Optical design in the terahertz (THz) waveband can be challenging, especially for high-precision applications. In this
paper we summarise our experience with the quasi-optical design and subsequent performance of astronomical
telescopes designed to measure the faint temperature and polarisation properties of the Cosmic Microwave Background
Radiation, in particular QUaD1, the PLANCK Surveyor2 and MBI3. These telescopes contain a range of quasi-optical
components including corrugated feed horns, on- and off-axis conic mirrors and lenses. Knowledge of their optical
performance and beam patterns is critical for understanding systematic effects in the reliable extraction of feeble
polarisation signals.
Although Physical Optics can be used to characterise electromagnetic systems to high accuracy, it is computationally
intensive at these frequencies and often not suitable for the initial design or preliminary analysis of large multi-element
optical systems. In general there is a lack of dedicated software tools for modelling the range of components and
propagation conditions encountered in typical systems and we have employed a variety of commercial and in-house
software packages for this task. We describe the techniques used, their predictions and the performance of the
telescopes that have been measured to-date.
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