Traceable, highly accurate calibration of wavefront sensors such as Shack–Hartmann sensors is a challenging task and an active field of research. We have developed a measurement system for the traceable calibration of wavefront sensors employing spherical wavefronts and a point light source in combination with a three-axis linear stage. We obtain an absolute calibration of sensor errors, including the reference spot position and wavefront gradient-dependent errors for each microlens individually. We discuss the main error influences and present an initial measurement uncertainty budget for the calibration. The calibration can be performed with an expanded measurement uncertainty (95% coverage interval) of better than 5  μrad for the wavefront gradient deviation.