Interferometric Visibility Nulls of the Two Shadows at 230 Gigahertz

A thin ring of angular diameter theta has visibility amplitude given by the zeroth-order Bessel function of pi theta times the baseline in wavelengths, so its visibility vanishes at definite baselines that measure the diameter without any imaging at all. At 230 gigahertz, wavelength 1.303 millimetres, the first null for the 42 microarcsecond M87* ring falls at a baseline of 4900 kilometres and for the 51.8 microarcsecond Sagittarius A* ring at 3973 kilometres, both comfortably inside the 21.1 microarcsecond fringe spacing of an Earth-diameter baseline. The second M87* null falls at 11248 kilometres, near the longest baselines the planet offers. On an Earth-diameter baseline the ring visibility magnitudes stand at 0.214 for M87* and 0.232 for Sagittarius A* in the thin-ring model. Null positions are diameter measurements immune to imaging-algorithm choices, and their accessibility is a concrete argument for dense long-baseline coverage. Computed under a thin-ring model; simulated, not measured.