1. An apparatus for measuring surface topography of a three-dimensional structure, the apparatus comprising: an optical probe moveable relative to the three-dimensional structure; an illumination unit configured to generate a plurality of incident light beams, each of the plurality of incident light beams comprising a first wavelength component; an optical system configured to focus the first wavelength component of each of the plurality of incident light beams to a respective fixed focal position relative to the optical probe while measuring the surface topography; and a detector unit configured to measure a characteristic of each of a plurality of returned light beams that are generated by illuminating the three-dimensional structure with the plurality of incident light beams.