Polarization experiments

Possible instrumental set--ups for the measurement of CMB polarization are reviewed in this article. We discuss existing and planned instruments, putting special emphasis on observational, instrumental, and data processing issues for the detection of very low polarization signals of prime cosmological interest. A short prospective summary is included

All-sky signals from recombination to reionization with the SKA

Cosmic evolution in the hydrogen content of the Universe through recombination and up to the end of reionization is expected to be revealed as subtle spectral features in the uniform extragalactic cosmic radio background. The redshift evolution in the excitation temperature of the 21-cm spin flip transition of neutral hydrogen appears as redshifted emission and absorption against the cosmic microwave background. The precise signature of the spectral trace from cosmic dawn and the epoch of reionization are dependent on the spectral radiance, abundance and distribution of the first bound systems of stars and early galaxies, which govern the evolution in the spin-flip level populations. Redshifted 21 cm from these epochs when the spin temperature deviates from the temperature of the ambient relic cosmic microwave background results in an all-sky spectral structure in the 40-200 MHz range, almost wholly within the band of SKA-Low. Another spectral structure from gas evolution is redshifted recombination lines from epoch of recombination of hydrogen and helium; the weak all-sky spectral structure arising from this event is best detected at the upper end of the 350-3050 MHz band of SKA-mid. Total power spectra of SKA interferometer elements form the measurement set for these faint signals from recombination and reionization; the inter-element interferometer visibilities form a calibration set. The challenge is in precision polarimetric calibration of the element spectral response and solving for additives and unwanted confusing leakages of sky angular structure modes into spectral modes. Herein we discuss observing methods and design requirements that make possible these all-sky SKA measurements of the cosmic evolution of hydrogen.Comment: Accepted for publication in the SKA Science Book 'Advancing Astrophysics with the Square Kilometre Array', to appear in 201

A Resistive Wideband Space Beam Splitter

We present the design, construction and measurements of the electromagnetic performance of a wideband space beam splitter. The beam splitter is designed to power divide the incident radiation into reflected and transmitted components for interferometer measurement of spectral features in the mean cosmic radio background. Analysis of a 2-element interferometer configuration with a vertical beam splitter between a pair of antennas leads to the requirement that the beam splitter be a resistive sheet with sheet resistance {\eta}o /2, where {\eta}o is the impedance of free space. The transmission and reflection properties of such a sheet is computed for normal and oblique incidences and for orthogonal polarizations of the incident electric field. We have constructed such an electromagnetic beam splitter as a square soldered grid of resistors of value 180 Ohms (approximately {\eta}o /2) and a grid size of 0.1 m, and present measurements of the reflection and transmission coefficients over a wide frequency range between 50 and 250 MHz in which the wavelength well exceeds the mesh size. Our measurements of the coefficients for voltage transmission and reflection agree to within 5% with physical optics modeling of the wave propagation, which takes into account edge diffraction.Comment: 14 pages,17 figure

CMB Telescopes and Optical Systems

The cosmic microwave background radiation (CMB) is now firmly established as a fundamental and essential probe of the geometry, constituents, and birth of the Universe. The CMB is a potent observable because it can be measured with precision and accuracy. Just as importantly, theoretical models of the Universe can predict the characteristics of the CMB to high accuracy, and those predictions can be directly compared to observations. There are multiple aspects associated with making a precise measurement. In this review, we focus on optical components for the instrumentation used to measure the CMB polarization and temperature anisotropy. We begin with an overview of general considerations for CMB observations and discuss common concepts used in the community. We next consider a variety of alternatives available for a designer of a CMB telescope. Our discussion is guided by the ground and balloon-based instruments that have been implemented over the years. In the same vein, we compare the arc-minute resolution Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT). CMB interferometers are presented briefly. We conclude with a comparison of the four CMB satellites, Relikt, COBE, WMAP, and Planck, to demonstrate a remarkable evolution in design, sensitivity, resolution, and complexity over the past thirty years.Comment: To appear in: Planets, Stars and Stellar Systems (PSSS), Volume 1: Telescopes and Instrumentatio

Tropospheric Phase Calibration in Millimeter Interferometry

We review millimeter interferometric phase variations caused by variations in the precipitable water vapor content of the troposphere, and we discuss techniques proposed to correct for these variations. We present observations with the Very Large Array at 22 GHz and 43 GHz designed to test these techniques. We find that both the Fast Switching and Paired Array calibration techniques are effective at reducing tropospheric phase noise for radio interferometers. In both cases, the residual rms phase fluctuations after correction are independent of baseline length for b > b_{eff}. These techniques allow for diffraction limited imaging of faint sources on arbitrarily long baselines at mm wavelengths. We consider the technique of tropospheric phase correction using a measurement of the precipitable water vapor content of the troposphere via a radiometric measurement of the brightness temperature of the atmosphere. Required sensitivities range from 20 mK at 90 GHz to 1 K at 185 GHz for the MMA, and 120 mK for the VLA at 22 GHz. The minimum gain stability requirement is 200 at 185 GHz at the MMA assuming that the astronomical receivers are used for radiometry. This increases to 2000 for an uncooled system. The stability requirement is 450 for the cooled system at the VLA at 22 GHz. To perform absolute radiometric phase corrections also requires knowledge of the tropospheric parameters and models to an accuracy of a few percent. It may be possible to perform an `empirically calibrated' radiometric phase correction, in which the relationship between fluctuations in brightness temperature differences with fluctuations in interferometric phases is calibrated by observing a celestial calibrator at regular intervals.Comment: AAS LATEX preprint format. to appear in Radio Science 199

Analytical evaluation of ILM sensors, volume 1

The functional requirements and operating environment constraints are defined for an independent landing monitor ILM which provides the flight crew with an independent assessment of the operation of the primary automatic landing system. The capabilities of radars, TV, forward looking infrared radiometers, multilateration, microwave radiometers, interferometers, and nuclear sensing concepts to meet the ILM conditions are analyzed. The most critical need for the ILM appears in the landing sequence from 1000 to 2000 meters from threshold through rollout. Of the sensing concepts analyzed, the following show potential of becoming feasible ILM's: redundant microwave landings systems, precision approach radar, airborne triangulation radar, multilateration with radar altimetry, and nuclear sensing

Current Status and Perspectives of Cosmic Microwave Background Observations

Measurements of the cosmic microwave background (CMB) radiation provide a unique opportunity for a direct study of the primordial cosmic plasma at redshift z ~1000. The angular power spectra of temperature and polarisation fluctuations are powerful observational objectives as they encode information on fundamental cosmological parameters and on the physics of the early universe. A large number of increasingly ambitious balloon-borne and ground-based experiments have been carried out following the first detection of CMB anisotropies by COBE-DMR, probing the angular power spectrum up to high multipoles. The recent data from WMAP provide a new major step forward in measurements percision. The ESA mission Planck Surveyor, to be launched in 2007, is the third-generation satellite devoted to CMB imaging. Planck is expected to extract the full cosmological information from temperature anisotropies and to open up new fronteers in the CMB field.Comment: 6 pages, 1 figure, to appear in "Proc of International Symposium on Plasmas in the Laboratory and in the Universe: new insights and new challenges", September 16-19, 2003, Como, Ital