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International audience; Lensing of the cosmic microwave background (CMB) is now a well-developed probe of the clustering of the large-scale mass distribution over a broad range of redshifts. By exploiting the non-Gaussian imprints of lensing in the polarization of the CMB, the CORE mission will allow production of a clean map of the lensing deflections over nearly the full-sky. The number of high-S/N modes in this map will exceed current CMB lensing maps by a factor of 40, and the measurement will be sample-variance limited on all scales where linear theory is valid. Here, we summarise this mission product and discuss the science that will follow from its power spectrum and the cross-correlation with other clustering data. For example, the summed mass of neutrinos will be determined to an accuracy of 17 meV combining CORE lensing and CMB two-point information with contemporaneous measurements of the baryon acoustic oscillation feature in the clustering of galaxies, three times smaller than the minimum total mass allowed by neutrino oscillation measurements. Lensing has applications across many other science goals of CORE, including the search for B-mode polarization from primordial gravitational waves. Here, lens-induced B-modes will dominate over instrument noise, limiting constraints on the power spectrum amplitude of primordial gravitational waves. With lensing reconstructed by CORE, one can "delens" the observed polarization internally, reducing the lensing B-mode power by 60 %. This can be improved to 70 % by combining lensing and measurements of the cosmic infrared background from CORE, leading to an improvement of a factor of 2.5 in the error on the amplitude of primordial gravitational waves compared to no delensing (in the null hypothesis of no primordial B-modes). Lensing measurements from CORE will allow calibration of the halo masses of the tens of thousands of galaxy clusters that it will find, with constraints dominated by the clean polarization-based estimators. The 19 frequency channels proposed for CORE will allow accurate removal of Galactic emission from CMB maps. We present initial findings that show that residual Galactic foreground contamination will not be a significant source of bias for lensing power spectrum measurements with CORE.

Carrying out astronomical observations at far-infrared wavelengths is critical in enabling further progress in the fields of cosmology and astrophysics. Such observations will allow additional insight into the birth and evolution of the Universe. To allow progress in these areas, it is necessary to improve the sensitivity and resolution of the instrumentation that is used to carry out these observations. At the high frequencies in question (terahertz), the instruments typically make use of horn antenna fed detector systems. To achieve the required performance, the horn designs must be highly optimised. Full electromagnetic solvers (CST, HFSS, COMSOL etc.) struggle to predict the performance of horn antennas at such high frequencies in a timely manner due to the large electrical size of the structures. It is therefore very challenging to perform the optimisation using such solvers particularly for multi-mode systems where each mode would have to be considered individually. In this paper we outline an alternative technique for modelling multi-mode (partially coherent) horn antennas based on the mode-matching technique, which allows electrically large structures to be modelled in a highly efficient manner. This technique returns a set of scattering matrices which gives a full vector definition of the transmission and re ection characteristics of the resulting design at a given frequency. We demonstrate how this can be used to extract field patterns and other figures of merit that are important for evaluating the electromagnetic performance of a horn design. An efficient genetic algorithm based optimisation technique (using mode-matching) is also presented. The optimisation process is based on a piecewise conical profile horn design and produces a geometry that is optimised with respect to some figure of merit that is of critical importance for the application in question. This allows the instrument to realise the high levels of optical performance that are required for astronomical applications.

The location of the phase centres of antenna feeds is critical for optimised sensitivity and resolution on reflector antennas and telescopes. While the measurement of the far-field intensity patterns of such feeds is relatively straightforward, the direct recovery of their phase patterns requires access to expensive phase sensitive instrumentation such as a vector network analyzer. We present an inexpensive alternative quasi-optical technique, analogous to off-axis holography at visible wavelengths, that allows for the phase curvature of the feed pattern, and thus the phase centre, to be recovered with sufficient accuracy for optimizing aperture efficiency and resolution on a reflector antenna. We discuss the accuracy of the technique and compare results for the case of a specialized horn antenna for CMB polarization operating at 100GHz, using both the quasi-optical method and a vector network analyzer as a bench mark measurement tool for verification of the approach. We also include some measurements made of a lens antenna fed by a bare waveguide radiator.

We report on an ongoing study where different optical configurations for a multi-beam limb-viewing (four to eight receiver channels at 340 and two channels at 625 GHz) spaceborne instrument for climate research are presented and compared. The optical configurations are analyzed in terms of optical performance (gain, side lobe levels, beam efficiency etc.), weight and size of the overall instrument envelope. Using ideal fundamental Gaussian beam modes and numerical tools relying on ray-tracing and physical optics methods, the different configurations are designed and evaluated. Preliminary results indicate that a 1.3 m x 0.65 m primary reflector can be used in a configuration that includes a relay optics system having two to four elements. In addition to the limb-viewing instrument, there will be an additional instrument operating at 640 GHz for observing clouds in nadir mode.

The main topic of this thesis is to examine the design and analysis of millimetre and sub- millimetre optical systems for use in future cosmic microwave background radiation and far infrared space based astronomy missions. Astronomical observations in these regions of the electromagnetic spectrum are vital for studying the formation and evolution of planets, stars, galaxies, and on an even larger scale the origin of the Universe, through an analysis of the Cosmic Microwave Background (CMB) radiation. In order to further our knowledge of these astronomical processes it will be necessary to use highly calibrated instruments with increased sensitivity, and the capability to measure the polarisation signal of the cosmic microwave background radiation. It will also be necessary to design instruments with higher angular resolution so as measurements can be made on a finer scale. This will allow our current understanding of the origin and evolution of our Universe to be better understood by placing further constraints on the various models that currently exist which serve to explain these processes, for example the various in ationary models. Significant efforts are being made to develop and characterise the technology required to realise these ambitions. Examples of missions that are currently being planned are COrE (Cosmic Origins Explorer) Plus and SPICA (SPace Infrared telescope for Cosmology and Astrophysics), which contains an instrument known as SAFARI, a far infrared Fourier transform spectrometer that makes use of a Mach- Zehnder optical configuration. Such missions require high performance focal plane antennas, a position traditionally filled using corrugated horns. Future missions will require large arrays of high performance anten- nas, and the thermal and mechanical penalties associated with high mass corrugated horns (in addition to their cost) can become prohibitive. In this thesis, the design of an alternative smooth-walled profiled horn antenna will be examined. The resulting simple design is eas- ier and cheaper to manufacture than its corrugated counterpart, and carries less mechanical penalties. The horn was designed using a genetic algorithm based optimisation process that was applied to an implementation of the mode matching technique (used to model the perfor- mance of horn antennas) that has been optimised for computational efficiency. A number of improvements in the execution of the mode matching code were made to increase its efficiency and are described, including singular value decomposition analysis and reducing the number of waveguide modes used in the analysis. The limitations of the approach are considered, in- cluding determining the ideal horn performance metric to optimise with respect to in order to maintain the required levels of performance in all of the important characteristics. A further goal of the optimisation process was to use only desktop computing power and to complete the process within a reasonable amount of time (within a day). A further area analysed is the multi-moded detector pixels for use in the SAFARI in- strument. Such pixels are currently being tested in the Space Research Organisation of the Netherlands (SRON), Groningen. Models already exist to attempt to describe such pixels, however in order to attempt to improve the agreement between the measured and simulated results it is necessary to refine the model. Such pixels contain, for mechanical reasons, a sec- tion between the horn and cavity which can be considered to be a free space gap. This thesis considers the implementation of such a gap within the mode matching technique, and examines the impact of the gap on the measured pixel efficiency. This is critical for understanding the optical performance of the receiver system. Future missions will make use of such multi-moded systems as they maximise the amount of power available to the detector, a critical feature when low strength signals are being measured. Measurements of multi-moded horns are not well understood. Consideration is given in this thesis to a method that may be used in the future, with further development, to measure the performance of multi-moded horns. Such measurements will lead to a better understanding of multi-moded systems. A brief descrip- tion of the background science is also described in the first chapter, along with the specific contributions of the author.