We describe an efficient and compact terahertz (THz) illumination system as part of an active THz imager. The design and fabrication methods are verified by comparing measurements with wave optical simulations that form a basis for the design of a Gaussian-to-tophat beam shaper. Introducing realistic alignment tolerances in the simulations led to an improved agreement with the experimental data. Furthermore, we propose a method to reduce the alignment effort for THz beam shaping elements by correlating measured results with simulated data of a misaligned system.