Efficient optical modelling in the far infrared is challenging because of the dominance of diffraction effects in typical astronomical instruments. With the development of the next generation of array imagers and multi-moded feed systems the necessity for computational efficiency has become critical to ensure an optimised design, comprehensive system and telescope analysis and image deconvolution. A multi-technique capability is necessary to simulate both efficiently and accurately the propagation of the signal collected by the telescope through the quasi-optical beam guide and feed structures using an appropriate combination of modelling tools, seamlessly passing from one regime to the next from detector to sky. Physical optics for example, although computationally intensive, is useful tool when detailed telescope beam analysis is required, particularly for providing cross-polarisation information. Modal analysis is often appropriate for modelling beam guide structures while analysing the detector feed coupling may rely on a more complete electromagnetic analysis because of the small sizes involved and the use of waveguide and planar structures. Image recovery ideally requires a deconvolution technique based on a modal approach and precise knowledge of the beams on the sky. In this paper we report on our work in the continued development of such appropriate techniques with the particular goal of prototyping powerful efficient computational tools for imaging arrays and partially coherent systems. In the presentation, we will discuss these issues and present examples from real instrumentation.