E. Gleeson

Planck Pre-Launch Status: HFI Beam Expectations from the Optical Optimisation of the Focal Plane

Planck is a European Space Agency (ESA) satellite, launched in May 2009, which will map the cosmic microwave background anisotropies in intensity and polarisation with unprecedented detail and sensitivity. It will also provide full-sky maps of astrophysical foregrounds. An accurate knowledge of the telescope beam patterns is an essential element for a correct analysis of the acquired astrophysical data. We present a detailed description of the optical design of the High Frequency Instrument (HFI) together with some of the optical performances measured during the calibration campaigns. We report on the evolution of the knowledge of the pre-launch HFI beam patterns when coupled to ideal telescope elements, and on their significance for the HFI data analysis procedure.

Multi-mode horn design and beam characteristics for the Planck satellite

The ESA Planck satellite has begun studying the anisotropies of the cosmic microwave background radiation over the whole sky with unprecedented sensitivity and high angular resolution. The High Frequency Instrument, HFI, on Planck is observing simultaneously in six bands in the range 100 GHz to 857 GHz. The inclusion of non-CMB bands allows for robust removal of foreground sources from the data. This paper is concerned with the design, modeling and predicted performances of the two highest frequency channels centered on 545 GHz and 857 GHz, which use specialized multi-mode feedhorns, and are dedicated to observing these foregrounds. Multi-mode systems have the advantage of increasing the throughput, and thus sensitivity, of the detection assembly when diffraction limited resolution is not required. The horns are configured in a back-to-back setup which transmits the signal through filters to a detector horn. The modeling of the broadband beam patterns on the sky is shown to require careful analysis. Simulations of the complex interactions of the horns is computationally challenging when the detector horn in the relay system is included. The paper describes the approach to modeling these high frequency channels and discusses how the optical requirements on the horn designs are met in terms of spillover, edge taper, illumination of the telescope aperture and beam patterns on the sky.

“Phase Centers” of Far Infrared Multi-Moded Horn Antennas

Multi-moded horn antennas can be used as high efficiency feeds for bolometers when diffraction limited resolution is not required. For example, such horns are proposed for the PLANCK Surveyor, a satellite telescope due to be launched in 2007 to make definite measurements of the Cosmic Microwave Background. In a previous paper we described an accurate approach involving electromagnetic modelling using a rigorous mode matching technique to obtain both the horn aperture fields and the corresponding far field radiation patterns. In this paper we extend this description to determine the “phase center” of such horns when used on large reflecting telescopes. The “phase center” is ill defined as the individual spatially coherent fields making up the far field pattern all appear to come from different phase centers. The best average phase center location is therefore redefined in terms of the virtual beam waist position behind the horn aperture at which the focus of the telescope should be located in order to optimise angular resolution and on-axis gain for the beam on the sky. A number of alternative techniques to locating the phase center are discussed in detail in thes paper.

Study of corrugated Winston horns

Smooth walled Winston horns have been extensively used as light collectors for bolometric instruments. Used in multimoded operation without a waveguide, the beam shape is top-hat like and a simple equation is sufficient to define its Full Width Half Max. On the other hand, it is well known that corrugated feed horns are more efficient than smooth walled horns and much better well behaved with respect to polarization characteristics and sidelobe rejection. We present in this paper a study of corrugated Winston feed horns which could be used for future Astronomical instruments devoted to Cosmic Microwave Background (CMB) polarization measurements. We show that in this case low cross-polarization can be expected in single moded operation and that they could produce lower sidelobes levels compared to conical or profile shapes.

The quasi-optical design of the QUaD Telescope

QUaD is a ground-based high-resolution (up to l ≈ 2500) instrument designed to map the polarisation of the Cosmic Microwave Background and to measure its E-mode and B-mode polarisation power spectra. QUaD comprises a bolometric array receiver (100 and 150 GHz) and re-imaging optics on a 2.6-m Cassegrain telescope 2. It will operate for two years and begin observations in 2005. CMB polarisation measurements will require not only a significant increase in sensitivity over earlier experiments but also a better understanding and control of systematic effects particularly those that contribute to the polarised signal. To this end we have undertaken a comprehensive quasi-optical analysis of the QUaD telescope. In particular we have modelled the effects of diffraction on beam propagation through the system. The corrugated feeds that couple radiation from the telescope to phase-sensitive bolometers need to have good beam symmetry and low sidelobe levels over the required bandwidth. It is especially important that the feed horns preserve the polarisation orientation of the incoming fields. We have used an accurate mode-matching model to design such feed horns. In this paper we present the diffraction analysis of the QUaD front-end optics as well as the electromagnetic design and testing of the QUaD corrugated feeds.

Planck-HFI focal plane concept

The future ESA space mission Planck Surveyor mission will measure the Cosmic Microwave Background temperature and polarisation anisotropies in a frequency domain comprised between 30GHz and 1THz. On board two instruments, LFI based on HEMT technology and HFI using bolometric detectors. We present the optical solutions adopted for this mission, in particular the focal plane design of HFI, concept which has been applied already to other instruments such as the balloon borne experiment Archeops.

Electromagnetic modelling of few-moded Winston cones in the far-infrared

Winston cones have traditionally been used as detector feeds in far-infrared cosmological experiments, such as SuZIe, the Sunyaev-Zel’dovich Infra-red Experiment [1] on the CSO. They are usually designed using ray tracing, which becomes a very poor approximation when the number of spatial modes propagated by the horn is small in number, often the case at the longest wavelengths. We describe a more accurate approach involving electromagnetic modelling of Winston cones using a rigorous electromagnetic mode matching technique. It is straightforward to also consider the case of few-moded corrugated Winston cones, which offer lower sidelobe levels than smooth walled cones which is important for high sensitivity experiments. Furthermore, the mode matching technique allows more complex structures such as back-to-back Winston cones and the detector cavities to also be analyzed.

Corrugated horn design for HFI on PLANCK

In this paper we report on the back-to-back corrugated feed horn design for the High Frequency Instrument on the PLANCK Surveyor. Special single moded Gaussian profiled horns have been developed for the 4 lowest frequency channels (100 GHz, 150 GHz, 217 GHz & 353 GHz). These feed structures produce very pure Gaussian radiation patterns with sidelobe levels reduced well below −30 dB. Similar few moded horn antennas are being proposed for the highest frequency channels (545 GHz and 850 GHz) to give non-diffraction limited performance with high throughput. The modelling of the horns uses a rigorous electromagnetic mode matching technique that is found to give good agreement with measurement.