No single telescope on Earth could ever photograph a black hole — at radio wavelengths, seeing the ring of light around M87∗ would take a dish thousands of kilometres across. Radio astronomers build one anyway: separate telescopes on different continents observe the same source, and each pair of them measures one Fourier component of the sky image. As the Earth rotates, the pairs sweep through the Fourier plane, and a computer assembles the picture a planet-sized dish would have seen. This page lets you run the whole pipeline: draw a sky, place the telescopes, let the Earth turn, and reconstruct your drawing from the interference fringes alone. A walkthrough below, aimed at undergrad physics students, develops the van Cittert–Zernike theorem, Earth-rotation synthesis, and the CLEAN deconvolution algorithm running in your browser.
Everything is live — change anything and the panels below recompute. Draw a bright source on the black sky or load the crescent preset for an M87∗-style ring, arrange the telescopes on the map (the page opens with the EHT+ layout — the 2017 sites plus stations added since), and press ▷ Observe to watch the Earth's rotation sweep out the uv-coverage and the image sharpen as it goes.
The following physics explainer was written by Claude Fable 5, a large language model by Anthropic, with the simulation's numerics validated against closed-form results. It has not been reviewed by a human radio astronomer. Treat it as an informed starting point, not as an authoritative reference.