REU Reports

2005

Simulating the Effects of the Ionosphere on MWA Measurements
(1.2 MB PDF)
Sean Ting, Stanford University

The epoch of reionization (EOR), a period in the early universe during which the formation of the first stars and/or active galactic nuclei ionized the surrounding intergalactic medium, has become a hot topic in astrophysics and cosmology.Presently, telescopes cannot take accurate readings at the frequencies necessary for probing the structure of the universe during this era. However, there is an effort to build an array in Western Australia, the Mileura Widefield Array, which should have the required sensitivity, precision, and frequency range. In order to test this, a simulation has been built at Haystack that mimics the measurements the MWA will take. This summer I have examined the effects of inserting an ionosphere into this simulation. This has included testing preexisting code, creating a model of the ionosphere, and exploring different methods for correcting distortions of the data due to the ionosphere.

2006

Advanced Radio Imaging Techniques
Presentation (PDF)
Dylan Nelson, University of California Berkeley

The science requirements of next-generation radio telescope arrays present a new set of challenges to traditional imaging and data processing techniques. Instruments such as the MWA, SKA, and other arrays implementing large numbers of small diameter dishes have a naturally large field-of-view due to the small diameter of individual telescopes. In order to achieve high image fidelity and dynamic ranges, noise contributions from off-center sources must be reduced — a task traditionally requiring imaging of the full field-of-view. However, implementation of this subtraction requires unreasonable computational speeds while also generating unmanageable volumes of data. One approach to this problem involves dynamic control over the field-of-view, implemented in software as a weighting function internal to the integration routine of the correlator. Using this technique one can effectively reduce the noise contribution levels from sources outside the region of interest, and in some cases dramatically reduce the volume of data exiting the correlator for post-processing. In this paper we focus on verifying the effectiveness of this technique, implemented through the MIT Array Performance Simulator (MAPS) using simulated data sets. Additionally, several dimensions of possible parameter space are explored in order to test limitations and determine design requirements of this approach, including the impact of variable levels of radio frequency interference (RFI) excision on image fidelity and off-center source signal rejection.

2007

Algorithms for Transient Detection
Presentation (PDF)
John Gilling, Boston University

One of the key science areas of the Mileura Wide-Field Array (MWA) will be to scan the sky for transient radio emissions, originating from sources in the sky ranging from large Jupiter-like planets to active galactic nuclei. A central element of the transient detection software collaboration is the All Sky Monitor (ASM), which will make continuous comparisons in real-time between the observed images and the reference sky model, appropriately modified to account for the array characteristics. This process consists of convolving the sky with an array beam pattern, in order to find the response of the array to the sky, and distinguishing RFI signals from astronomically notable transients. I have written a series of programs that calculate a beam pattern for a single tile, for a 32-tile prototype array, and for a 496-tile sample array, as well as programs that convolve a given reference sky with the calculated beam pattern and plot the brightness distribution. This software has been used to evaluate different approaches to the convolution and detection tasks, and will eventually be incorporated into the operational ASM software.

2008

Analysis and Simulations of 6T MWA Solar Data
Presentation (PDF)
Pauli Kehayias, Tufts University

The first set of interferometric data for the Murchison Widefield Array (MWA) prototype was collected in April, 2008 using six of the 32 tiles currently installed. Two hours of solar observations at 85 MHz and 180 MHz were analyzed to search for solar activity and to evaluate the instrument's performance. Four of the twelve input signals (two inputs per tile) were determined to be bad. No solar events were found, though four types of strongly monochromatic short-lived transient events, believed to be radio frequency interference, were identified and characterized. The bandshape stability over time for several baselines was examined. Bandshapes were found to maintain their shapes but had time-varying amplitudes. Simulations were done to determine whether the background radio sky, the Sun, or the instrument are responsible for the bandshape variation. The background radio sky explanation is unlikely to be the primary cause, and changing baseline fringe spacings and array responses at different orientations are being investigated.

Searching for Transients with the MWA
Presentation (PDF)
Munier Salem, Cornell University

The Murchison Widefield Array (MWA)'s unprecedented field of view and sensitivity allow observations across an entire hemisphere of sky. The All Sky Monitor (ASM) software of the MWA will utilize this novel capability to provide real-time detection of low frequency radio transients. This summer the ASM code was improved in multiple areas. The beam formation code was optimized by expressing the beam as a product of two factors, thus dramatically decreasing the run time. The ASM's calculated array beam was made more accurate by including a dipole-response model and instrumental gains and phases ascertained from calibration. The ASM uses this beam to produce the theoretical response of the array, a so-called "dirty image" of the night sky, in the array's pointing direction. Differences between the snapshot dirty images and the theoretical images are then used to detect deviations from the static sky. A new binning algorithm that sums snapshot images facilitates integration of data over a variety of timescales and with staggered intervals; the ASM can now perform rudimentary statistical analyses of these integrations to detect radio transients. We will present preliminary results obtained by applying the ASM detection software to a six-tile dataset.