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Berlin Brandenburg

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• 1
Article
In: Radio Science, December 2012, Vol.47(6), pp.n/a-n/a
Description: A new method of absolute calibration of sky noise temperature using a three‐position switched spectrometer, measurements of antenna and low noise amplifier impedance with a vector network analyzer, and ancillary measurements of the amplifier noise waves is described and compared with other methods of calibration. An initial test of the method has been made using brief observations from 55 to 110 MHz at West Forks, Maine, to estimate the sky noise spectral index. Estimates are made of the accuracy that might ultimately be achieved with observations to detect or set limits on the red‐shifted 21 cm line at a radio quiet site. It is concluded that an antenna reflection coefficient better than about −20 dB is required to avoid being limited by the accuracy of the antenna reflection coefficient measurements using a vector network analyzer. Absolute calibration of antenna and spectrometer for sky noise measurements
Keywords: Antenna ; Calibration ; Spectrometer
ISSN: 0048-6604
E-ISSN: 1944-799X
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• 2
Article
In: Monthly Notices of the Royal Astronomical Society, 2017, Vol. 470(4), pp.4720-4731
Description: We present the E-field Parallel Imaging Calibration (EPICal) algorithm, which addresses the need for a fast calibration method for direct imaging radio astronomy correlators. Direct imaging involves a spatial fast Fourier transform of antenna signals, alleviating an $\mathcal {O}(N_{\mathrm{a}} ^2)$ computational bottleneck typical in radio correlators, and yielding a more gentle $\mathcal {O}(N_{\mathrm{g}} \log _2 N_{\mathrm{g}})$ scaling, where N a is the number of antennas in the array and N g is the number of gridpoints in the imaging analysis. This can save orders of magnitude in computation cost for next generation arrays consisting of hundreds or thousands of antennas. However, because antenna signals are mixed in the imaging correlator without creating visibilities, gain correction must be applied prior to imaging, rather than on visibilities post-correlation. We develop the EPICal algorithm to form gain solutions quickly and without ever forming visibilities. This method scales as the number of antennas, and produces results comparable to those from visibilities. We use simulations to demonstrate the EPICal technique and study the noise properties of our gain solutions, showing they are similar to visibility-based solutions in realistic situations. By applying EPICal to 2 s of Long Wavelength Array data, we achieve a 65 per cent dynamic range improvement compared to uncalibrated images, showing this algorithm is a promising solution for next generation instruments.
Keywords: Instrumentation: Interferometers ; Techniques: Image Processing ; Techniques: Interferometric
ISSN: 0035-8711
E-ISSN: 1365-2966
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• 3
Article
In: Monthly Notices of the Royal Astronomical Society, 2017, Vol. 467(1), pp.715-730
Description: Modern radio telescopes are favouring densely packed array layouts with large numbers of antennas ( N A ≳ 1000). Since the complexity of traditional correlators scales as $\mathcal {O}(N_{\rm A}^2)$ , there will be a steep cost for realizing the full imaging potential of these powerful instruments. Through our generic and efficient E-field Parallel Imaging Correlator ( epic ), we present the first software demonstration of a generalized direct imaging algorithm, namely the Modular Optimal Frequency Fourier imager. Not only does it bring down the cost for dense layouts to $\mathcal {O}(N_{\rm A}\log _2N_{\rm A})$ but can also image from irregular layouts and heterogeneous arrays of antennas. epic is highly modular, parallelizable, implemented in object-oriented python , and publicly available. We have verified the images produced to be equivalent to those from traditional techniques to within a precision set by gridding coarseness. We have also validated our implementation on data observed with the Long Wavelength Array (LWA1). We provide a detailed framework for imaging with heterogeneous arrays and show that epic robustly estimates the input sky model for such arrays. Antenna layouts with dense filling factors consisting of a large number of antennas such as LWA, the Square Kilometre Array, Hydrogen Epoch of Reionization Array, and Canadian Hydrogen Intensity Mapping Experiment will gain significant computational advantage by deploying an optimized version of epic . The algorithm is a strong candidate for instruments targeting transient searches of fast radio bursts as well as planetary and exoplanetary phenomena due to the availability of high-speed calibrated time-domain images and low output bandwidth relative to visibility-based systems.
Keywords: Instrumentation: Interferometers ; Techniques: Image Processing ; Techniques: Interferometric ; Telescopes
ISSN: 0035-8711
E-ISSN: 1365-2966
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• 4
Article
Language: English
In: The Astrophysical Journal, 2017, Vol.835(1), p.49 (13pp)
Description: The EDGES High-Band experiment aims to detect the sky-average brightness temperature of the 21 cm signal from the epoch of reionization in the redshift range . To probe this redshifted signal, EDGES High-Band conducts single-antenna measurements in the frequency range 90–190 MHz from the Murchison Radio-astronomy Observatory in Western Australia. In this paper, we describe the current strategy for calibration of the EDGES High-Band receiver and report calibration results for the instrument used in the 2015–2016 observational campaign. We propagate uncertainties in the receiver calibration measurements to the antenna temperature using a Monte Carlo approach. We define a performance objective of 1 mK residual rms after modeling foreground subtraction from a fiducial temperature spectrum using a five-term polynomial. Most of the calibration uncertainties yield residuals of 1 mK or less at confidence. However, current uncertainties in the antenna and receiver reflection coefficients can lead to residuals of up to 20 mK even in low-foreground sky regions. These dominant residuals could be reduced by (1) improving the accuracy in reflection measurements, especially their phase, (2) improving the impedance match at the antenna-receiver interface, and (3) decreasing the changes with frequency of the antenna reflection phase.
Keywords: Astrophysics - Instrumentation And Methods For Astrophysics ; Astrophysics - Cosmology And Nongalactic Astrophysics;
ISSN: 0004-637X
E-ISSN: 1538-4357
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• 5
Article
Description: We report absolutely calibrated measurements of diffuse radio emission between 90 and 190 MHz from the Experiment to Detect the Global EoR Signature (EDGES). EDGES employs a wide beam zenith-pointing dipole antenna centred on a declination of -26.7$^\circ$. We measure the sky brightness temperature as a function of frequency averaged over the EDGES beam from 211 nights of data acquired from July 2015 to March 2016. We derive the spectral index, $\beta$, as a function of local sidereal time (LST) and find -2.60 〉 $\beta$ 〉 -2.62 $\pm$0.02 between 0 and 12 h LST. When the Galactic Centre is in the sky, the spectral index flattens, reaching $\beta$ = -2.50 $\pm$0.02 at 17.7 h. The EDGES instrument is shown to be very stable throughout the observations with night-to-night reproducibility of $\sigma_{\beta}$ 〈 0.003. Including systematic uncertainty, the overall uncertainty of $\beta$ is 0.02 across all LST bins. These results improve on the earlier findings of Rogers & Bowman (2008) by reducing the spectral index uncertainty from 0.10 to 0.02 while considering more extensive sources of errors. We compare our measurements with spectral index simulations derived from the Global Sky Model (GSM) of de Oliveira-Costa et al. (2008) and with fits between the Guzm\'an et al. (2011) 45 MHz and Haslam et al. (1982) 408 MHz maps. We find good agreement at the transit of the Galactic Centre. Away from transit, the GSM tends to over-predict (GSM less negative) by 0.05 〈 $\Delta_{\beta} = \beta_{\text{GSM}}-\beta_{\text{EDGES}}$ 〈 0.12, while the 45-408 MHz fits tend to over-predict by $\Delta_{\beta}$ 〈 0.05.
Keywords: Astrophysics - Instrumentation And Methods For Astrophysics ; Astrophysics - Astrophysics Of Galaxies
ISSN: 00358711
E-ISSN: 13652966
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• 6
Article
In: The Astrophysical Journal, 09/20/2017, Vol.847(1), p.64
Description: We report constraints on the global $21$ cm signal due to neutral hydrogen at redshifts $14.8 \geq z \geq 6.5$. We derive our constraints from low foreground observations of the average sky brightness spectrum conducted with the EDGES High-Band instrument between September $7$ and October $26$, $2015$. Observations were calibrated by accounting for the effects of antenna beam chromaticity, antenna and ground losses, signal reflections, and receiver parameters. We evaluate the consistency between the spectrum and phenomenological models for the global $21$ cm signal. For tanh-based representations of the ionization history during the epoch of reionization, we rule out, at $\geq2\sigma$ significance, models with duration of up to $\Delta z = 1$ at $z\approx8.5$ and higher than $\Delta z = 0.4$ across most of the observed redshift range under the usual assumption that the $21$ cm spin temperature is much larger than the temperature of the cosmic microwave background (CMB) during reionization. We also investigate a `cold' IGM scenario that assumes perfect Ly$\alpha$ coupling of the $21$ cm spin temperature to the temperature of the intergalactic medium (IGM), but that the IGM is not heated by early stars or stellar remants. Under this assumption, we reject tanh-based reionization models of duration $\Delta z \lesssim 2$ over most of the observed redshift range. Finally, we explore and reject a broad range of Gaussian models for the $21$ cm absorption feature expected in the First Light era. As an example, we reject $100$ mK Gaussians with duration (full width at half maximum) $\Delta z \leq 4$ over the range $14.2\geq z\geq 6.5$ at $\geq2\sigma$ significance.
Keywords: Astrophysics - Cosmology And Nongalactic Astrophysics ; Astrophysics - Instrumentation And Methods For Astrophysics;
ISSN: The Astrophysical Journal
ISSN: 0004637X
E-ISSN: 1538-4357
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• 7
Conference Proceeding
Language: English
In: 2015 USNC-URSI Radio Science Meeting (Joint with AP-S Symposium), July 2015, pp.296-296
Description: Removal of the sky foreground is critical to the success of the Experiment to Detect the Global Epoch of reionization Signature (EDGES) program. We evaluate terrestrial locations for the ease of removing the sky foreground from the convolution of the antenna beam with the frequency scaled Haslam skymap. We use two dipole antennas: 1) a ½-wavelength dipole whose beam can be expressed in a closed-form equation and 2) the EDGES dipole antenna, both over ground planes.
Keywords: Communication, Networking and Broadcast Technologies ; Fields, Waves and Electromagnetics ; Photonics and Electrooptics;
Source: IEEE Conference Publications
Source: IEEE Xplore
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• 8
Conference Proceeding
Language: English
In: 2014 United States National Committee of URSI National Radio Science Meeting (USNC-URSI NRSM), January 2014, pp.1-1
Description: Recent interest in high redshift cosmology observations with the redshifted 21cm line has rekindled exploration of the VHF radio band (50-200MHz) for radio astronomy. Single antenna instruments like the ground-based EDGES and the proposed lunar orbiting DARE have the goal of characterizing the global HI signal and extracting astrophysical and cosmological information. One limitation over much of the band is strong man-made and naturally occurring interference, which DARE avoids by observing as it orbits the far side of the moon. Another advantage of space-based observing is avoidance of the ionosphere which becomes increasingly reflective at the lower end of the VHF band. Technical challenges to this type of mission include development of lower power wide-band spectrometers, better mapping of Earth originating interference, and incorporation of lessons learned from ongoing ground-based experiments. One of the main challenges faced by EDGES, observing the narrower but clean stretch of bandwidth found in Western Australia, is calibrating the spectral response of the antenna at the required 0.01dB level.
Keywords: Space Vehicles ; Antennas ; Interference ; Bandwidth ; Educational Institutions ; Earth ; Propulsion
Source: IEEE Conference Publications
Source: IEEE Xplore
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• 9
Article
Language: English
In: The Astrophysical Journal, 2009, Vol.695(1), pp.183-199
Description: Subtraction of astrophysical foreground contamination from dirty sky maps produced by simulated measurements of the Murchison Widefield Array (MWA) has been performed by fitting a third-order polynomial along the spectral dimension of each pixel in the data cubes. The simulations are the first to include the unavoidable instrumental effects of the frequency-dependent primary antenna beams and synthesized array beams. They recover the one-dimensional spherically binned input redshifted 21 cm power spectrum within 1% over the scales probed most sensitively by the MWA (0.01 k 1 Mpc 1 ) and demonstrate that realistic instrumental effects will not mask the epoch of reionization signal. We find that the weighting function used to produce the dirty sky maps from the gridded visibility measurements is important to the success of the technique. Uniform weighting of the visibility measurements produces the best results, whereas natural weighting significantly worsens the foreground subtraction by coupling structure in the density of the visibility measurements to spectral structure in the dirty sky map data cube. The extremely dense uv -coverage of the MWA was found to be advantageous for this technique and produced very good results on scales corresponding to | u | 500 in the uv -plane without any selective editing of the uv -coverage.
Keywords: Astrophysics;
ISSN: 0004-637X
E-ISSN: 1538-4357
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• 10
Article
In: 2012 ApJ 759 17
Description: We introduce the Fast Holographic Deconvolution method for analyzing interferometric radio data. Our new method is an extension of A-projection/software-holography/forward modeling analysis techniques and shares their precision deconvolution and widefield polarimetry, while being significantly faster than current implementations that use full direction-dependent antenna gains. Using data from the MWA 32 antenna prototype, we demonstrate the effectiveness and precision of our new algorithm. Fast Holographic Deconvolution may be particularly important for upcoming 21 cm cosmology observations of the Epoch of Reionization and Dark Energy where foreground subtraction is intimately related to the precision of the data reduction. Comment: ApJ accepted
Keywords: Astrophysics - Instrumentation And Methods For Astrophysics ; Astrophysics - Cosmology And Extragalactic Astrophysics
ISSN: 0004637X
E-ISSN: 15384357
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