Freecad Visualisation Of Realistic 3d Physical Optics Beams Within A Cad System-Model

The facility to realise the shape and extent of optical beams within a telescope or beamcombiner can aid greatly in the design and layout of optical elements within the system. It can also greatly facilitate communication between the optical design team and other teams working on the mechanical design of an instrument. Beyond the realm where raytracing is applicable however, it becomes much more difficult to realise accurate 3D beams which incorporate diffraction effects. It then is another issue to incorporate this into a CAD model of the system.A novel method is proposed which has been used to aid with the design of an optical beam combiner for the QUBIC (Q and U Bolometric Interferometer for Cosmology) (1) experiment operating at 150 GHz and 220 GHz. The method combines calculation work in GRASP (2), a commercial physical optics modelling tool from TICRA, geometrical work in Mathematica, and post processing in MATLAB. Finally, the Python console of the open source package FreeCAD(3) is exploited to realise the 3D beams in a complete CAD system-model of the QUBIC optical beam combiner.This paper details and explains the work carried out to reach the goal and presents some graphics of the outcome. 3D representations of beams from some back-to-back input horns of the QUBIC instrument are shown within the CAD model. Beams of the -3dB and -13dB contour envelope are shown as well as envelopes enclosing 80% and 95% of the power of the beam. The ability to see these beams in situ with all the other elements of the combiner such as mirrors, cold stop, beam splitter and cryostat widows etc. greatly simplified the design for these elements and facilitated communication of element dimension and location between different subgroups within the QUBIC group.