This paper presents the key findings of an ESA-funded programme of work to investigate refractive systems and their application to precision polarimetry experiments. We briefly summarize the derivation of requirements on the optical system for CMB polarimetry, and the design of a refractive telescope system which meets these stringent requirements. An extensive programme of experimental work was undertaken in order to better understand the optical, thermal and mechanical characteristics of the lens material, and of lenses made from this material. A repeatable and controllable antireflection coating procedure was developed and validated, and used to coat lenses used in this study. Optical measurements before and after coating have been used to validate a new module for an industry-standard antenna modelling software package.
The Atacama Large Millimetre Array will be a single research instrument composed of up to 50 high precision antennas,
located at the Chajnantor plain in the district of San Pedro de Atacama, 5000m above sea level. Each ALMA telescope
will contain 10 frequency channels/bands, ranging from 30 to 950GHz. Radiation from the secondary reflector is
collected to the receivers of each wavelength channel through their accompanying front end optics. We present a full
electromagnetic treatment of the front end optics for band 5 (163 - 211 GHz) and band 9 (602 - 720 GHz). A full quasi
optical and physical optics analysis of the band 5 front end optics, using the antenna analysis tool, GRASP9  is
presented. Potential optimisation for the system is presented, namely a reflector edge taper and a comparison of two
surface geometries. A similar analysis of the band 9 system is presented. Full electromagnetic simulations are compared
with cold beam pattern measurements made at the Space Research Organisation of the Netherlands [2, 3]. Analysis of the
effect of the polarizing grid is presented, with suggested modifications to improve cross polar levels.
HIFI is one of the three instruments for the Herschel Space Observatory, an ESA cornerstone mission. HIFI is a high resolution spectrometer operating at wavelengths between 157 and 625 μm. The need for a compact layout reducing the volume and mass as much as possible has important consequences for the optical design. Many mirrors are located in the near-field of the propagating beam. Especially in the long wavelength limit diffraction effects might therefore introduce significant amplitude and phase distortions. A classical geometrical optical approach is consequently inadequate. In this paper we present a rigorous quasi-optical analysis of the entire
optical system including the signal path, local oscillator path and onboard calibration source optical layout. In order to verify the results of the front-to-end coherent propagation of the detector beams, near-field measurement facilities capable of measuring both amplitude and phase have beam developed. A remarkable feature of these facilities is that the absolute coordinates of the measured field components are known to within fractions of a wavelength. Both measured and simulated fields can therefore compared directly since they are referenced to one single absolute position. We present a comparison of experimental data with software predictions obtained from the following packages: GRASP (Physical Optics Analysis) and GLAD (Plane Wave Decomposition).
We also present preliminary results for a method to correct for phase aberrations and optimize the mirror surfaces without changing the predesigned mechanical layout of the optical system.