perforated-panels-that-make-up-the-primary-spherical-dish-One-of-our_fig4_241551316 https://www.researchgate.net/figure/A-close-view-of-the-perf... and https://www.localguidesconnect.com/t5/Share-Your-Photos-and/Arecibo-Observatory-In-Puerto-Rico/td-p/815131 https://www.localguidesconnect.com/t5/Share-Your-Photos-and/...