FIGURE 8.1 Diagram illustrating that there is little justification for astronomical interferometry on baselines longer than the Earth-Sun distance (1 AU). Sloping black lines: angular resolution as a function of baseline (ordinate) and photon wavelength λ, frequency υ, or energy E (abscissas). One arcsecond is 1/3600 degree. Right ordinate: bottom to top, baselines of the Keck Telescope, Palomar Testbed Interferometer (PTI), Very Large Array (VLA), Earth’s radius , the distance to the Earth-Sun L2 point , the Earth-Sun separation (1 AU), and the semi-major axis of Pluto’s orbit. Lightface labels—angular diameters of a Sun-like star at 100 pc, of the horizon of the 3 billion solar-mass black hole in the nearby galaxy M87 (the black hole in the center of the Milky Way appears 3 times larger), of the 100 million solar-mass black holes in typical quasars at redshift of 2, and of a nearby neutron star at 1-kpc distance. Boldface labels: resolution needed to obtain a 1 percent parallax (π) or resolve a 1-year period solar-mass binary at 10 kpc and to measure a 1 percent parallax at a distance of 100 Mpc. Scattering in the interstellar medium causes the images formed by combinations of wavelength and baseline falling in the hatched region (upper left) to be blurred to the size of the region’s lower boundary line. Thin horizontal lines: strong multipath scattering. Dotted vertical lines: weak scattering, with images appearing as scattered speckles; some information about source size may remain.