A novel on-axis interferometric alignment scheme, especially applicable to x-ray lithography, is described which combines the position sensitivity of interferometry and the robustness of imaging. It employs broadband illumination, and hence should be relatively immune to many of the effects that tend to corrupt alignment signals in conventional interferometric systems. In its initial demonstration a standard deviation (sigma) of 6 nm was achieved in both X and Y. Ultimate limits are calculated to be below 1 nm. On an Apple Quadra 800 computer, the spatial-phase information that measures misalignment is fully analyzed in 200 ms.