This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
In order for astronomers to have the highest possible resolution to detect objects of the smallest angular size, they require large telescopes, or they combine the light from telescopes that are separated from one another. As the separation between the telescopes increases, so does the ability to resolve finer details. The largest platform that is currently in use for astronomy is the earth itself. Radio astronomers use radio telescopes that are spread by thousands of miles in what is called the Very Large Baseline Array (VLBA). A large number of important scientific problems are investigated with the VLBA which also provides an important service in determining the positions and motions of objects on the sky, a branch of astronomy called astrometry. Astrometry is important for all astronomers, but it also serves commercial and defense interests for satellite navigation and calibration of the GPS system.
Radio astronomers at the National Radio Astronomy Observatory (NRAO) have realized that they can significantly increase the sensitivity of the VLBA by adding computing power and data storage capabilities to the signal processing equipment that correlates the signals from the telescopes. Their work, sponsored by the National Science Foundation's Major Research Instrumentation program, will allow scientists to see fainter objects to probe farther and deeper into space. It will also allow them to improve the accuracy of measurements made for astrometry. The work will be led by Dr. R. Craig Walker of NRAO in Socorro, NM.
The Very Long Baseline Array (VLBA) is a facility of the National Science Foundation that is part of the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities Inc (AUI). The VLBA was built for very high resolution radio astronomy and astrometry. It is used by astronomers from throughout the world with observing projects selected on the basis of the quality of their scientific proposals. The array has 10 antennas of 25m diameter each, spread across the United States from Hawaii to the Virgin Islands. The raw signals at each antenna are recorded on hard disk drives and shipped to Socorro, New Mexico. There a computer cluster known as a correlator is used to combine the signals from all possible pairs of antennas to determine the interference patterns (fringes). Those fringes are used to make images and to measure accurate positions (astrometry). The image resolution is better than 0.001 arcseconds, far higher even than the 0.04 arcsecond resolution of the Hubble Space Telescope, let alone the roughly 60 arcsecond resolution of the human eye. For measurements of the relative positions of sources that are less than a few degrees apart on the sky, accuracies on the order of 0.00001 arcseconds can be achieved. With the help of distant calibration sources, such as quasars, the distance to closer objects can be measured using triangulation based on the slight changes in their apparent position caused by the motion of the Earth around the Sun. This is the parallax method. It is the gold standard of astronomical distance measurements because it relies solely on pure geometry and is not dependent on complex models of the target objects. It has long been used optically to measure the distances to nearby stars, but the additional accuracy of the VLBA allows parallax distance measurements to be made for objects throughout our galaxy. Also, since the VLBA works at radio frequencies, it can make such measurements of objects in star formation regions that are obscured optically by dust. The astrometric accuracy of the VLBA also allows transverse motions to be measured, which, combined with Doppler measurements of spectral lines from gas or stars, allows the 3D motions of nearby galaxies to be measured. The usefulness of the high resolution imaging and precision astrometry depends on being able to observe a significant number of objects. For example, distances to nearby star formation regions can be measured with an accuracy near 1 percent, far better than possible with other techniques. Such distances are critical for understanding the physics of star formation because all energy measurements depend on the square of the distance. But the parallax distances are measured to stars that have weak radio emission. The current VLBA sensitivity is sufficient to measure a few such stars and very interesting initial results have been obtained. But an increase in sensitivity of even a factor of a few would allow many more stars to be observed, providing distances to many more regions and providing the 3D structure of nearby regions with multiple stars. In fact, nearly all areas of science done using the VLBA will benefit from an increase in sensitivity. There is an on-going project to increase the sensitivity of the VLBA by a factor of about 3 for all observations of broad-band sources by increasing the bandwidth. That would be equivalent to increasing the sizes of the antennas from 25m to 43m. The major elements of the upgrade are new digital electronics at the antennas to process more bandwidth, a new recording system to allow more bits per second to be recorded, and a new software correlator that can deal with all of those bits, while providing a variety of enhanced capabilities compared to the old correlator. These engineering developments have been funded internally by NRAO operations and by AUI. The most expensive aspect of the upgrade is simply the purchase of all the hard disk drives needed to record the bits at the antennas and ship them to the correlator. This proposal provided the funding for the hard drives required to offer a standard bandwidth of 256 MHz per polarization to the entire VLBA user community. It also funded some enhancements to the correlator. The cost share funding from AUI provided additional correlator capability and enhanced electronics at the antennas. The overall project has also benefited from contributions from some international partners, especially Mexico. The engineering developments for the upgrade, funded outside this proposal, are not yet complete so new astronomical results cannot be reported yet. The new equipment has been deployed to all of the VLBA antennas plus the Green Bank Telescope in West Virginia (another NRAO facility). The recording media and computers funded by this proposal are in hand. Initial successful tests have been done, but full testing and system integration is still in progress. It is expected that scientific observing with the new equipment will begin during 2011.
