Optical and mechanical design of the HiCAT testbed. The point source and the collimator simumlate a star located at infinity. The aperture stop corresponds to a pupil mask and can be circular, contain spiders or be shaped such as segmented telescopes. It is conjugated is the Iris AO, here called Hexagonal Segmented DM and together, they can simulate a segmented primary mirror at its full complexity. The apodizer (pupil plane), the FPM (focal plane), and the Lyot stop form a full APLC, which can easily be downgraded to a classical Lyot corongaraph when the apodizer is replaced with a flat mirror or even be removed if no coronagraph is needed. DM1 (pupil plane) and DM2 (out of pupil plane) are used for wavefront control to correct for wavefront aberrations or apply a diversity phase. At the end of the optical beam, two cameras can be found: a pupil plane camera and the science camera, which is also used as the post-coronagraph wavefront sensor when focal plane wavefront sensing techniques are tested. The phase retrieval camera offers an alternative as a pre-coronagraph detector.