Snapshot from the Zemax simulation used to optimize the optical system of NIKA. In the order of photon travel, the NIKA optics consist of a flat mirror at the top of the cryostat, an off-axis biconic-polynomial curved mirror, a 300 K window lens, a field stop, a 4 K lens, an aperture stop, a dichroic, a 100 mK lens, and two band- defining filters in front of the back-illuminated KID arrays. Top panel: ray tracing from the entrance of the receiver cabin of the 30 m telescope (which is simulated but not shown in the image), view from the elevation axis (which is symbolized by the big circles and contains the 2 mirrors of the Nasmyth system). The NIKA cryostat and entrance nose are represented by the rectangles. Bottom panel: zoomed plot of the optical system inside the NIKA cryostat. The angular size, the beam efficiency and the overall optical efficiency of the system are presented in Table 1.