A New Generation of Technology
The ngVLA, or Next Generation Very Large Array of telescopes, is the direct successor to the VLA and VLBA. The planned observatory will feature instruments ten times more sensitive with wider frequency reception and ability to resolve radio sources to within a fraction of a milliarcsecond. Specifically, the ngVLA will detect between submillimeter and decimeter wavelengths. The ngVLA will analyze frequencies from 1.2 – 116 GHz, which is in the centimetric range of 25 to 0.26 centimeter waves, with plenty of room for outside collaboration to experiment with co-located arrays to cover overlapping frequencies. For instance, the U.S. Low Frequency Radio Community is proposing a neighboring array that will be able to cover 20 MHz to 150 MHz. According to the FAQ from the ngVLA website, “An option to add a prime focus feed to the ngVLA antennas to cover 150 MHz to 800 MHz is also possible.”
Fundamentally in a coherent array it is necessary to continuously align the wavefront from the source prior to correlation and beamforming. In the conventional approach, this is done using coherent LOs and digitizer clocks to do the high temporal-precision portion of this alignment, followed by digital delay and phase tracking according to a model of the wavefront delay for a known LO, and then periodic calibrator source observations to facilitate final alignment such that the “white light fringe”, on every correlated baseline, stays near zero relative delay. However, the first part of this process can be done differently by using free-running and incoherent clocks at each antenna, measuring the phase/frequency of each one in a common clock domain, and then digitally correcting the data—amounting to delay and phase corrections—prior to wavefront delay+phase correction and correlation and beamforming.
NRAO, Enabling Tech
Around the World
The array of receivers will span around the world, and the operations will be based at the existing VLA in San Augustin, New Mexico with mid-baseline stations in Mexico, Arizona and Texas. The observation sites for the ngVLA will take place in Hawaii, Washington, California, Iowa, West Virginia, New Hampshire, Puerto Rico, the U.S. Virgin Islands, and Canada. There will be 10 baseline array stations (located at five VLBA sites in addition to radio facilities already in existence). The architecture of the ngVLA system will allow the array to be operated individually, as a whole, or divided into subarrays. There will be a total of 263 dishes, with 244 dishes at 18 meters diameter and 19 at 6 meters in diameter. 214 of these dishes are to be placed in the US Southwest, and make up the primary array. Thirty of the 18 meter dishes are located across the US at baselines of up to 9,000 km or nearly 6,000 miles apart. This Long Baseline Array, or LBA, will be integrated into the ngVLA system. The 18 meter design of the antennas has a low feed arm in an off set Gregorian geometry. This gives the array a longer arc across the sky and opens up possibilities for longer times and thus more detailed observations. The 18 meter antenna will be prototyped starting in 2022, as funded by the National Science Foundation. The additional 19 dishes of 6 meters diameter will create a Short Baseline Array, or SBA. The SBA is designed to detect parts of the viewing angle that are inaccessible or missed by the primary 18-meter dishes that make up the LBA. In addition to its modular ability to observe independently of the primary array, the SBA can team up with 4 of the LBA antennas in a total power mode to compensate for any gaps in the viewing plane of the smaller, 6 meter dish array.
Open Access
Construction of the ngVLA is proposed to begin starting in 2025, with initial Early Science experiments beginning in 2028, to be fully operational by 2034. Although the VLA will remain a part of the new project, there will be time of reduced operations at the facility. ngVLA scientists say that a plan is in the works to transition operations between the VLA and ngVLA, as well as set up collaborations. The new array was designed with collaboration in focus, able to complement other arrays such as ALMA, US-ELT, James Webb, and the proposed low frequency project, the Square Kilometer Array (SKA). The enhanced sensitivities and wide radiofrequency range of the ngVLA are indispensable to comparing infrared and optical data to its results, making it the second highest priority NRAO project. The ngVLA will make it possible to observe: pulsars to test relativity, the evolution of black holes and galaxies, the formation of exoplanets, and the formation of the Universe’s earliest galaxies and stars. In addition to providing an inter-collaborative environment, the new facility will take part in the Open Skies program just like its predecessor, the VLA – allowing any astronomer, regardless of institutional association, to propose view time at the ngVLA.
NRAO: ngVLA Strongly Endorsed https://public.nrao.edu/news/ngvla-strongly-endorsed-by-decadal-survey/
NRAO: ngVLA FAQ https://ngvla.nrao.edu/page/faq
NRAO: ngVLA Enabling Tech https://ngvla.nrao.edu/page/enabling-tech
S. Weinreb. Cryogenic 1.2 to 116 GHz receiver for large arrays. https://ieeexplore.ieee.org/document/8568499
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