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Edward Fomalont led a precise radio test of gravitational lensing near the Sun. The team used the Very Long Baseline Array to compare four radio sources over eighteen days in 2005, measuring changes in their relative apparent directions as the Sun moved across the sky.

Observations at several frequencies helped assess refraction in the solar corona. The 43 gigahertz data, sufficiently far from the Sun, largely avoided that disturbance. Fitting the gravitational signal gave the post-Newtonian parameter $\gamma =0.9998\pm 0.0003$, consistent with general relativity. The experiment illustrates why controlling propagation through plasma matters even in a test of gravity.^[ref]

In general relativity, light follows null geodesics. A weak gravitational field outside an isolated spherical mass gives the Schwarzschild light deflectionfor impact parameter $b\gg GM/c^2$. At the solar limb this is approximately $1.75$ arcseconds. A Newtonian corpuscular calculation gives half this value; the full result depends on spatial curvature as well as the gravitational time dilation. The parametrized post-Newtonian formalism predicts $\alpha =2(1+{\gamma }_{\mathrm{PPN}})GM/(b{c}^2)$, with ${\gamma }_{\mathrm{PPN}}=1$ in general relativity.

The Eddington experiment compared photographic star positions near the eclipsed Sun with reference exposures taken without the Sun in that field. Rays bend toward the Sun, but their backward extrapolations place the apparent stellar images farther from it. Plate scale, focus changes, atmospheric refraction and comparison-field alignment must be separated from a radial displacement proportional to $1/b$. The original report discusses the instruments, plate reductions and competing predicted deflections; the test is a quantitative comparison with uncertainty, not a unique proof of a gravitational theory.^[ref]

radio interferometry permits an independent test without an eclipse. A 2005 Very Long Baseline Array experiment observed four compact sources at 43, 23 and 15 GHz. Differential astrometry suppressed common antenna errors, while a fit $\mathbf{p}(\nu )={\mathbf{p}}_0+\mathbf{I}{\nu }^{-2}$ separated refraction in the solar corona from achromatic gravitational deflection. Tropospheric errors and radio-source structure also required calibration. The reported ${\gamma }_{\mathrm{PPN}}=0.9998\pm 0.0003$ was consistent with general relativity.^[ref]

The Very Long Baseline Array is a radio-telescope array used for very-long-baseline interferometry. In a solar-deflection experiment, differential astrometry of nearby compact sources compares their apparent directions as their angular separation from the Sun changes. Measurements at several frequencies can distinguish dispersive refraction in the solar corona from achromatic gravitational deflection.^[ref]