# Very Long Baseline Array

The eastern terminus of the VLBA, on Saint Croix, U.S. Virgin Islands
The VLBA telescope in Owens Valley, California

The Very Long Baseline Array (VLBA) is a system of ten radio telescopes which are operated remotely from their Array Operations Center located in Socorro, New Mexico, as a part of the National Radio Astronomy Observatory (NRAO). These ten radio antennas work together as an array that forms the longest system in the world that uses very long baseline interferometry. The longest baseline available in this interferometer is about 8,611 kilometres (5,351 mi).[1]

The construction of the VLBA began in February 1986 and it was completed in May 1993. The first astronometrical observation using all ten antennas was carried out on May 29, 1993. The total cost of building the VLBA was about 85 million dollars.

Each receiver in the VLBA consists of a parabolic dish antenna 25 meters (82 feet) in diameter, along with its adjacent control building. This contains the supporting electronics and machinery for the receiver, including low-noise electronics, digital computers, data storage units, and the antenna-pointing machinery. Each of the antennas is about as tall as a ten-story building when the antenna is pointed straight up, and each antenna weighs about 218 metric tons (240 short tons).

The signals from each antenna are recorded on a bank of approximately one-terabyte hard disc drives, and the information is time-stamped using atomic clocks. Once the disc drives are loaded with information, they are carried to the Pete V. Domenici Science Operations Center at the NRAO in Socorro. There the information undergoes signal processing in a powerful set of digital computers that carry out the interferometry. These computers also make corrections for the rotation of the Earth, the slight shifts in the crust of the Earth over time, and other small measurement errors

## Observations by the VLBA

The Very Long Baseline Array usually makes radio observations at wavelengths from three millimeters to 28 centimeters, or in other words, at frequencies from 0.3 gigahertz to 96 gigahertz. Within this frequency range, the VLBA observes in eight different frequency bands that are useful for radio astronomy. The VLBA also makes observations in two narrow radio bands below one gigahertz that include spectral lines produced by bright maser emissions.

The VLBA radio telescopes are located at:

Hancock
North Liberty
Fort Davis
Los Alamos
Pie Town
Kitt Peak
Owens Valley
Brewster
Green Bank
VLA
VLBA locations (red) and HSA locations (blue) in the contiguous United States
St. Croix
Mauna Kea
Arecibo
Effelsberg
VLBA locations (red) and HSA locations (blue) around the world

The use of the VLBA can be scheduled dynamically, and its sensitivity can be improved by a factor of five by including other radio telescopes such as the Very Large Array (VLA) in New Mexico and the Green Bank Telescope in West Virginia with the VLBA. Four other sites are brought online for as much as 100 hours per four-month trimester. In this configuration, the entire array is known as the High-Sensitivity Array (HSA).[2] These sites, with coordinates, are as follows:

## Baseline distance and angular resolution

Distance between each VLBA baseline (km):[3] The longest baseline in the array is 8,611 kilometres (5,351 mi).[1]

SC HN NL FD LA PT KP OV BR MK EB AR GB Y27
SC ... 2853 3645 4143 4458 4579 4839 5460 5767 8611 6822 238 2708 4532
HN 2853 ... 1611 3105 3006 3226 3623 3885 3657 7502 5602 2748 829 3198
NL 3645 1611 ... 1654 1432 1663 2075 2328 2300 6156 6734 3461 1064 1640
FD 4143 3105 1654 ... 608 564 744 1508 2345 5134 8084 3922 2354 515
LA 4458 3006 1432 608 ... 236 652 1088 1757 4970 7831 4246 2344 226
PT 4579 3226 1663 564 236 ... 417 973 1806 4795 8014 4365 2551 52
KP 4839 3623 2075 744 652 417 ... 845 1913 4466 8321 4623 2939 441
OV 5460 3885 2328 1508 1088 973 845 ... 1214 4015 8203 5255 3323 1025
BR 5767 3657 2300 2345 1757 1806 1913 1214 ... 4398 7441 5585 3326 1849
MK 8611 7502 6156 5134 4970 4795 4466 4015 4398 ... 10328 8434 7028 4835
EB 6822 5602 6734 8084 7831 8014 8321 8203 7441 10328 ... 6911 6335 8008
AR 238 2748 3461 3922 4246 4365 4623 5255 5585 8434 6911 ... 2545 4317
GB 2708 829 1064 2354 2344 2551 2939 3323 3326 7028 6335 2545 ... 2516
Y27 4532 3198 1640 515 226 52 441 1025 1849 4835 8008 4317 2516 ...

Minimum angular resolution:

Wavelength (cm) 90 50 21 18 13 6 4 2 1 0.7
$\theta_{HPBW}$ (milliarcseconds) 22 12 5.0 4.3 3.2 1.4 0.85 0.47 0.32 0.17