Abstract: Defocus and astigmatism compensation methods and apparatuses for use in an aberration measurement system. The apparatuses including reflectors for altering the optical distance between a pair of lenses passing a wavefront without changing the physical distance between the lenses, thereby compensating for defocus in the wavefront; and cylindrical mirrors for adding and removing curvature from a curved wavefront, thereby compensating for astigmatism in the wavefront. The methods including passing a wavefront having defocus through a first lens on a first path, reflecting the wavefront from the first path to a second path, reflecting the wavefront from the second path to a third path, and passing the wavefront through a second lens as a defocus compensated wavefront; and passing a wavefront through first and second cylindrical lens, and orienting the first and second cylindrical lenses with respect to the wavefront and to one another to compensate for astigmatism in the wavefront.