Interferometric Radio Telescope Research Articles

Low-Cost, Low-Loss, Ultra-Wideband Compact Feed for Interferometric Radio Telescopes

We have developed, manufactured, and tested a new feed design for interferometric radio telescopes with “large-N, small-D” designs. Such arrays require low-cost and low-complexity feeds for mass production on reasonable timescales and budgets, and also require those feeds to be compact to minimize obstruction of the dishes, along with having ultra-wide frequency bands of operation for most current and future science goals. The feed presented in this paper modifies the exponentially tapered slot antenna (Vivaldi) and quad-ridged flared horn antenna designs by having an oversized backshort, a novel method of maintaining a small size that is well-suited for deeper dishes ([Formula: see text]). It is made of laser cut aluminum and printed circuit boards, such that it is inexpensive ([Formula: see text][Formula: see text]USD per feed in large-scale production) and quick to build; it has a 5:1 frequency ratio, and its size is approximately a third of its longest operating wavelength. We present the science and engineering constraints that went into design decisions, the development and optimization process, and the simulated performance. A version of this feed design was optimized and fabricated for the Canadian Hydrogen Observatory and Radio-transient Detector (CHORD) prototypes. When simulated on CHORD’s very deep dishes ([Formula: see text]) and with CHORD’s custom first-stage amplifiers, the on-sky system temperature [Formula: see text] of the complete receiving system from dish to digitizer remains below 30[Formula: see text]K over most of the 0.3–1.5[Formula: see text]GHz band, and maintains an aperture efficiency [Formula: see text] between 0.4 and 0.6. The entire receiving chain operates at ambient temperature. The feed is designed to slightly under-illuminate the CHORD dishes, in order to minimize coupling between array elements and spillover.

Detection of interstellar cyanamide (NH$$_{2}$$CN) towards the hot molecular core G10.47$$+$$0.03

In the interstellar medium, the amide-type molecules play an important role in the formation of the prebiotic molecules in the hot molecular cores or high-mass star formation regions. The complex amide-related molecule cyanamide (NH$_{2}$CN) is known as one of the rare interstellar molecule which has played a major role in the formation of urea (NH$_{2}$CONH$_{2}$). In this article, we presented the detection of the emission lines of cyanamide (NH$_{2}$CN) towards the hot molecular core G10.47+0.03 between the frequency range 158.49$-$160.11 GHz using the Atacama Large Millimeter/submillimeter Array (ALMA) interferometric radio telescope. The estimated column density of the emission lines of NH$_{2}$CN using the rotational diagram model was $N$(NH$_{2}$CN) = (6.60$\pm$0.1)$\times$10$^{15}$ cm$^{-2}$ with rotational temperature ($T_{rot}$) = 201.2$\pm$3.3 K. The fractional abundance of NH$_{2}$CN with respect to H$_{2}$ towards G10.47+0.03 was $f$(NH$_{2}$CN) = 5.076$\times$10$^{-8}$. Additionally, we estimated the NH$_{2}$CN/NH$_{2}$CHO abundance ratio towards the G10.47+0.03 was 0.170, which was nearly similar with NH$_{2}$CN/NH$_{2}$CHO abundance ratio towards IRAS 16293$-$2422 B and Sgr B2 (M). We found that the observed abundance of NH$_{2}$CN with respect to H$_{2}$ towards G10.47+0.03 fairly agrees with the theoretical value predicted by Garrod (2013). We also discussed the possible formation and destruction pathways of NH$_{2}$CN.

The Deployable Low‐Band Ionosphere and Transient Experiment

AbstractThe Deployable Low‐Band Ionosphere and Transient Experiment (DLITE) is a four‐element interferometric radio telescope made from mostly commercial off‐the‐shelf parts to minimize costs and maximize ease of deployment. It operates in the high frequency and very high frequency (VHF) regimes, nominally in a 30–40 MHz band, but with good sensitivity (sky‐noise dominated) in the 20–80 MHz range. Its configuration is optimized to probe ionospheric structure using the so‐called “A‐Team,” exceptionally bright sources of cosmic radio emission. Methods have been developed to track the apparent positions and intensities of A‐Team sources without the need for beam forming to enable measurements of VHF scintillations as well as total electron content gradients. Time difference of arrival and frequency difference of arrival methods have been adapted for all‐sky imaging to facilitate both statistical measurements of scintillation levels and time domain astronomy. This study provides a detailed description of the system design, the analysis algorithms, and the science that can be conducted using results from two prototype DLITE systems in Maryland and New Mexico.

The Properties and Origins of Corotating Plasmaspheric Irregularities: Part II—Tomography With Compact Arrays of GPS Receivers

AbstractThis paper describes insights into the nature of corotating plasmaspheric irregularities (CPIs) enabled by a newly developed method for range‐ and time‐resolved tomographic images of these structures. This was originally developed using high‐precision measurements of total electron content (TEC) gradients toward cosmic radio sources using an interferometric radio telescope, the Very Large Array (VLA). Here, the method has been adapted to work with TEC measurements from compact arrays of Global Positioning System (GPS) receivers in California and Hawaii. Because the VLA is much more sensitive to line‐of‐sight density fluctuations and the background density within the plasmasphere drops quickly with radial distance, the GPS‐based analysis has a much more limited range (maximum of ∼5,000 km with GPS versus ∼15,000 with the VLA). However, because the GPS arrays collect data continuously, they offer much more complete temporal coverage and range resolution. This enabled a thorough comparison between CPI behavior near solar maximum and that near solar minimum, which was not possible with the VLA. The solar minimum data largely agree with previously reported VLA‐based data, which were mostly confined to times near solar minimum. These indicated that the primary drivers of CPIs during these conditions are perturbations within the background electric field associated with so‐called electro‐buoyancy waves within the midlatitude ionosphere. However, the detection rates of and properties of CPIs were often markedly different near solar maximum. This points to different CPI drivers related to solar/geomagnetic activity such as substorms and/or the interchange instability at lower L‐shells (∼3–4).

The Properties and Origins of Corotating Plasmaspheric Irregularities as Revealed Through a New Tomographic Technique

AbstractThis paper describes insights into the nature of corotating plasmaspheric irregularities (CPIs) enabled by a newly developed method for range‐ and time‐resolved tomographic images of these structures. The method is based on high‐precision measurements of total electron content gradients measure toward cosmic radio sources using an interferometric radio telescope, the Very Large Array (VLA). Exploiting hundreds of hours of data from the VLA Low‐band Ionosphere and Transient Experiment, we confirm that CPIs are predominantly detected at night and at shells 2–4. Combining this large data set with other ionospheric, atmospheric, and space weather databases, we have established that CPI detection rates are much higher when levels of solar and geomagnetic activity are low. We also demonstrate that higher detection rates coincide with drivers of perturbations in the ionospheric electric field, including sporadic E and gravity waves. The gravity wave sources most commonly associated with CPIs are jet stream bend/shear and substorm‐triggered atmospheric disturbances. We also show that simulations of the plasmaspheric impact of both gravity wave‐driven electric field perturbations and polarization electric fields associated with electrobuoyancy waves/sporadic E produce results that are very similar to what is observed with the VLA.

Robust profile decomposition for large extragalactic spectral-line surveys

We present a novel algorithm that is based on a Bayesian Markov Chain Monte Carlo (MCMC) technique for performing robust profile analysis of a data cube from either single-dish or interferometric radio telescopes. It fits a set of models comprised of a number of Gaussian components given by the user to individual line-of-sight velocity profiles, then compares them and finds an optimal model based on the Bayesian Inference Criteria computed for each model.The decomposed Gaussian components are then classified into bulk or non-circular motions as well as kinematically cold or warm components. The fitting based on the Bayesian MCMC technique is insensitive to initial estimates of the parameters, and suffers less from finding the global minimum in models given enough sampling points and a wide range of priors for the parameters. It is found to provide reliable profile decomposition and classification of the decomposed components in a fully automated way, together with robust error estimation of the parameters as shown by performance tests using artificial data cubes. We apply the newly developed algorithm to the HI data cubes of sample galaxies from the Local Volume HI galaxy Survey (LVHIS). We also compare the kinematically cold and warm components, and bulk velocity fields with previous analyses made in a classical method.

Online radio interferometric imaging: assimilating and discarding visibilities on arrival

The emerging generation of radio interferometric (RI) telescopes, such as the Square Kilometre Array (SKA), will acquire massive volumes of data and transition radio astronomy to a big-data era. The ill-posed inverse problem of imaging the raw visibilities acquired by RI telescopes will become significantly more computationally challenging, particularly in terms of data storage and computational cost. Current RI imaging methods, such as CLEAN, its variants, and compressive sensing approaches (sparse regularisation), have yielded excellent reconstruction fidelity. However, scaling these methods to big-data remains difficult if not impossible in some cases. All state-of-the-art methods in RI imaging lack the ability to process data streams as they are acquired during the data observation stage. Such approaches are referred to as online processing methods. We present an online sparse regularisation methodology for RI imaging. Image reconstruction is performed simultaneously with data acquisition, where observed visibilities are assimilated into the reconstructed image as they arrive and then discarded. Since visibilities are processed online, good reconstructions are recovered much faster than standard (offline) methods which cannot start until the data acquisition stage completes. Moreover, the online method provides additional computational savings and, most importantly, dramatically reduces data storage requirements. Theoretically, the reconstructed images are of the same fidelity as those recovered by the equivalent offline approach and, in practice, very similar reconstruction fidelity is achieved. We anticipate online imaging techniques, as proposed here, will be critical in scaling RI imaging to the emerging big-data era of radio astronomy.

PL612 LOFAR station sensitivity measurements in the context of its application for pulsar observations

The Low Frequency Array (LOFAR) is relatively new interferometric radio telescope. LOFAR operates in poorly explored part of the spectrum (10–240 MHz). In this frequency range pulsars are bright, and LOFAR observations of pulsars are import for probing fundamental physics and the interstellar medium. In this work we present the description of the international LOFAR station frontend and backend parameters related to pulsar observations with a single LOFAR station, based on observations with the Polish station, located in Bałdy near Olsztyn. We present new results from observations aimed at better characterizing the telescope sensitivity. These measurements are important for efficient planning of pulsar observating campaigns. Observations show good sensitivity for pulsar measurements above 25° elevation and show the noise level rises significantly for elevations below 25°.

Detecting radio frequency interference in radio‐antenna arrays with the recurrent neural network algorithm

Signal artifacts due to radio frequency interference (RFI) are a common nuisance in radio astronomy. Conventionally, the RFI‐affected data are tagged by an expert data analyst in order to warrant data quality. In view of the increasing data rates obtained with interferometric radio telescope arrays, automatic data filtering procedures are mandatory. Here, we present the results from the implementation of a RFI‐detecting recurrent neural network (RNN) employing long short‐term memory (LSTM) cells. For the training of the algorithm, a discrete model was used that distinguishes RFI and non‐RFI data, based on the amplitude information from radio interferometric observations with the Giant Metre‐Wave Radio Telescope (GMRT) at 610 MHz. The performance of the RNN is evaluated by analyzing a confusion matrix. The true positive and true negative rates of the network are ∼99.9 and ∼97.9%, respectively. However, the overall efficiency of the network is ∼30% because of the fact that a large amount non‐RFI data is classified as being contaminated by RFI. Matthews correlation coefficient is 0.42, suggesting that a still more refined training model is required.

Direction-dependent Corrections in Polarimetric Radio Imaging. II. A-solver Methodology: A Low-order Solver for the A-term of the A-projection Algorithm

Abstract The effects of the antenna far-field power pattern limits the imaging performance of modern wide-bandwidth, high-sensitivity interferometric radio telescopes. Given a model for the aperture illumination pattern (AIP) of the antenna, referred to as the A-term, the wide-band (WB) A-Projection algorithm corrects for the effects of its time, frequency, and polarization structure. The level to which this correction is possible depends on how accurately the A-term represents the true AIP. In this paper, we describe the A-Solver methodology that combines physical modeling with optimization to holographic measurements to build an accurate model for the AIP. Using a parametrized ray-tracing code as the predictor, we solve for the frequency dependence of the antenna optics and show that the resulting low-order model for the Karl G. Jansky Very Large Array antenna captures the dominant frequency-dependent terms. The A-Solver methodology described here is generic and can be adapted for other types of antennas as well. The parameterization is based on the physical characteristics of the antenna structure and optics and is therefore arguably a compact representation (minimized degrees of freedom) of the frequency-dependent structure of the antenna A-term. In this paper, we also show that the parameters derived from A-Solver methodology are expected to improve sensitivity and imaging performance out to the first side-lobe of the antenna.

Building Models for Extended Radio Sources: Implications for Epoch of Reionisation Science

AbstractWe test the hypothesis that limitations in the sky model used to calibrate an interferometric radio telescope, where the model contains extended radio sources, will generate bias in the Epoch of Reionisation power spectrum. The information contained in a calibration model about the spatial and spectral structure of an extended source is incomplete because a radio telescope cannot sample all Fourier components. Application of an incomplete sky model to calibration of Epoch of Reionisation data will imprint residual error in the data, which propagates forward to the Epoch of Reionisation power spectrum. This limited information is studied in the context of current and future planned instruments and surveys at Epoch of Reionisation frequencies, such as the Murchison Widefield Array (MWA), Giant Metrewave Radio Telescope and the Square Kilometre Array (SKA1-Low). For the MWA Epoch of Reionisation experiment, we find that both the additional short baselineuv-coverage of the compact Epoch of Reionisation array, and the additional long baselines provided by TGSS and planned MWA expansions, are required to obtain sufficient information on all relevant scales. For SKA1-Low, arrays with maximum baselines of 49 km and 65 km yield comparable performance at 50 MHz and 150 MHz, while 39 km, 14 km, and 4 km arrays yield degraded performance.

Possible gamma-ray burst radio detections by the Square Kilometre Array. New perspectives

The next generation interferometric radio telescope, the Square Kilometre Array (SKA), which will be the most sensitive and largest radio telescope ever constructed, could greatly contribute to the detection, survey and characterization of Gamma Ray Bursts (GRBs). By the SKA, it will be possible to perform the follow up of GRBs even for several months. This approach would be extremely useful to extend the Spectrum Energetic Distribution (SED) from the gamma to the to radio band and would increase the number of radio detectable GRBs. In principle, the SKA could help to understand the physics of GRBs by setting constraints on theoretical models. This goal could be achieved by taking into account multiple observations at different wavelengths in order to obtain a deeper insight of the sources. Here, we present an estimation of GRB radio detections, showing that the GRBs can really be observed by the SKA. The approach that we present consists in determining blind detection rates derived by a very large sample consisting of merging several GRB catalogues observed by current missions as Swift, Fermi, Agile and INTEGRAL and by previous missions as BeppoSAX, CGRO, GRANAT, HETE-2, Ulysses and Wind. The final catalogue counts 7516 distinct sources. We compute the fraction of GRBs that could be observed by the SKA at high and low frequencies, above its observable sky. Considering the planned SKA sensitivity and through an extrapolation based on previous works and observations, we deduce the minimum fluence in the range 15-150 keV. This is the energy interval where a GRB should emit to be detectable in the radio band by the SKA. Results seem consistent with observational capabilities.

The Low-Frequency Environment of the Murchison Widefield Array: Radio-Frequency Interference Analysis and Mitigation

AbstractThe Murchison Widefield Array is a new low-frequency interferometric radio telescope built in Western Australia at one of the locations of the future Square Kilometre Array. We describe the automated radio-frequency interference detection strategy implemented for the Murchison Widefield Array, which is based on the aoflagger platform, and present 72–231 MHz radio-frequency interference statistics from 10 observing nights. Radio-frequency interference detection removes 1.1% of the data. Radio-frequency interference from digital TV is observed 3% of the time due to occasional ionospheric or atmospheric propagation. After radio-frequency interference detection and excision, almost all data can be calibrated and imaged without further radio-frequency interference mitigation efforts, including observations within the FM and digital TV bands. The results are compared to a previously published Low-Frequency Array radio-frequency interference survey. The remote location of the Murchison Widefield Array results in a substantially cleaner radio-frequency interference environment compared to Low-Frequency Array’s radio environment, but adequate detection of radio-frequency interference is still required before data can be analysed. We include specific recommendations designed to make the Square Kilometre Array more robust to radio-frequency interference, including: the availability of sufficient computing power for radio-frequency interference detection; accounting for radio-frequency interference in the receiver design; a smooth band-pass response; and the capability of radio-frequency interference detection at high time and frequency resolution (second and kHz-scale respectively).

Advanced spectral analysis of ionospheric waves observed with sparse arrays

This paper presents a case study from a single, 6h observing period to illustrate the application of techniques developed for interferometric radio telescopes to the spectral analysis of observations of ionospheric fluctuations with sparse arrays. We have adapted the deconvolution methods used for making high dynamic range images of cosmic sources with radio arrays to making comparably high dynamic range maps of spectral power of wavelike ionospheric phenomena. In the example presented here, we have used observations of the total electron content (TEC) gradient derived from Very Large Array (VLA) observations of synchrotron emission from two galaxy clusters at 330MHz as well as GPS‐based TEC measurements from a sparse array of 33 receivers located within New Mexico near the VLA. We show that these techniques provide a significant improvement in signal‐to‐noise ratio (S/N) of detected wavelike structures by correcting for both measurement inaccuracies and wavefront distortions. This is especially true for the GPS data when combining all available satellite/receiver pairs, which probe a larger physical area and likely have a wider variety of measurement errors than in the single‐satellite case. In this instance, we found that the peak S/N of the detected waves was improved by more than an order of magnitude. The data products generated by the deconvolution procedure also allow for a reconstruction of the fluctuations as a two‐dimensional waveform/phase screen that can be used to correct for their effects.

Revisiting the spread spectrum effect in radio interferometric imaging: a sparse variant of the w-projection algorithm

Next-generation radio interferometric telescopes will exhibit non-coplanar baseline configurations and wide field-of-views, inducing a w-modulation of the sky image, which in turn induces the spread spectrum effect. We revisit the impact of this effect on imaging quality and study a new algorithmic strategy to deal with the associated operator in the image reconstruction process. In previous studies it has been shown that image recovery in the framework of compressed sensing is improved due to the spread spectrum effect, where the w-modulation can act to increase the incoherence between measurement and sparsifying signal representations. For the purpose of computational efficiency, idealised experiments were performed, where only a constant baseline component w in the pointing direction of the telescope was considered. We extend this analysis to the more realistic setting where the w-component varies for each visibility measurement. Firstly, incorporating varying w-components into imaging algorithms is a computational demanding task. We propose a variant of the w-projection algorithm for this purpose, which is based on an adaptive sparsification procedure, and incorporate it in compressed sensing imaging methods. This sparse matrix variant of the w-projection algorithm is generic and adapts to the support of each kernel. Consequently, it is applicable for all types of direction-dependent effects. Secondly, we show that for w-modulation with varying w-components, reconstruction quality is significantly improved compared to the setting where there is no w-modulation (i.e. w=0), reaching levels comparable to the quality of a constant, maximal w-component. This finding confirms that one may seek to optimise future telescope configurations to promote large w-components, thus enhancing the spread spectrum effect and consequently the fidelity of image reconstruction.

An AIPS-based, distributed processing method for large radio interferometric datasets

The data output rates of modern radio interferometric telescopes make the traditional data reduction process impractical in many cases. We report on the implementation of a lightweight infrastructure, named AIPSLite, that enables the deployment of AIPS interferometric processing routines on distributed systems in an autonomous and fault tolerant manner. We discuss how this approach was used to search for sources of 6.7 GHz methanol maser emission in the Cep A region with the European VLBI Network (EVN). The field was searched out to a radius of 1.25 min^(−1) at milli-arcsecond spatial resolution and 1024 frequency channels with 0.088 km s^(−1) velocity resolution. The imaged data was on the order of 30 TB. Processing was performed on 128 processors of the Irish Centre for High End Computing (ICHEC) linux cluster with a run time of 42 h, and a total of 212 CPU days.

Compressed sensing for wide-field radio interferometric imaging

For the next generation of radio interferometric telescopes it is of paramount importance to incorporate wide field-of-view (WFOV) considerations in interferometric imaging, otherwise the fidelity of reconstructed images will suffer greatly. We extend compressed sensing techniques for interferometric imaging to a WFOV and recover images in the spherical coordinate space in which they naturally live, eliminating any distorting projection. The effectiveness of the spread spectrum phenomenon, highlighted recently by one of the authors, is enhanced when going to a WFOV, while sparsity is promoted by recovering images directly on the sphere. Both of these properties act to improve the quality of reconstructed interferometric images. We quantify the performance of compressed sensing reconstruction techniques through simulations, highlighting the superior reconstruction quality achieved by recovering interferometric images directly on the sphere rather than the plane.

Primary-Beam Shape Calibration from Mosaicked, Interferometric Observations

ABSTRACTImage quality in mosaicked observations from interferometric radio telescopes is strongly dependent on the accuracy with which the antenna primary beam is calibrated. The next generation of radio telescope arrays such as the Allen Telescope Array (ATA) and the Square Kilometer Array (SKA) have key science goals that involve making large mosaicked observations filled with bright point sources. We present a new method for calibrating the shape of the telescope’s mean primary beam that uses the multiple redundant observations of these bright sources in the mosaic. The method has an analytical solution for simple Gaussian beam shapes but can also be applied to more complex beam shapes through χ2 minimization. One major benefit of this simple, conceptually clean method is that it makes use of the science data for calibration purposes, thus saving telescope time and improving accuracy through simultaneous calibration and observation. We apply the method both to 1.43 GHz data taken during the ATA Twenty Centimeter Survey (ATATS) and to 3.14 GHz data taken during the ATA’s Pi Gigahertz Sky Survey (PiGSS). We find that the beam’s calculated full width at half-maximum (FWHM) values are consistent with the theoretical values, the values measured by several independent methods, and the values from the simulation we use to demonstrate the effectiveness of our method on data from future telescopes such as the expanded ATA and the SKA. These results are preliminary and can be expanded upon by fitting more complex beam shapes. We also investigate, by way of a simulation, the dependence of the accuracy of the telescope’s FWHM on antenna number. We find that the uncertainty returned by our fitting method is inversely proportional to the number of antennas in the array.

Solving for closure errors due to polarization leakage in radio interferometry of unpolarized sources

Mechanical and electronic imperfections can result into polarization leakage in individual antennas of a radio interferometer. Such leakages manifest themselves as closure errors even in co-polar visibility measurements of unpolarized sources. This paper describes and tests a method for the study of polarization leakage for radio interferometric telescopes using only the nominally co-polar visibilities for unpolarized calibrators. Interpretation of the resulting closure phases on the Poincar e sphere is presented. Since unpolarized sources are used, the actual solutions for leakage parameters is subject to a degeneracy which is discussed. This however, does not aect the correction of closure errors in our scheme.

An unusually strong Einstein ring in the radio source PKS1830–211

RADIO observations of the strong, flat-spectrum radio source PKS1830–211 revealed a double structure, with a separation of 1 arcsec, suggesting that it might be a gravitationally lensed object1. We have now obtained high-resolution radio images of PKS1830–211 from several interferometric radiotelescope networks, which show an unusual elliptical ring-like structure connecting the two brighter components. The presence of the ring, and the similarity of the two brighter spots, argue strongly that this is indeed a gravitationally lensed system, specifically an Einstein ring in which lens and lensed object are closely aligned. Although the source is close to the galactic plane, it seems that both the lens and background (lensed) object are extragalactic. This object is one hundred times brighter than either of the two previously discovered radio Einstein rings, and is among the six brightest flat-spectrum sources in the sky. Its brightness makes it a peculiar object: it must involve either a chance alignment of a lensing object with an unusually bright background source, or an alignment with a less bright object but amplified to an unusual degree.