1,144 research outputs found
Angular and Polarization Response of Multimode Sensors with Resistive-Grid Absorbers
High sensitivity receiver systems with near ideal polarization sensitivity
are highly desirable for development of millimeter and sub-millimeter radio
astronomy. Multimoded bolometers provide a unique solution to achieve such
sensitivity, for which hundreds of single-mode sensors would otherwise be
required. The primary concern in employing such multimoded sensors for
polarimetery is the control of the polarization systematics. In this paper, we
examine the angular- and polarization- dependent absorption pattern of a thin
resistive grid or membrane, which models an absorber used for a multimoded
bolometer. The result shows that a freestanding thin resistive absorber with a
surface resistivity of \eta/2, where \eta\ is the impedance of free space,
attains a beam pattern with equal E- and H-plane responses, leading to zero
cross polarization. For a resistive-grid absorber, the condition is met when a
pair of grids is positioned orthogonal to each other and both have a
resistivity of \eta/2. When a reflective backshort termination is employed to
improve absorption efficiency, the cross-polar level can be suppressed below
-30 dB if acceptance angle of the sensor is limited to <60degrees. The small
cross-polar systematics have even-parity patterns and do not contaminate the
measurements of odd-parity polarization patterns, for which many of recent
instruments for cosmic microwave background are designed. Underlying symmetry
that suppresses these cross-polar systematics is discussed in detail. The
estimates and formalism provided in this paper offer key tools in the design
consideration of the instruments using the multimoded polarimeters.Comment: 22 pages, 15 figure
High Absorptance Coatings for THz Applications
High absorptance materials find application throughout the electromagnetic spectrum as radiation terminations, calibration standards, and glint reduction coatings. Successful use of materials at millimeter through submillimeter wavelengths requires an accurate knowledge and control over their thermal, mechanical, and electromagnetic properties in order to achieve the desired response while minimizing mass and volume. In practice, the achieved blackness is intimately linked to the material properties and geometry. Here, we summarize the characteristics of a variety of tunable artificial dielectric mixtures appropriate for THz applications at room and cryogenic temperatures. Theoretical guidelines for their application will be provided in the context of the effective-medium mean-field-approximation. The performance of these coatings as elements of reflectance standards, radiometric flux calibrators, passive thermal radiators, and stray light suppression baffles for imaging systems will be reviewed
Precision Continuum Receivers for Astrophysical Applications
Cryogenically cooled HEMT (High Electron Mobility Transistor) amplifiers find widespread use in radioastronomy receivers. In recent years, these devices have also been commonly employed in broadband receivers for precision measurements of the Cosmic Microwave Background (CMB) radiation. In this setting, the combination of ultra-low-noise and low-spectral-resolution observations reinforce the importance achieving suitable control over the device environment to achieve fundamentally limited receiver performance. The influence of the intrinsic amplifier stability at low frequencies on data quality (e.g., achievable noise and residual temporal correlations), observational and calibration strategies, as well as architectural mitigation approaches in this setting will be discussed. The implications of device level 1/f fluctuations reported in the literature on system performance will be reviewed
A Cryogenic Infrared Calibration Target
A compact cryogenic calibration target is presented that has a peak diffuse
reflectance, , from m).
Upon expanding the spectral range under consideration to m) the observed performance gracefully degrades to at the band edges. In the implementation described, a
high-thermal-conductivity metallic substrate is textured with a pyramidal
tiling and subsequently coated with a thin lossy dielectric coating that
enables high absorption and thermal uniformity across the target. The resulting
target assembly is lightweight, has a low-geometric profile, and has survived
repeated thermal cycling from room temperature to K. Basic design
considerations, governing equations, and test data for realizing the structure
described are provided. The optical properties of selected absorptive materials
-- Acktar Fractal Black, Aeroglaze Z306, and Stycast 2850 FT epoxy loaded with
stainless steel powder -- are characterized and presented
The Anisotropy in the Cosmic Microwave Background At Degree Angular Scales
We detect anisotropy in the cosmic microwave background (CMB) at degree
angular scales and confirm a previous detection reported by Wollack et al.
(1993). The root-mean-squared amplitude of the fluctuations is K. This may be expressed as the square root of the angular power spectrum
in a band of multipoles between . We find K. The measured spectral
index of the fluctuations is consistent with zero, the value expected for the
CMB. The spectral index corresponding to Galactic free-free emission, the most
likely foreground contaminant, is rejected at approximately .
The analysis is based on three independent data sets. The first, taken in
1993, spans the 26 - 36 GHz frequency range with three frequency bands; the
second was taken with the same radiometer as the first but during an
independent observing campaign in 1994; and the third, also take in 1994, spans
the 36-46 GHz range in three bands. For each telescope position and radiometer
channel, the drifts in the instrument offset are K/day over a period
of one month. The dependence of the inferred anisotropy on the calibration and
data editing is addressed.Comment: 16 pages, 2 figures. Saskatoon 1993/1994 combined analysi
Impedance Matched Absorptive Thermal Blocking Filters
We have designed, fabricated and characterized absorptive thermal blocking
filters for cryogenic microwave applications. The transmission line filter's
input characteristic impedance is designed to match and its
response has been validated from 0-to-50\,GHz. The observed return loss in the
0-to-20\,GHz design band is greater than dB and shows graceful
degradation with frequency. Design considerations and equations are provided
that enable this approach to be scaled and modified for use in other
applications
WMAP - A Portrait of the Early Universe
A host of astrophysical observations suggest that early Universe was incredibly hot, dense, and homogeneous. A powerful probe of this time is provided by the relic radiation which we refer to today as the Cosmic Microwave Background (CMB). Images produced from this light contain the earliest glimpse of the Universe after the 'Big Bang' and the signature of the evolution of its contents. By exploiting these clues, constraints on the age, mass density, and geometry of the early Universe can be derived. A brief history of the evolution of the microwave radiometer systems and map making approaches used in advancing these aspects our understanding of cosmological will be reviewed. In addition, an overview of the results from NASA's Wilkinson Microwave Anisotropy (WMAP) will be presented
Offset balancing in pseudo-correlation radiometers for CMB measurements
Radiometeric CMB measurements need to be highly stable and this stability is
best obtained with differential receivers. The residual 1/f noise in the
differential output is strongly dependent on the radiometer input offset which
can be cancelled using various balancing strategies. In this paper we discuss a
software method implemented in the Planck-LFI pseudo-correlation receivers
which uses a tunable "gain modulation factor, r, in the sky-load difference.
Numerical simulations and experimental data show how proper tuning of the
parameter r ensures a very stable differential output with knee frequencies of
the order of few mHz. Various approaches to calculate r using the radiometer
total power data are discussed with some examples relevant to Planck-LFI.
Although the paper focuses on pseudo-correlation receivers and the examples are
relative to Planck-LFI, the proposed method and its analysis is general and can
be applied to a large class of differential radiometric receivers.Comment: 12 pages, 8 figures, accepted for publication in A&A (updated version
with few editorial changes
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